Susana Filipa Antunes Rodrigues. Intermetallic layer formation and influence in solder joints reliability. Universidade do Minho Escola de Engenharia

Transcrição

1 Universidade do Minho Escola de Engenharia Susana Filipa Antunes Rodrigues Intermetallic layer formation and influence in solder joints reliability UMinho 2015 Susana Filipa Antunes Rodrigues Intermetallic layer formation and influence in solder joints reliability October 2015

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3 Universidade do Minho Escola de Engenharia Susana Filipa Antunes Rodrigues Intermetallic layer formation and influence in solder joints reliability Master Thesis Integrated Master in Materials Engineering Supervised by: Professor Doctor Delfim Fernandes Soares Professor Doctor José Carlos Fernandes Teixeira Dr. José Ricardo de Barros Alves October 2015

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5 ACKNOWLEDGEMENTS This section is addressed to acknowledge all those who gave their contribute to this work and were always available for me. I would like to acknowledge my supervisor, Professor Delfim Fernandes Soares for their guidance and support, and for being available to help during the development of the present work. I would like to thank Bosch Car Multimedia, especially to the Engineer José Luís de Sousa Ribas, for his contribution, opportunity, and guidance to carry out this final work. I would also like to all persons from Bosch Car Multimedia by the support during my internship. I would like to give special thanks to José Ricardo de Barros Alves for all the knowledge, support, dedication and patience. journey. To my family and friends, a special thanks for encouraging and motivating me throughout this Thank you all, Susana Rodrigues iii

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7 ABSTRACT Over the years, industries have been experiencing an increasing tendency to reduce the use of nitrogen during the soldering process. Due to this fact, the electronics industry has been investing in the research of cheaper and alternative manufacturing process. This necessity became the motivation for this work, whose purpose is to study of the intermetallic layer formation during soldering processes, and therefore, analyze with accuracy the reliability of solder joints. The main focus of this study was the analysis of an oxidant atmosphere influence in solder joints reflowed. In the initial stage, a soldering study with an oxidant atmosphere was performed, and the influence of the atmosphere was analyze in specific parameters, such as, the thermal properties, solder joints microstructure, formation and growth of the intermetallic layer. From this analysis, the results revealed that different thermal cycles (heating rates, atmospheres, and maximum temperature) affect the melting temperatures and the thickness of the intermetallic layer. In the second stage, the effect of the oxidant atmosphere was studied in a Bosch Car Multimedia production line. The product, a PCB for a solar panel, was subjected to several inspections steps along the line, namely, solder printing inspection, automatic optical inspection and in circuit test. The results of these inspections showed that the oxidant atmosphere did not have a significant influence in the product reliability and all production was successful. However, it was important to analyze the solder joints in detail. For that, a visual inspection was performed using an optical microscope and microstructural characterization of the solder joints after the soldering process with different atmospheres were conducted. The experimental results indicate that different atmospheres during reflow soldering did not have influenced the formation and growth of the intermetallic layer. It was also performed an evaluation concerning wettability in a different component in order to study the influence of the oxidant atmosphere. The effect of oxidant atmosphere on contact angle and solder height indicated that the absence of the nitrogen during reflow soldering caused an oxidation of component side and reduce the solder wettability. Keywords: Intermetallic layer, Oxidation atmosphere, Solder joint, Lead-free solders, Thermal cycle v

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9 RESUMO Ao longo dos anos as indústrias têm vindo a registar uma tendência crescente para reduzir o uso de azoto durante o processo de soldadura. Devido a este facto, a indústria eletrónica tem vindo a investigar processamentos mais baratos. Esta necessidade tornou-se a motivação para este trabalho, cujo objetivo centra-se no estudo da formação da camada intermetálica durante os processos de soldadura e, portanto, analisar com precisão a fiabilidade das juntas de soldadura. O foco principal deste estudo consistiu em analisar a influência da atmosfera oxidante durante a soldadura por Reflow nas juntas de soldadura. Na fase inicial, foi realizado um estudo da solda com uma atmosfera oxidante e na qual a influência foi estudada nos parâmetros como, as propriedades térmicas, microestrutura das juntas de soldadura e na formação e crescimento da camada intermetálica. A partir desta análise, os resultados revelaram que para diferentes ciclos térmicos (variação da velocidade de aquecimento, temperatura máxima e atmosfera) as temperaturas de fusão e as camadas intermetálicas foram afetadas. Na segunda fase foi estudado também o efeito de uma atmosfera oxidante numa linha de produção da Bosch Car Multimedia. O produto escolhido, uma placa de circuito impresso de um painel solar, foi submetido a várias etapas de inspeção ao longo da linha, ou seja, a inspeção da impressão de pasta, uma inspeção ótica automática às juntas de soldadura e um teste ao circuito da placa. Os resultados destas inspeções mostraram que o uso de uma atmosfera oxidante não teve uma influência significativa na fiabilidade uma vez que toda a produção foi realizada com sucesso. No entanto, é necessário realizar uma análise em detalhe às juntas de soldadura. Para tal, foi realizada uma inspeção visual e uma caracterização microestrutural das juntas de soldadura através do uso do microscópio ótico e eletrónico. Os resultados experimentais obtidos indicam que diferentes atmosferas durante a soldadura por Reflow não teve uma influência significativa no crescimento e formação da camada intermetálica. Foi também realizada uma avaliação da molhabilidade em diferentes componentes a fim de estudar a influência da atmosfera oxidante. O efeito da atmosfera oxidante sobre o ângulo de contacto da solda e altura mostrou que a ausência de azoto durante a soldadura por Reflow causou uma oxidação do lado do componente uma vez que molhabilidade reduziu. Palavras-chave: Camada intermetálica, Atmosfera oxidante, Junta de soldadura, Soldas sem chumbo, Ciclo térmico vii

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11 INDEX Acknowledgements... iii Abstract... v Resumo... vii List of Figures... xi List of Tables... xv List of Abbreviations and Acronyms... xvii Chapter 1 Introduction Motivation Objectives Structure of the Thesis... 2 Chapter 2 Literature review Printed Circuit Board (PCB) Classification of PCBs PCB surface finishes PCB assembled (PCBA) Electronic components Surface mount technology (SMT) Through-hole technology (THT) Fluxes Solder alloys Commercial solder alloys Soldering techniques in electronic packaging industry Reflow soldering process Wave soldering Solder joints Intermetallic compound (IMC) Joint defects ix

12 Chapter 3 Experimental Procedures Introduction Soldering study with an oxidant atmosphere Materials Methods Characterization techniques Morphological analysis Oxidant atmosphere study in a production line Materials Methods Product validation Morphological characterization Chapter 4 Results and discussions Soldering study with an oxidant atmosphere Thermal analysis Microstructural analysis of the interface Formation and growth of intermetallic layer at the interface between SAC and Sn substrate Oxidant atmosphere study in a production line Solder paste inspection Automated optical inspection In circuit test Visual Inspection Characterization of solder joints Chapter 5 Conclusions Chapter 6 Future work References Annex I Annex II x

13 LIST OF FIGURES Figure 2.1 PCB schematic representation [6] Figure 2.2 Three main categories of printed circuit board: a) single-sided, b) double-sided and c) multi-layered [5] Figure 2.3 The function of the flux in soldering [19] Figure 2.4 Market solders lead: a) Different lead-free solders in the market and b) different types of SAC alloys [11] Figure 2.5 PCBAs production line [6] Figure 2.6 Schematic representation of the stencil printing machine [16] Figure 2.7 Typical reflow profile for lead-free Sn-Ag-Cu solder paste [21] Figure 2.8 Wave soldering of a PCB [24] Figure 2.9 Constituents of the solder joints Figure 2.10 Optical micrograph of the solder/substrate interconnection [29] Figure 2.11 The ternary phase diagram of Sn-Ag-Cu alloys [11] Figure 3.1 SEM image for IMC thickness measurement Figure 3.2 Schematic representation about the Solar A production Figure 3.3 Reflow process steps: a) EKRA X4 professional machine Bosch edition, b) Siemens Siplace S20, and c) REHM Inert Gas Reflow Soldering System VXP Figure 3.4 Reflow profile and the results obtained on the different zones Figure 3.5 SMT adhesive curing: temperature profile Figure 3.6 Wave soldering process: a) Universal Radial type 8, b) Siemens G2 equipment and c) EPM CIG Figure 3.7 Wave thermal profile Figure 3.8 Koh-Young Machine (SPI equipment) Figure 3.9 AOI equipments. a) Viscom M, b) Marantz M22XDL Figure 3.10 Reer ICT equipment Figure 3.11 Cross-sectional zones Figure 3.12 Contact angle measurement: a) resistor, b) capacitor and c) QFP xi

14 Figure 3.13 SEM image for IMC thickness measurement between solder and substrate Figure 3.14 SEM image for IMC thickness measurement between solder and component Figure 3.15 Micrograph and SEM image of electrolytic with specific zones where were made IMC thickness measurement Figure 3.16 SEM image for solder height measurement Figure 4.1 DSC curves, with different heating rates, under an oxidant atmosphere Figure 4.2 DSC curves, with the same heating rate, under an inert and oxidant atmosphere Figure 4.3 Graph about start of melting temperature as a function of the heating rate for SAC Figure 4.4 Graph about the end of melting temperature as a function of the heating rate for SAC Figure 4.5 SEM micrographs of SAC305 for different temperatures, 240, 250 and 260 ºC, under inert and oxidant atmospheres Figure 4.6 SEM micrograph of the solder joint with SAC305 and Sn substrate a) and EDS element mapping for all the elements b), for Sn c) Cu d) Ag e) and Al f) Figure 4.7 EDS spectra of IMC layer Figure 4.8 SEM micrographs of SAC405 for different temperatures, 240, 250 and 260 ºC, under inert and oxidant atmospheres Figure 4.9 a) SEM micrograph of the solder joint with SAC405 and Sn substrate a) and EDS element mapping for all the elements b), for Sn c) Cu d) Ag e) and Al f) Figure 4.10 EDS spectra of intermetallic layer Figure 4.11 Average of IMC thickness for SAC305 as a function of maximum temperature Figure 4.12 Average of IMC thickness for SAC405 as a function of maximum temperature Figure 4.13 Results about SPI (height) for different components Figure 4.13 Results about SPI (height) for different components (continued) Figure 4.14 Number of pseudo-errors detected in AOI Figure 4.15 Visual inspection of PCBAs after reflow and wave in an inert and oxidant atmosphere Figure 4.16 Evaluation of solder joints in capacitor component, under different atmospheres and after reflow and wave soldering xii

15 Figure 4.17 Evaluation of solder joints in resistor component, under different atmospheres and after reflow and wave soldering Figure 4.18 Evaluation of solder joints in QFP component, under different atmospheres and after reflow and wave soldering Figure 4.19 Evaluation of solder joints in the electrolytic component, under different atmospheres and after reflow and wave soldering Figure 4.20 SEM micrograph of capacitor solder joint a) and EDS element mapping corresponding Sn b), Cu for c), Ag d), Ni e) and Al f) Figure 4.21 EDS general mappings of capacitor solder joint with different conditions: a) Reflow (inert atmosphere), b) Reflow (oxidant atmosphere), c) Wave (after reflow with an inert atmosphere) and d) Wave (after reflow with an oxidant atmosphere) Figure 4.22 Average of IMC/PCB thickness in solder joints for SMD components as a function of soldering processes Figure 4.23 Average of IMC/PCB thickness in solder joints for TH component in different zones as a function of soldering processes Figure 4.24 Average of IMC/Component thickness in solder joints for SMD components as a function of soldering processes Figure 4.25 SEM micrograph of QFP solder joint a) and EDS element mapping general b), Sn for c), Cu d), Ag e) and Ni f) Figure 4.26 Contact angles of the resistor, capacitor and QFP components measured as a function of atmosphere used Figure 4.27 Solder height in the QFP component fabricated in inert and oxidant atmosphere xiii

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17 LIST OF TABLES Table 2.1 SMD and TH components... 8 Table 2.2 Different IMCs formed between SAC alloy and substrates [2, 30] Table 2.3 Defects which can appear in PCBAs after soldering [33-35] Table 3.1 Composition, melting point and density corresponding to SAC305 and SAC Table 3.2 Thermal cycle used for SAC305 and SAC Table 3.3 Heating-cooling DTA profile used for SAC Table 3.4 Data about the soldering process, reflow and wave Table 3.5 The temperatures defined on reflow oven Table 4.1 Results of DTA analysis for different heating rates and different atmosphere Table 4.2 Number of pseudo-errors after reflow and wave soldering with an inert and oxidant atmosphere Table 4.3 Average IMC/PCB thickness for the capacitor, resistor and QFP components with different conditions Table 4.4 Average IMC thickness of electrolytic component with different conditions Table 4.5 Average IMC/Component thickness for the capacitor, resistor and QFP components with different conditions xv

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19 LIST OF ABBREVIATIONS AND ACRONYMS Ag Silver Al Aluminum AOI Automatic Optical Inspection Cu Copper DTA Differential Thermal Analysis EDS Energy-dispersive X-ray Spectroscope ENIG Electroless Nickel Immersion Gold FR4 Flame Retardant 4 g Gramm IMC Intermetallic compound Imm. Sn Immersion Tin kv Kilovolt LFHASL Lead-Free Hot Air Solder Leveled m Meter mg Milligram min Minute mm Millimeter Ɵ Angle ºC Degree Celsius OSP Organic Solderability Preservatives PCB Printed Circuit Board PCBA Printed Circuit Board assembled ppm Parts per million PTHs Plated through-hole QFP Quad Flat Pack s Second SAC Sn-Ag-Cu SEM Scanning Electron Microscope SMD Surface Mounted Device xvii

20 SMT Sn SPI TH THT μm Surface Mount Technology Tin Solder Printing Inspection Through-hole Through-hole Technology Micrometer xviii

21 1. CHAPTER 1 INTRODUCTION 1.1. Motivation Currently, the market is more competitive and requires companies to be more effective and flexible on price. With the emergence of reducing the costs in an electronics manufacturing, the electronic industry has studied cheaper alternatives. Nitrogen atmosphere has been used for a long time in the production line and the using this atmosphere has as objective the oxidation reducing of the solder and improve the solderability [1]. However, the costs associated to the use of nitrogen atmosphere are high and the Bosch Car multimedia is spending 1 million euros by year in nitrogen. The solder joints play an important role in electronic manufacturing, serving both as electrical interconnections and mechanical support between the components and the PCB [2]. The change of inert atmosphere for oxidant atmosphere in the soldering process can bring a substantial impact in the solder joint reliability. Therefore, when the industry thinks into changing the atmosphere, is necessary to understand the causes behind this change. The use of oxidant atmosphere during soldering process has many disadvantage such as solder oxidation, wetting reducing, increase the number of defects, among others [3]. There are several issues that need to be reevaluated and readjusted in the process windows. For such, the evaluating of the atmosphere effect on the soldering performance of lead-free solders is an important factor. Therefore, this evidence became the motivation for this work, whose purpose is to study the intermetallic layer formation and the influence in solder joints reliability, which may posteriorly serve as a basis for others investigations Objectives Due to the fact of the high cost associated to the nitrogen used during soldering processes, alternatives have been investigated for reducing these costs. Thus, an alternative proposed by Bosch Car Multimedia was the removel of the nitrogen during reflow soldering process. Therefore, with this project it is intended to achieve the following objectives: To study the thermal properties of the SAC405 with different conditions (heating rates, maximum temperatures, atmosphere ) in the start and end of melting temperatures; 1

22 Evaluation of the intermetallic layer in function of different conditions, such as, thermal cycles, atmospheres and the solder type; To study the effect of reflow atmosphere on the intermetallic layer formation and the solder joint reliability Structure of the Thesis The present thesis includes two different projects which were developed in two institutions, University of Minho and Bosch Car Multimedia. This thesis is a result of collaborative research work carried out by me, Susana Rodrigues, Professor Delfim Soares and José Ricardo Alves. The Master s thesis is based in the study of the intermetallic layer formation and the influence of oxidant atmosphere in the solder joints reliability. This way, the thesis is divided into 6 chapters. In Chapter 1 is presented the motivation to performed this study and objectives to achieve during the work. Chapter 2 approaches the literature review where it is explained the manufacturing processes and the essential background concepts concerning the soldering. A description of the work carried out are presented in Chapter 3 and this are divided into two projects, soldering study with an oxidant atmosphere (performed at University of Minho) and oxidant atmosphere study in a production line (performed at Bosch Car Multimedia). Chapter 4 presents the results and related discussions and Chapter 5 presents a conclusion of the work performed along this Master Degree. Finally, Chapter 6 presents possible future areas to be worked on. 2

23 2. CHAPTER 2 LITERATURE REVIEW 2.1. Printed Circuit Board (PCB) Printed circuit boards are the products used in the interconnection technology in electronic products. It is through this product that can be made the connection between the components in an electronic circuit. Thus, these are products with a vast expansion in the market due to its function [4]. The essential components in the printed circuit board are the base and conducting tracks (circuits). The PCB base, which is a thin board of insulating material, rigid or flexible, provides mechanical support to all components which make up a circuit. Regarding to the circuit, this is formed by a thin layer of conducting material, usually copper, deposited on the surface the substrate. These circuits provide the electrical connection between components and base [4, 5]. The base used is constituted by glass fiber reinforced epoxy resin designated Flame Retardant (FR4). The conductors, pads, and vias are constituted by copper. The components soldered to pads on the surface of the circuit board are designated as surface mounted device (SMD). However, there are vias were can be inserted and soldered component to the PCB which are designated as through-hole (TH), as shown in Figure 2.1 [4]. SMD Component Pads FR4 Copper layers Vias (TH) Tracks Figure 2.1 PCB schematic representation [6] Classification of PCBs The PCB classification is based on the number of layers printed and to the presence or absence of plated-through holes. Thus, there are three types of PCBs construction (Figure 2.2): 3

24 Single Sided PCB s consists in the simplest and the cheapest type of boards that contain a single layer of copper on one side of an insulating base material. This type of PCB can be used for through-hole and surface mount components. In a through-hole single-sided printed circuit board, components are situated on the non-track side of the circuit board while their leads go through the through-holes to the other side, where they are soldered to lands. In a surface mounted printed circuit board, components are situated on the same side of the board as the copper track [4, 5]. Double Sided PCB s consist of two copper layers on both sides of the insulating base material. Thus, in this PCB is possible to have more density of components and the conductor tracks are higher than the single sided boards. This type of PCB can be used for both through-hole and surface mount components. In a through-hole double-sided assembly, components are usually situated on just one side and soldered on the other. Where through-holes are required to interconnect top and bottom copper track layers, they are placed inside the hole with copper and are called plated through vias. Where a component lead also goes through a via it is called a plated through-hole (PTHs). In a surface mounted double-sided assembly, components are on the same side as boards are soldered. Components may be mounted on both sides of the board. It is important to note that only there is an electrical connection between the two sides when used plated through-hole [4, 5]. Multi-layered PCB s is constituted by a substrate with layers of printed circuits separated by layers of insulation. The current products tend to add greater numbers of features in a smaller PCB area. Therefore, these PCBs are used where the density of connections needed is too high to be handled by two layers. Plated through-holes can be used for either component terminal connection, or just as electrical connections (in which case they are called vias). Vias passing from one outside track to the other are called through vias while those connecting internal track layers are called blind, or buried vias [4, 5]. 4

25 Figure 2.2 Three main categories of printed circuit board: a) single-sided, b) double-sided and c) multi-layered [5]. The PCBs can also be classified on the basis of the type of insulating material used, i.e. rigid, flexible or rigid-flex. The rigid boards is composed of a variety of materials and the flexible boards are constituted by a substrate material like polyester or polyamide. The rigid-flex boards consist in a rigid and flexible substrates laminated together into a single structure [4] PCB surface finishes The surface finish can be defined as a coating, either metallic or organic in nature. The effect of surface finish is the important factor that causes differences in the microstructure and mechanical properties between solder joints. Copper is used as a base metal conductor in the fabrication of printed circuit board. This metal has excellent properties as a conductor of heat and electricity. However, the copper oxidizes and deteriorates in the presence of water and air. This way, if the copper surface on the PCB is not coated or treated with a protective agent, the exposed area would rapidly become unsolderable [4]. 5

26 Therefore, the surface finish is applied to a PCB in order to assure solderability of the metal. This protective finish must be solderable and act as a barrier for preventing the copper from oxidizing. Thus, it is possible to provide a solderable surface when assembling the components to the printed circuit board [4, 7]. The most commonly PCB surface finishes used in Bosch Car Multimedia are detailed below. Immersion Tin (Imm. Sn) Immersion Tin is a metallic surface finish that is considered to be very good in terms of corrosion resistance and its cost is competitive. This process consists in immersion tin bath, it continues to build up in thickness over time. The thickness is on the order of 0.1 to 1.5 μm [8]. This surface finish is used for replacing and protect the solderability of copper surfaces and in that way extend the shelf life of the board [9]. Over time, the tin forms an intermetallic compound with Cu at a rate that is dependent on temperature. If the intermetallic reaches the coating surface, it oxidizes rapidly resulting in poor solderability of the solder joint area [8, 10]. Lead-Free Hot Air Solder Leveled (LFHASL) Lead-free HASL is a novel surface finish which is currently being implemented by industry in order to respect the new directives implemented by the European Union. These new regulations require that lead has to be removed from any end-of-life electrical or electronic components [11]. The LFHASL process consists in the application of flux to the PCB, and subsequently, immersing it in a molten pot solder at approximately 265ºC. Afterwards, the hot air knives blow the excess molten solder of the pads. The typical thickness of the LFHASL may vary between 0.6 to 51 μm [12, 13]. LFHASL has the key advantage of achieving excellent solderability because Nothing solders like solder. The products with LFHASL surface finishes possess an excellent shelf life and has high solderability even after two surface mount profiles. The LFHASL surface finish is slightly more planar than HASL and it gets wet faster [8]. Organic Solderability Preservatives (OSP) OSP, as the name implies, is an organic coating that protects the copper from oxidation on the solderability process. The typical thickness is 0.2 to 0.5 µm. In the soldering process, the organic coating is penetrated, dissolved and removed by the flux during soldering. Therefore, at high temperatures, OSP is not as robust as the other metal finishes and has a smaller process window [8, 14]. 6

27 The HASL tends to be able replaced by OSP because this surface finish has advantages as stronger solder joint, substrate low price, is environment-friendly, simple process, provides a surface planarity equivalent to the plated copper finish, and require very low equipment maintenance [7, 9, 14]. Electroless Nickel Immersion Gold (ENIG) In this surface finish (according to the Bosch specifications), a layer of electroless nickel with a thickness of 4 to 9 µm is deposited on the copper and is the surface to which the soldering occurs. The immersion gold is the layer that protect the nickel from oxidation during storage and has a thickness of 0.05 to 0.15 µm [9, 15]. This surface finish are attractive because it presents a good flat coplanar surface, has excellent solder wettability characteristics, is an ideal surface for pad contacts and improve shelf life over other surface finishes [9]. ENIG is known to have good solderability but, on the other hand, it has high cost and the susceptibility to black pad issues. Black pad is a phenomenon related to some weak solder joints on ENIG surface finish [12] PCB assembled (PCBA) The PCBA refers to the PCB after the soldering process. It means that on this stage the components are electrical and mechanical connected to the board [4]. The Bosch Car Multimedia is one of the companies responsible for PCBA mounting since PCBs, components and solders are purchased from other companies. The PCBA can be done manually or by using machines which make the assembly process automatic, fast and reliable. It is important to refer that exist different products which play different functions, the components and the soldering process used can be different. Thus, the final product of assembled PCB consists of a lot of different types of components with many configurations, an assembly process must be selected of according with cost and the reliability [4] Electronic components The electronic components can be defined as a material that handles electricity. The electronic industry makes use of a variety of components. These components can have many 7

28 shapes, sizes and perform different electrical functions depending upon the purpose for which they are used [4]. Components can be classified into two types, surface mount device, and through-hole. This classification is based on the basis of the method of their attachment to the circuit board. The components leaded directly onto the surface of the board are designed by surface mount components. When the components leaded by inserted through mounting holes in the circuit board these are considerate through-hole components [4]. On the Table 2.1 can see different types of components, SMD, and TH, used in electronic equipment. Table 2.1 SMD and TH components. Surface mount components QFP (Quad Flat Pack) Through-hole components Electrolytic Chip capacitor Ceramic capacitor Surface mount technology (SMT) Surface mount technology is a revolutionary change in the electronics industries. Surface mount technology dates back to the 1960s when it was developed for hybrid microcircuit assemblies for which it was difficult to put holes into the ceramic substrates. The advance of surface mount technology for the laminate substrate, though, is relatively recent. Thus, the surface mount technology emerged due to advantages of being able to place components on both sides of the PCBs. Surface mount technology is based in the components soldered to pads on the surfaces of the circuit board. This technology present advantages as a higher degree of automation that 8

29 result in a lower manufacturing cost, inclusion the smaller components which imply smaller volume, higher circuitry, and better performance [4, 16, 17] Through-hole technology (THT) Through-hole technology consists on the process by which component leads are inserted into holes in the circuit board and soldered into place. This technology has been used since early in the electronic industrial (the 1920s) when the component was soldering point-to-point. In 1960, this process had one progress with the emergence of wave soldering process. The main disadvantage of this technique is the low assembly densities, which led to the introduction of surface-mount technology. On the other hand, through-hole components are used for high-reliability products that require strong connections between layers. There are many reasons why through-hole technology is used over surface mount including the size and shape of the component, the leads placement, and even the sensitivity and stress of component. This technology is used by military and aerospace products because are exposed to extreme accelerations, collisions, or high temperatures [9] Fluxes Flux is a substance used in the soldering process which cleans the surfaces to be soldered. The flux function is to remove oxides and other nonmetallic impurities (Figure 2.3). The majority of the metals tend to form compounds with atmospheric oxygen which leaves a coating of oxide even at room temperature, gold and platinum are exception. The oxidation rate can increase if there is a temperature and humidity increase. However, soldering flux has several functions to perform, such as: React with oxide and other contamination on the surface to be soldered Dissolve the metal salts formed during the reaction with the metal oxides Protect the surface from re-oxidation during the soldering process Provide a thermal blanket to spread the heat evenly during soldering Reduce the surface tension of the melted solder in order to enhance wetting These fluxes contain different types of ingredients capable of performing these functions, such as: 9

30 Activators: are present in the flux formulation to enhance the removal of metal oxide presents in the surfaces to be soldered. The activators are usually acids or compounds that release acids at elevated temperature. The activity of the activators increases with temperature during the preheat step of the soldering process, up to a certain value where their activity concludes, either due to thermal decomposition or excessive volatilization. Vehicle: is a solid or nonvolatile liquid that acts as an oxygen barrier protecting the metal surface against oxidation. This ingredient dissolves the metal salts formed in the reaction of the activators with the surface metal oxides and carry them away from the metal surface. Provide to a heat transfer medium between the solder and the components or PCB substrate. Solvent: serves to dissolve the vehicle, activators, and other additives. Solvents evaporate during preheating and before the soldering operation. Additives: can be other ingredients that serve a specialized function. These ingredients can enhance the wetting properties by decrease the interfacial surface tension between the molten solder and the PCB, provide good viscosity, low slump during the preheat step, among others effects [5, 9, 18]. Figure 2.3 The function of the flux in soldering [19]. The fluxes can be classified as, rosin fluxes, water-soluble fluxes or no-clean fluxes. The rosin fluxes can be non-activated (R), mildly activated (RMA) and activated (RA). RA and RMA are the fluxes which contain rosin combined with an activating agent (an acid) which increases the wettability of metals by removing the oxides presents. Regarding the water soluble fluxes, they need to be cleaned after the soldering because these fluxes present higher activity. The no-clean fluxes are mild enough to not require removal due to their non-conductive and non-corrosive residue [5, 9]. 10

31 2.4. Solder alloys With the development of electronics industry, Sn-Pb solder was used as the interconnect material of electronic components to the PCB. These solders were predominant choice of the electronics industry because provide excellent solderability and reliability of the solder joint. However, the lead present in solder was considered a toxic substance that causes several negative impacts on the environment and human body. Thus, due to the problems that this causes the legislation required the removal of lead in the electronics industry. The European Union in June 2000 adopted two directives, the "Waste of Electrical and Electronic Equipment (WEEE)" and "Restriction of the Use of Certain Hazardous Substances (RoHS)". The first requires the removal of lead in all the electronic and electrical components at the end of life and the second directive prohibits the use of lead in electrical components and electronic manufactured after July 1, 2006 [11]. To be considered an alternative to Sn-Pb solders these should be lead-free candidates with similar or better properties. Therefore, for a Sn-Pb solder to be replaced the electronic industry to take account of the advantages that this solder (Sn-Pb) guarantee, such as the following: Reduction the surface tension of pure tin in that way improving wettability Provide ductility to Sn-Pb solders Allowing the rapid formation of intermetallic compounds by tin and copper diffusion Allowing the use of low processing temperature due to its low melting point Low cost and very abundant material Therefore, as lead has the disadvantage of toxicity electronics industry had to replace to other alternatives. Thus, it appeared on the market lead-free solders which presents similarities to Sn-Pb solder [11] Commercial solder alloys The lead-free solder needs to have a low melting point, in order to have a similar reflow profile during the manufacturing process; good wetting ability to ensure good metallization; good electrical properties to efficiently transmit electrical signals; and adequate mechanical properties to preserve the reliability of solder joints, i.e. good bonding of electronic components on the printed circuit board. The new lead-free solders also need to be non-toxic and low-cost. Thus, after several types of research and studies, the solders that have similarities with Sn-Pb solders are the Sn-Ag, 11

32 Sn-Ag-Cu (SAC), and other alloys containing elements such as tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), indium (In) and zinc (Zn), as shown in Figure 2.4a [11]. Several SAC alloys have been proposed by electronic industry. These included SAC305 ( Ag-0.5Cu), to SAC387 (95.5Sn-3.8Ag-0.7Cu), SAC396 (95.5Sn-3.9Ag-0.6Cu) and the SAC405 (95.5Sn-4.0Ag-0.5Cu), as shown in Figure 2.4b. These solders have some benefits for examples, low melting temperatures compared with the 96.5Sn-3.5Ag, as well as their superior mechanical and solderability properties when compared to other lead-free solders [11]. a) b) Figure 2.4 Market solders lead: a) Different lead-free solders in the market and b) different types of SAC alloys [11] Soldering techniques in electronic packaging industry Soldering is a method used to produce permanent electrical and mechanical connection between substrate and components. There are many different soldering practices employed in the electronic packaging industry. They all involve four basics ingredients: base metals, solder material to the joining components, flux and heat. By far the commonest methods of soldering used by electronic packaging industry are reflow soldering and wave soldering Reflow soldering process The reflow soldering process consists in solder electronic components (SMD) on the PCB. This process consists in three main steps: the solder printing, the components placement and the reflow soldering. Figure 2.5 shows an integrated manufacture line for electronics boards. 12

33 SPI (Solder printed inspection) AOI (Automated optical inspection) Solder printing Component placement Reflow soldering Figure 2.5 PCBAs production line [6] Solder paste printing The first step, stencil printing process, consists in the application of the solder paste on the PCB using a stencil. The stencil is formed of metal foil with a pattern of aperture matching the footprint on the PCB. This stencil is positioned above the board and a squeegee drawn over the stencil forcing the solder paste through the screen apertures and the solder paste is deposited on top of the corresponding pads (Figure 2.6). It is important to refer that the solder paste provides some adhesive qualities during transportation of the board before soldering to prevent component misalignment or loss. Figure 2.6 Schematic representation of the stencil printing machine [16]. This process requires a flat substrate for the stencil to be laid on and present advantages as higher speed, higher throughput, and better solder paste volume and better pattern registration [16, 20] Component placement After the solder paste has been printed, the PCB is then transported for the production line to the component pick and place station. Firstly, the machine selects the proper component, orient 13

34 correctly and placed it onto the PCB pads, where the solder paste secures the component in place during other processes of the board assembly. This process used automatic vision viewing and alignment technology. These machines are used because they present high speed in order to maximize the production volume and high precision placement of a whole range of electronic components [9, 16, 20] Reflow soldering Reflow soldering is the step in which a PCB, which has the components placed on the solder deposit is passed through an oven in order to melt the solder and form the solder joints. The methods used for reflow soldering are forced convection, infrared, and vapor phase. The method used at Bosch Car Multimedia is forced convection reflow. Reflow equipment contains typically about 8 to 10 heating zones with temperature control to handle large complex assemblies. During the soldering, the assemblies are transported through the oven by a conveyor line. Forced convection reflow consist in an oven, where the heating of the PCB is made through the forced convection. This soldering process is based in the transfer of heat via a flow of gases [10, 16]. Reflow ovens are divided into the following four zones: preheat, soak, reflow and cooling (Figure 2.7). Preheat During this stage the volatile material in the flux evaporate. Initially, the PCB and components are heated up gradually from room temperature to about 170 ºC. The ramp rate is a critical parameter so it should be controlled between 1-3 ºC/sec. This way, slow heating minimize thermal shock to the PCB and the components electronic [10]. Soak This stage should be considered as a continuation of the preheat stage. The flux is activated and it cleans the surface oxides on the solder particles, pads, and components leads. Also, activated flux continues to keep the metal surfaces away from re-oxidizing. The temperature is maintained between 170 to 220 ºC, during an extended period. This stage had as aim to reduce the temperature difference between the large and small mass components on the assembly during reflow [10]. Reflow The PCBA enters the reflow stage where the solder particles melt and result in the soldering between the PCB and components terminations. On this stage the temperature rises over the liquidus temperature of the solder paste between 217 to 221 ºC for lead-free alloys. Normally the 14

35 range of peak temperature is 235 to 250 ºC and the range of time above liquidus is between seconds, this way it is possible to provide adequate time for wetting and formation of a quality solder joint. This stage should be controlled because if the temperature and times above referred are exceed it can result in a potential PCB/component damage. Other possible failure is the excessive intermetallic compound (IMC) formation, making the solder joint more brittle [10]. Cooling After the formation of all solder joints, the assembly is cooled down to room temperature. This step determines the grain structure during solidification. A fast cooling rate forms a finer grain structure (normally 3 to 4 ºC/sec.). A slow cooling results in a larger grain size that may have poor fatigue resistance. This way, the fast cooling is chosen because it presents better mechanical properties due to the fine grain structure [10]. Figure 2.7 Typical reflow profile for lead-free Sn-Ag-Cu solder paste [21] Inert and oxidant atmosphere To avoid high rate oxidation during the soldering process an inert gas can be used. At Bosch Car Multimedia the inert gas used is nitrogen. Nitrogen is a common gas found in the atmosphere (about 78% of the Earth s atmosphere). The soldering process with an inert atmosphere protect metal surfaces from oxidation during heatup and assuring proper action of flux. Nitrogen is used to displace oxygen because it is the 15

36 chemically unreactive [22]. This way, nitrogen is used for many years in the soldering process and it presents several benefits, such as: Reduce oxidation during the joining process ensuring the solder wetting Elimination of the oxides formation improving the wettability Reduce the defects number Avoid the use of solder paste with high fluxing capacity which improve quality and cleanliness of the solder joints [22, 23] However, at Bosch Car multimedia are spending about 1 million euros by year in nitrogen. The used of nitrogen is considered an additional cost in soldering process and this way it was decided to study the soldering process without nitrogen. Thus, an alternative to reduce costs is to use an air atmosphere in reflow. Although the costs reduction an oxidant atmosphere, especially at high temperatures, cause most metals to oxidize, this way the soldering flux in the system has to work more to remove the oxidation on the pad. Thus, the presence of oxygen during the reflow soldering results in poorer solderability. The soldering process with an oxidant atmosphere is a problem for wettability, mainly for the second side to be soldered once that after the first reflow the PCB surface of the second side is already oxidized [16]. On the other hand, in the first reflow the oxidation problems can appear in the components lead and the other side of the PCB Wave soldering The wave soldering is a method used to solder through-holes components and some surface mounted components onto PCBs (Figure 2.8). These components are soldering when the PCB passes over a wave solder. Wave soldering is used for both, through-hole and surface mount printed circuit assemblies. Firstly, there is the insertion of components in the PCB. This insertion can be by radial leaded and manual assembly. The radial leaded components is an automatic insertion which insert the components into the printed circuit board in the proper order. Regarding the manual assembly these components are installed manually by operators. This operators install a few components and then slide the PCB to the next operator via a conveyor [9]. The wave soldering process consists of three main stages, such as: 16

37 Fluxing The first stage consists in the application of the flux on the surface of the PCB assembly. The PCB is moved by conveyor where is to be applied flux by foam or spray. This flux is applied for improve the wetting of surfaces and to protect the metal parts from oxidation during soldering. Preheating This stage consists in preheating the PCB where the flux is activated and evaporation of its volatiles. This stage has the role in minimizing the thermal shock before the wave soldering operation. The top of the board should reach a minimum of ºC before the next step, wave soldering, in order to avoid thermal shock. Wave soldering This stage, the components are soldered to the board using the solder wave. The wave soldering process is ideal for the through-holes components and some surface mount boards that use larger components. For lead-free the solder temperature should be between of ºC. There are several types and shapes of waves that can be used for single and dual waves. In this case, the process used is a dual wave where the first wave is a turbulent and second wave is laminar. The turbulent wave ensures the wetting of all leads while the laminar wave removes excessive solder the way to control the meniscus of the molten solder at each joint. This process is usually used when SMDs are to be soldered in the bottom of the board. This machine can have coupled a hot air knife. The force of the air helps to remove excess solder reducing solder bridges as shown in Figure 2.8 [4, 5, 16]. Figure 2.8 Wave soldering of a PCB [24]. 17

38 2.6. Solder joints The formation of solder interconnections in electronic packages implicates the melting of solder in contact with a metal surface. Thus, the solder joints play an important role in electronics packaging, serving both as electrical interconnections and mechanical support between the substrate (PCB) and the components. As technology advanced, the size of the components used have decreased and, as a result, the number of solder joints increased. These solder joints are also required to have the capacity to remove heat from joined devices. In addition, one of the most important factors which are known to influence solder joint reliability is the intermetallic compound layer formed between the solder and the substrate. Figure 2.9 shows the parts that constitute a solder joint [2, 25]. Figure 2.9 Constituents of the solder joints Intermetallic compound (IMC) In the soldering process, there are interfacial reactions between two different materials. The liquid solder reacts with the base metal and forms an IMC layer, which is the bond between the two materials. The solder joint formed at the solder/substrate interface is affected by diffusion rate of a metal substrate (Cu) into the molten solder. This way, the intermetallic layer presence is an indicator of the metallic bonding and, therefore, represents one the most important factors in the soldering process because they influence solder joint reliability [20, 26]. It is important to study the dissolution rate of the metal substrate into the molten solder because it can result on the formation on an excessive amounts of intermetallics. These IMCs can be very brittle, causing a region of weakness, which can result in solder joint failure. Moreover, the 18

39 dissolution may also result in de-wetting due to exposure of an unsolderable intermetallic layer on the substrate surface. Furthermore, the formation of a thick intermetallic layer result in a weak joint, which could reduce the service life of a solder joint. Regarding to the formation of intermetallic compounds, when liquid dissolution interfacial reactions stop upon solidification of the solder. After solidification the formation of the intermetallic layer is slowly due to the low diffusion rate of the metals [26]. The amount of IMCs formed depends on the solubility of the solder in the base metal, the time and the temperature of the soldering process cycle. A binary phase system that forms IMCs normally, provides good adhesion because the metal in the base has a limited mutual solubility, i.e. there is a mutual attraction between atoms of different species [25]. The temperature plays a fundamental role in the IMCs formation because elevated temperatures can result in excessive IMCs growth causing embrittlement of solder joint and decrease fatigue strength resulting reliability problems. However, if the formation of the IMCs is insufficient or non-uniform it can cause solder defects and weak solder joints [27]. In SAC alloys, the Ag and Cu additives forms different IMCs with Sn. These primary elements can affect all the properties of the alloys. According to the binary phase diagram it is possible to verify the existence of three intermetallic compounds formed: Ag3Sn forms due to the reaction between Sn and Ag and this intermetallic compound are dispersed into the liquid solder; Cu6Sn5 form due to the Sn and Cu reaction and forms adjacent to the solder alloy, followed by Cu3Sn adjacent to the Cu substrate present at the interface between the solder and substrate. In a solder joint a layer Cu6Sn5 appears over Cu3Sn. This phase is formed preferentially when there is an excess of copper and at high temperatures because Cu3Sn is more stable at high temperatures than the Cu6Sn5. These IMCs form a strong bond between solder/substrate. These particles of the intermetallic compounds (Ag3Sn and Cu6Sn5) present much higher strength than the bulk solder joint. Fine intermetallic particles in Sn matrix can strengthen the alloys improving the fatigue life of the solders. These particles can also serve for to block the movement of dislocations. Figure 2.10 shows an optical micrograph of the solder/substrate interconnection. It can be seen the formation of large Ag3Sn plates, Cu6Sn5 in the copper interface followed by Cu3Sn [11, 25, 28]. 19

40 Figure 2.10 Optical micrograph of the solder/substrate interconnection [29]. In Figure 2.11, is shown the Sn-Ag-Cu ternary phase diagram near the tin corner. This area in the red box is the near eutectic region. Most of the SAC alloys compositions presently in the market are in this region [11]. Figure 2.11 The ternary phase diagram of Sn-Ag-Cu alloys [11]. The types of IMCs formed depends on the surface finish type used in the PCB substrate. Table 2.2 shows the different intermetallic compounds between SAC alloy and various substrates. 20

41 Table 2.2 Different IMCs formed between SAC alloy and substrates [2, 30]. Substrates IMCs Imm.Sn LFHASL OSP Cu3Sn; Cu6Sn5 Cu3Sn; Cu6Sn5 Cu3Sn; Cu6Sn5 ENIG (Cu, Ni)6Sn5; (Cu, Ni, Au)6Sn3 The composition of the IMC layer formed on bare copper and on copper coated with immersion tin, lead-free HASL and OSP are basically the same [31]. If the pads (Cu) are protected with an ENIG coating, the Au dissolves and migrates into the solder, promoting the intermetallic layer formation between the liquid solder elements (Sn, Cu...) and Ni. The thickness of the intermetallic layer should be between 1 and 5 µm [32] Joint defects When soldered one electronic product is necessary to know what constitutes a good joint, and how to correct soldering defects. A defect is a condition that may be insufficient to ensure the form, fit or function of the assembly in its end-use environment. In this sub-section will be presented some common defects that can appear during the assembled process. These defects are present in IPC-A-610E, Acceptability of Electronic Assemblies, where illustrates the requirements for many types of solder connections [33]. The requirements are divided into three classes of acceptance criteria: Class 1 General electronic products Includes products suitable for applications where the major requirement is function of the completed electronic assembly; Class 2 Dedicated service electronic products Includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically the end-use environments would not cause failures; Class 3 High-performance electronic products 21

42 Includes products where continued high performed or performance-on-demand is critical, equipment downtime cannot be tolerated, the end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support or other critical systems [33]. Table 2.3 are presented some common defects in PCBAs: Table 2.3 Defects which can appear in PCBAs after soldering [33-35]. Defect Solder ball defects Description Solder ball defect appears after reflow in form spherical particles with various diameters. This defect can be caused by the incompatibility between the solder paste and solder resist. Solder shorts/bridging This defect appears mostly in wave process in which there is solder connecting between two or more pads before the solder solidifies, creating a short (pads come into contact to form a conductive path). Poor or incomplete filling The hole is poorly filled or is not filled completely and the IPC standard states describe that 75% of the hole must be filled. This incomplete filling occurs due to flux and heat issues. Voids Voids are classified as spaces within the joint. The contributors for appearing of voids are outgassing of flux entrapped and excessive oxidation. 22

43 Tombstoning A tombstone is a defect characterized by a chip component that has partially or completely lifted off one end of the pad surface. Solder beading Solder beading occurs when solder paste gets under of the component softens due to heat input, and flows to the underside of chip components. 23

44 24

45 3. CHAPTER 3 EXPERIMENTAL PROCEDURES 3.1. Introduction The starting point of the present work was the study of the inert and oxidant atmosphere influence in the reliability of the solder joint. This thesis includes two different projects which were developed in two institutions: University of Minho (Campus Azurém, Guimarães) and Bosch Car Multimedia (Braga). The present chapter describes the materials, methods, and inspection and characterization techniques. The work performed at the University of Minho included the study of the thermal properties of the solder SAC405. Furthermore, the possibility to solder with an oxidant atmosphere was also analyzed (using SAC305 and SAC405). The second project was developed at a Bosch production line and consisted in the oxidant atmosphere influence study in a real product. The performance of the final product was also evaluated Soldering study with an oxidant atmosphere In the present project, the main objective consisted in the study and comparison of the intermetallic layer obtained after soldering using different atmospheres, inert and oxidant. The materials that were used in the first project were supplied by Bosch Car Multimedia. The characterization techniques that were used to study the intermetallic layer were the DTA and optical and electronic microscopy Materials Two types of solders SAC305 and SAC405 were used in this project. These are typically used at an industrial scale by Bosch Car Multimedia in the soldering processes. The wave soldering process uses SAC305 and the reflow soldering process implies the use of SAC405. Table 3.1 presents the solders characteristics, such as composition, melting point and density. 25

46 Table 3.1 Composition, melting point and density corresponding to SAC305 and SAC405. Solder Composition (wt. %) Melting point (ºC) Density (g/cm 3 ) SAC Sn-3.0Ag-0.5Cu SAC Sn-4.0Ag-0.5Cu It is important to refer that SAC305 was supplied as solder wire and the SAC405 was supplied as solder paste (with flux). Regarding the PCB used in the project, this was constituted by a FR4 base, a Cu layer and the surface finish in immersion tin. This PCB presented a thickness of the Cu layer of 35 µm and the thickness of the surface finish of 1.9 µm (Sn) Methods This project consisted in the study of the solders properties when an oxidant atmosphere was used during soldering. Therefore, the thermal properties of the solders SAC305 and SAC405 with and without substrate were studied using the DTA technique. The melting and solidification temperatures of the solders were only measured for the solders without substrate. Regarding the solder/substrate reactivity study, the DTA was also used to study the influence of the atmosphere and temperature on intermetallic layer formation Solder/substrate reactivity The study of the intermetallic layer is essential in this study because the IMC layer is responsible for solder joint reliability. Thus, to study the solder/substrate reactivity, the solder alloys used in this project were melted in a resistance furnace (using DTA) under inert and oxidant atmosphere by a constant flow of argon and air, respectively. A number of tests were made for studying the formation and growth of the intermetallic layer. The experiments were conducted with the two different solders. So, the IMC layer thickness were analyzed at three different temperatures. Table 3.2 present the thermal cycle used in this study. The entire process was conducted under N2 and air atmospheres. A high heating rate (30 ºC/min) was used, in the equipment allowed range, to approach it to the thermal cycle used at Bosch Car Multimedia. 26

47 Table 3.2 Thermal cycle used for SAC305 and SAC405. Solder Thermal cycle Temperature max. (ºC) Heating rate (ºC/min) Stage (s) Cooling rate (ºC/min) SAC305 and SAC Characterization techniques Differential thermal analyze (DTA) DTA is a technique in which the difference to a temperature between a sample and a reference material is measured as a function of temperature while the sample and reference are subjected to controlled temperature programmer [36]. The basic principle behind its technique is that when a sample is heated, it undergoes reactions and phase changes that involve absorption or emission of heat. DTA was used to determine the melting temperature for SAC405 without substrate. Regarding the solders with the substrate, it was also used the DTA for simulation of a cycle thermal. Preparation of the solders without substrate: SAC405 was supplied in form the solder paste and this was removed from the container and then dropped into aluminum crucibles. The quantity of solder used was approximately 65-80mg. Afterward, the samples were placed in DTA under argon and air atmosphere to induce the melting of the solder. This study was performed only in SAC405. These tests were performed under different atmospheres, argon and air, and different heating rates. Table 3.3 shows the conditions used in the study. It is important to note that were used the same conditions to different atmospheres. Table 3.3 Heating-cooling DTA profile used for SAC405. Solder Thermal cycle Heating rate (ºC/min) Temperature max. (ºC) Stage (s) Cooling rate (ºC/min) SAC

48 Preparation of the solders with substrate: In this case, firstly the substrate with a surface finish in immersion tin was prepared. The substrate had approximately 3 mm diameter because the dimensions of the aluminum crucibles were 5 mm diameter. After the preparation of the substrates, flux (no-clean, Cobar 94QMB) was applied in the surface of the substrate and the solder was placed. Samples of the solder SAC305 were cut and polish to obtain a plan surface and follow was placed on the substrate surface. The samples had approximately mg each. As the solder SAC405 was removed a few solder of the container and placed on the surface of the substrate. Samples were placed in aluminum crucibles and placed in DTA oven under argon and air atmosphere Morphological analysis The inspection and characterization of the samples can be made through electronic microscopy. In this project the samples had a small size, so for a facility the handling the samples were mounted in resin. The samples were grouped with respect to solder composition for detailed morphological analysis. After the mounting, the samples were subjected to grinding, using a different grip papers with 180, 600 and 1200 mesh. Then, the samples were polished using an abrasive diomond with 6 and 1 µm. To reveal the microstructure the samples were etched using a solution of the ferric chloride (96 ml of the ethyl alcohol more 2 ml of the hydrochloric acid and 5 g of the ferric chloride) Scanning electron microscopy (SEM) Scanning electron microscopy is a technique for high-resolution imaging of surfaces. The samples were analyzed using a scanning electron microscope to perform a morphological evaluation. SEM micrographs can be found in all Chapter 4. SEM uses a beam incident of electrons then interact with the atomic structure of the specimen and generate topographic images. Different types of electrons are produced from the beam, secondary and backscattered. The secondary electrons are responsibly by morphology and topography of the samples surfaces and backscattered electrons to achieve phase discrimination. The SEM is also capable of performing analyzes in a specific location using an energy-dispersive X-ray spectroscope (EDS). With the attachment of the EDS, the elemental composition of materials can be determined [37]. 28

49 A Hitachi TM3030 plus scanning electron microscope was used in this work for characterization of the samples. An accelerating voltage of 15 kv was used to obtain the micrographs at different magnifications. The microstructural analysis was conducted to quantify and qualify composition of the samples, namely the solder composition and intermetallic layer composition. With the SEM was also measured the interface thickness between the solder and substrate, as shown in Figure 3.1. Figure 3.1 SEM image for IMC thickness measurement. 29

50 3.3. Oxidant atmosphere study in a production line This sub-section describes the project that was performed in a production line at Bosch Car Multimedia. The main objective of this project was to study the influence of the oxidant and inert atmosphere in intermetallic formation and evaluation of the solder joint at a Bosch production line. For this study, was projected a work plan where were defined the steps to perform in the production. For such, this project was divided into two main groups, as shown in the following scheme (Figure 3.2): Figure 3.2 Schematic representation about the Solar A production Materials In the reflow soldering process was used the solder paste SAC405 which was development by Heraeus (F620Cu0.5-88M3). Regarding the wave soldering process was used the solder SAC305 which was development by Stannol. These are the same solders used in the first project, therefore the characteristics are the same (presented in Table 3.1). 30

51 Regarding the PCBs used in this project was supplied by Pacific Fame and have the following characteristics: PCB material: FR4 PCB thickness: 1.6±0.14 mm Cu layer thickness: µm Surface finish: Immersion tin Surface finish thickness: µm Panel dimension: 185 x 230 mm PCB dimension: 104 x mm The components used in the soldering process was supplied by Bosch Car Multimedia according to the product specifications Methods For this project was used the product existent at Bosch Car Multimedia. The product produced for this study was a solar panel defined by NSC Solar A. The product was chosen presents a double sided assembly, constituted by surface mount components and through-hole components, and is a lead-free product. Therefore, the production involves two soldering processes: reflow and wave soldering. Surface mount components were reflow soldered to the PCB, followed by wave soldering of the through-hole components and some surface mount components. The objective of the project was to study the solder joint with different atmospheres in reflow soldering process. Besides, with this project it was possible to study the intermetallic layer formation and growth, in various steps of the production as previously described Soldering with an inert and oxidant atmosphere Regarding the thermal profiles, it was used the normal production profile of the NSC Solar A, according to the requirements Bosch Car Multimedia. Table 3.4 presents the soldering data of this product. Table 3.4 Data about the soldering process, reflow and wave. Process Alloy Surface finish Flux Reflow SAC405 Immersion Tin No-clean, REL0 Wave SAC305 Immersion Tin No-clean, Cobar 94 QMB 31

52 Reflow Soldering In the experimental work, the solder paste was deposited onto the specified upper side volume of the PCB by EKRA X4 professional machine Bosch edition (the stencil printing machine), as represented in Figure 3.3a. The printing was performed by a metal squeegee where the solder paste was pre-applied on a stencil with openings corresponding to the pad on the PCB. The solder paste used in this soldering process was SAC405. After the solder printing process, in the second step, the components were automatically removed from the transport packaging and mounted in the position provided on the PCB using a Siemens Siplace S20 (pick and place machine) as shown in Figure 3.3b. After mounting components in solder paste on the side of the PCB, these are soldered by the application of heat in an inert (with nitrogen) and oxidant (air) atmosphere. This way, the reflow oven used in this production was a REHM Inert Gas Reflow Soldering System VXP 3500, as shown in Figure 3.3c. During the reflow, when using an inert atmosphere, the content of residual oxygen had to be less than 1500 ppm. The PCB was carried on along a conveyor belt system inside of the reflow oven where the transport speed was 0.85 m/min. The reflow profile used in this production was indicated by Bosch Car Multimedia, and this presented the following stages, heating, soak, and peak and cooling. Stencil printing machine Pick and place machine a) b) c) Reflow oven Figure 3.3 Reflow process steps: a) EKRA X4 professional machine Bosch edition, b) Siemens Siplace S20, and c) REHM Inert Gas Reflow Soldering System VXP This way, the thermal profile used in this production was according to Bosch Car Multimedia requirements. Figure 3.4 shows the reflow profile obtained for the REHM Inert Gas Reflow soldering System VXP

53 Figure 3.4 Reflow profile and the results obtained on the different zones. Table 3.5 presents the temperatures defined on reflow oven for to 7 heating zones. Table 3.5 The temperatures defined on reflow oven. Zones/Peak/Cooling Z1 Z2 Z3 Z4 Z5 Z6 Z7 P1 P2 P3 C1 Temperature (ºC) Wave soldering Wave soldering was a process used to solder the components in face b of the PCB. Therefore, after reflow soldering, the through-hole components were inserted into a specific hole in the PCB using the Universal Radial type 8 (radial lead components) as shown in Figure 3.6aFigure 3.6 Wave soldering process: a) Universal Radial type 8, b) Siemens G2 equipment and c) EPM CIG 400. After the insertion of the TH components in the holes, the SMD components were applied in the PCB underside, using the process SMT adhesive with the Siemens G2 equipment (Figure 3.6b). Firstly, the components were glued onto the PCB surface before running through the molten solder wave. The underside of the printed circuit board was heated to cure the SMT adhesive. The material (glue) used in SMT adhesive was supplied by Henkel Loctite E3621. During temperature curing, the process must be controlled because the radial electrolytic capacitors are temperature sensitive components. Figure 3.5 shows the curing profile used in this production. The conveyor speed used in this process was 0.8 m/min. 33

54 Figure 3.5 SMT adhesive curing: temperature profile. After the soldering process, some components were inserted in the PCBs manually. Thereafter the PCBAs were controlled by an automatic inspection system. At last, after the components have been inserted into or placed on the PCBs, these were loaded across a conveyor for waves of molten (Figure 3.6c). The solder wets the exposed metallic areas of the board creating a reliable mechanical and electrical connection. Inserting TH components Inserting SMD components Wave soldering a) b) c) Figure 3.6 Wave soldering process: a) Universal Radial type 8, b) Siemens G2 equipment and c) EPM CIG 400. The parameters used in wave soldering are permitted according to the Bosch Car Multimedia requirements. Figure 3.7 presents the thermal cycle used in this production in a double wave soldering. Production parameters used: Pre-heating: ºC Contact time: 10 s Conveyor speed: 1.2 m/min 34

55 Temperature (ºC) Figure 3.7 Wave thermal profile. Time (s) Product validation The product validation was performed through the following techniques: SPI: All the PCBAs in the reflow were evaluated after the solder paste printing for control the position of the solder paste. AOI: All the PCBAs were subjected to various control points after the soldering steps for check the existence of the defects. In circuit test: After the wave soldering, the electric system of all the PCBAs were checked. Visual inspection: All the solder joints of the PCBAs were analyzed using the optical microscope. This inspection consisted in the control of possible anomalies that were not detected by AOI. Scanning electronic microscopy/ Energy-dispersive X-ray spectroscopy: The SEM was used to analyze and measure the IMC layer. EDS was used to identify the chemical composition of the solder and IMC. It was also used for measurement of the contact angles Solder paste inspection (SPI) and automated optical inspection (AOI) SPI In the process of the solder paste inspection, the limits are defined for the product in the study. With this inspection, it is possible to control and to evaluate the solder paste printing in the reflow process, avoiding the appearance of defects in the solder joints. 35

56 Solder paste inspection can be categorized into 2D and 3D inspections. In 2D solder paste inspection consists in measuring and analyzing the area, bridging and X and Y offset. In 3D mode, the performed measures are the height, area, volume, bridging, shape deformation and X and Y offset [38].The evaluation of the solder printing was made by 3D Koh-Young machine (Figure 3.8). In this project, SPI technique was used to evaluate if the solder paste printing was successfully performed. Figure 3.8 Koh-Young Machine (SPI equipment). AOI The main task of AOI systems is to inspect all the PCBs and solder joints so that defects can be detected and removed from the production line. The automatic optical inspection was used after reflow soldering, wave soldering, and selective soldering. This system can detect failures types, such as missing components, misaligned, tombstoning, solder bridging, non-wetting, among others. After reflow, the final product was inspected by Viscom M (Figure 3.9a) and after radial and wave soldering it was used Marantz M22XDL350 machine (Figure 3.9b) [39]. 36

57 a) b) Figure 3.9 AOI equipments. a) Viscom M, b) Marantz M22XDL In circuit test (ICT) ICT is a technique that uses a bed-of-nails test fixture to access multiple test points on the PCBs bottom side. ICT can transmit test signals into and out of PCBs for measure the performance of the components and circuits. In the present project, the final product (after wave soldering) was tested using a Reer machine so to detect failures in the components and the circuits (Figure 3.10). The ICT evaluation can be classified by pass or fail. Figure 3.10 Reer ICT equipment Visual Inspection The visual inspection was performed after the soldering process so that it is possible to identify defects which cannot be detected in AOI, for example, extra components, solder balls, solder beads, damage PCBs, and components, among others. The PCBAs inspection were made through optical microscopy using a Leica M205C microscope. This inspection was performed according to the Car Multimedia acceptance criteria and were used a zoom of 7.8x and 25x for PCBAs visual inspection. 37

58 Morphological characterization In electronic industry is essential to evaluate the intermetallic layer formation and to study the solder joint reliability to identify defects present after to soldering, such as voids, cracks, among others. For morphological characterization, some products were chosen to be sectioned and inspected by optical and electronic microscopy. Therefore, the preparation of metallographic sections of solder joints involved cutting out the area of interest, mounting in resin, grinding, polishing and finally etching. In this study, four components types were selected to analyze the solder joints (QPF, a chip capacitor, resistor and electrolytic). The solder joint and intermetallic formation was analyzed in section 1, 2, 3 and 4 represented in Figure Figure 3.11 Cross-sectional zones. In this work, the metallographic preparation process was performed by Marques Ferreira Laboratory and consisted in the following steps: a) The cutting was performed in the intended section using the cutting tool. After cutting, the samples were grinding to guarantee the correct position during the mounting in resin. b) The second step was mounting the samples. This way, the samples with support clips were embedded in resin (7.5 mg of the epoxicure to 1.5 mg of the hardener) to facilitate their management. After adding the resin, the samples were placed in a 38

59 vacuum camera to eliminate any air present in the resin. Regarding the cure time of the resin, it was of approximately 8 hours. c) The grinding step consisted in the remove of material that was damaged or deformed surface material the form to produce a plane surface in the samples. Thus, were used the grit papers with 320, 500, 800, 1200 and 2500 mesh (in this order) for the various grinding stages. d) The polishing process was similar to grinding and it was used to remove the scratches produced from the finest grinding stage. In this study, it was used a soft cloth with 3 and 1 µm. The equipment used for the step c) and d) was a Struers RotoPol-21. e) The last step was etching. The etching was used to reveal the microstructure of the solder joint and to evaluate the intermetallic layer. This way, the chemical attack used to reveal the tin was 93 ml of the distilled water more 5 ml of the nitric acid and 2 ml of the hydrochloric acid. For to reveal the copper chemical attack was used a solution constituted by 20 ml of the ammoniac plus 20 drops of the hydrogen peroxide (20% volume). The metallographic preparation was performed in order to evaluate the intermetallic compound formation and solder joints with different conditions. This way, 36 crosses sectional were inspected using an optical and scanning electron microscope Optical microscope An optical microscope was used to study and characterize the solder joint carried out in 4 components described above. After sectioning these components, the solder joint was inspected and evaluated according to IPC-A-610E [33]. In this study, the samples were examined by a Keyence VHX-2000 microscope with a 200x and 700x magnification. The cross-sectional presented in this study were analyzed at the level of the joint appearance, the wettability, and present defects. By optical microscope analysis, it was also studied the contact angle between the solder and component. Contact angle measurements were performed on the samples after the cross sectional for SMD components, as shown in Figure It is important to refer that the measurements were performed on the both sides of samples (left and right). 39

60 Figure 3.12 Contact angle measurement: a) resistor, b) capacitor and c) QFP Scanning electron microscopy (SEM) Scanning electron microscope (Hitachi TM3030 plus) was used to study the IMC formation and the morphology of the samples. By recurring to EDS it was possible to identify the present elements in the IMC layer and solder, and also their composition. An accelerating voltage of 15kV was used to obtain the micrographs at different magnifications. With the SEM was also used to measure the IMC thickness between the solder and substrate, and the IMC thickness between the solder and component, as shown in Figure 3.13 and Figure 3.14, respectively. These measurements were performed for the capacitor, resistor, and QFP components. Figure 3.13 SEM image for IMC thickness measurement between solder and substrate. 40

61 Figure 3.14 SEM image for IMC thickness measurement between solder and component. The measurements between the solder and substrate were performed by drawing two lines, and thus determining its thickness. The measurements between the solder and component the method were different. The IMC thickness, in this case, was measured from 5 to 10 points along the intermetallic layer. Regarding the TH component, the electrolytic, the measurements were performed in 3 zones, as shown in Figure Figure 3.15 Micrograph and SEM image of electrolytic with specific zones where were made IMC thickness measurement. By SEM analysis was also studied the solder height for QFP component because it is a requirement of the soldering. This study consisted in to investigate the influence of oxidant atmosphere in the solder height. Figure 3.16 shows as were performed the measurements of the solder height. 41

62 42 Figure 3.16 SEM image for solder height measurement.

63 4. CHAPTER 4 RESULTS AND DISCUSSIONS 4.1. Soldering study with an oxidant atmosphere The study of the soldering process with an oxidant atmosphere is increasing in the electronic industry. So the aim of the present study consists into investigation of the effect of the oxidant atmosphere in the intermetallic layer formation using lead-free solders. This sub-section presents the results of the soldering study performed with SAC305 and SAC405 under inert and oxidant atmosphere. This way, the sub-section also presents the results regarding the thermal analysis of SAC405, microstructural analysis of the interface between solder (SAC305 and SAC405) and substrates and study the oxidant atmosphere effect on the formation and growth of intermetallic layer Thermal analysis The melting temperature is an important physical property of any solder. It is an important factor in the electronics industries due to its influence in the soldering process as well as in PCBA. A good solder should have a low melting temperature [40]. The thermal properties, such as melting and solidification temperature, were determined by DTA in SAC405 without substrate. These tests were performed under different atmospheres, argon and air, and different heating rates. After performing the tests, melting temperatures were determined from the onset points in the DTA curves. Figure 4.1 and 4.2 show single endothermic peaks corresponding to the melting temperatures of the SAC405 using different three heating rates and different atmospheres (argon and air). The measured melting temperatures (Tm) of the SAC405 are listed in Table

64 0,2 Wt corrected Heat Flow (W/g) 0,0-0,2-0,4-0,6-0,8 10ºC/min (Air) 20ºC/min (Air) 30ºC/min (Air) Temperature (ºC) Figure 4.1 DSC curves, with different heating rates, under an oxidant atmosphere. 0,2 Wt corrected Heat Flow (W/g) 0,0-0,2-0,4-0,6-0,8 10ºC/min (Ar) 10ºC/min (Air) Temperature (ºC) Figure 4.2 DSC curves, with the same heating rate, under an inert and oxidant atmosphere. The melting temperature of SAC solder ranges between 217 to 221 ºC depending on the composition of Ag and Cu. The onset melting temperatures are 20 ºC higher than above the traditional Sn-Pb alloys. The results of the thermal analysis show that the shapes of melting peaks are similar for all conditions. However, the intensity of the peaks increases with the increasing of the heating rate. Yet, the resolution of the peaks melting is more pronounced with the heating rate increasing. This can be explained by the melting being faster and, consequently the solder releasing more heat in more time (changing the heat dissipation characteristics, the peak shape and area). 44

65 Table 4.1 shows the melting range that was determined by considering the start of melting, determined as the onset point in the DSC curve, and the curve peak temperature, corresponding to the end of the melting, for the used heating rate. Table 4.1 Results of DTA analysis for different heating rates and different atmosphere. Solder Atmosphere Temperature (ºC) Weight (mg) Start of melting End of melting Argon SAC Air When comparing the melting temperatures with different atmospheres, the Table 4.1 shows that the results are similar. On the other hand, for low heating rates the solder melts in the range of ~2 ºC, while, for higher heating rates, the solder melts in a greater interval (~6-9 ºC). These results show that with increase the heating rate, the solder melting is slower. This aspect should be considered in the design of a PCB, in the soldering thermal simulation and analysis. In Figure 4.3 and 4.4, are presented the melting temperatures that were measured considering the start of melting, determined as the onset point in the DSC curve, and the curve peak temperature, corresponding to the end of melting, for the used heating rates and atmospheres. 45

66 Temperature(ºC) y= x y= x Start of melting temperature (Ar) Start of melting temperature (Air) Heating rate (ºC/min) Figure 4.3 Graph about start of melting temperature as a function of the heating rate for SAC Temperature(ºC) y= x y= x Heating rate (ºC/min) End of melting temperature (Ar) End of melting temperature (Air) Figure 4.4 Graph about the end of melting temperature as a function of the heating rate for SAC405. The results show that the melting temperature increases directly with the heating rate. By comparing the results between an inert and oxidant atmosphere, the oxidant atmosphere presents a higher start of melting temperature (approximately 1 ºC), as shown in the results. Under an oxidant atmosphere, the elements of liquid solder, having a lower free energy of oxide formation, are oxidized at the surface of the solder. Therefore, to dissolve these oxides is necessary more 46

67 energy and consequently the increase of initial melting temperature. The oxides presence is more elevated under oxidant atmosphere. So, the start of melting temperature increased because for dissolve oxides are necessary more energy when compared with an inert atmosphere. On the other hand, the solder melts later so the end of melting temperature will be lower as shown the results Microstructural analysis of the interface The solder microstructure and the intermetallic layer thickness are determined by the maximum achieved temperature and time of contact between the molten solder and the substrate (immersion tin). The micrographs of the interfacial intermetallic layer resulting from the three different temperatures, 240, 250 and 260 ºC with different atmospheres are presented in Figure

68 Figure 4.5 SEM micrographs of SAC305 for different temperatures, 240, 250 and 260 ºC, under inert and oxidant atmospheres. By observation of the micrographs, it is possible to see the intermetallic layer formation for all conditions. The formation of IMC layer after soldering indicates a good metallurgical bonding. A thin, uniform, and continuous IMC layer is essential for a good bonding. If the IMC layer does not form, the solder joint is weak because no metallurgical interactions were established between solder and substrate [27]. In this study, it was observed that the obtained IMC layer always have a similar interface structure, but with increasing temperature the intermetallic layer thickness increase. Regarding the results about IMC thickness, these are present in the section

69 In Figure 4.6 is illustrates the intermetallic layer composition obtained after soldering in the micrographs presented in Figure 4.5 (the first micrograph, 240 ºC and under an inert atmosphere). Figure 4.6 SEM micrograph of the solder joint with SAC305 and Sn substrate a) and EDS element mapping for all the elements b), for Sn c) Cu d) Ag e) and Al f). It is also possible to observe from these figures that the IMC layer morphology is of the scallop-type. By SEM/EDS analysis was revealed that the composition of the IMC layer presents Cu and Sn. By literature, the intermetallic layer present two types of IMC, the Cu6Sn5 and Cu3Sn. The Cu6Sn5 can be observed as large grains with a scallop type. While Cu3Sn is a thin phase present between the phase Cu6Sn5 and the copper layer. This phase was generally not observed or detected because Cu3Sn phase appears with increased holding time although it may exist [27, 41]. The mapping performed in the samples also showed the presence of the Ag3Sn phase. However, this phase appears within the solder alloy. The EDS spectra shown in Figure 4.7 indicates the intensities of Cu and Sn on IMC layer. 49

70 Figure 4.7 EDS spectra of IMC layer. The EDS results about the IMC layer chemical constitution showed that the distribution of Cu and Sn atomic percentages were 27.5%Cu and 56.9%Sn. This presence is explained by the Cu atoms dissolved from the substrate which reacted with the Sn atoms to form the IMC layer. Regarding the presence of Ag in the intermetallic layer was minimal and did not take part in the formation of the IMC at the interface. The other elements, such as carbon, oxygen, aluminum and silicon present in the samples correspond to the PCB constituents. Regarding SAC405, was performed the same procedure and the results about microstructure analysis are presented in Figure

71 Figure 4.8 SEM micrographs of SAC405 for different temperatures, 240, 250 and 260 ºC, under inert and oxidant atmospheres. The results about the microstructure of SAC405 present similar characteristics when compared with SAC305. The micrographs showed that for all conditions the intermetallic layer formed. However, the main difference between the solders is the intermetallic layer thickness. This difference is explained in the next section, In micrographs after the soldering with SAC405 was possible to identify the presence of black spots. These black spots were not seen in soldering with SAC305. Though not important for the study, the black spots in the solder may be flux that was retained (that because in this study 51

72 were not respected the temperatures thermal profiles recommended by the supplier). Moreover may be residues of polishing or a subsequent contamination to polishing. The interface chemical composition study of the intermetallic layer, for SAC405, is presented in Figure 4.9. Figure 4.9 a) SEM micrograph of the solder joint with SAC405 and Sn substrate a) and EDS element mapping for all the elements b), for Sn c) Cu d) Ag e) and Al f). The intermetallic layer composition about SAC405 presents similarity with SAC305.By observation of micrographs, the Ag presence was found in a higher percentage in the solder, but this due to the fact the SAC405 had a higher amount of Ag present initially. In Figure 4.10, are presented the EDS spectra regarding SAC405. Figure 4.10 EDS spectra of intermetallic layer. The EDS results about the IMC layer also showed no significant differences compared to SAC305. The percentage of the Cu and Sn in the intermetallic layer were approximately 29.1%Cu 52

73 and 43%Sn. The Sn percentage decrease compared with SAC305, but this explicated by increasing of the others elements (carbon, oxygen, aluminum and silicon) Formation and growth of intermetallic layer at the interface between SAC and Sn substrate In order to determine the effect of the temperature and oxidant atmosphere, it is important to analyze the growth of the IMC. The formation and growth of intermetallic layer are controlled by a diffusion process where, essentially, atoms from the substrate move into the solder matrix [2]. In this way, the thickness of the intermetallic layer formed after the reactions between substrate and solders, SAC305 and SAC405 was studied. Scanning electron microscopy technique was employed in the analysis of the intermetallic layer formation. So, the microstructures concerning the reactions between substrate and solder under various thermal conditions were evaluated. In Figure 4.11, are displayed the results of the IMC thickness vs maximum temperature under different atmospheres IMC Thickness ( m) Inert atmosphere Oxidant atmosphere Temperature (ºC) Figure 4.11 Average of IMC thickness for SAC305 as a function of maximum temperature. The results show that for SAC305, the IMC thickness increases with the increasing of the temperature. This indicates that the dissolution rate of Cu into the molten solder is higher with increasing temperature. 53

74 Under inert atmosphere, the results showed a thickness of 4.38, 5.41 and 6.83 µm for a temperature of 240, 250 and 260 C, respectively. However, under oxidant atmosphere the IMC thickness was 3.17, 4.55 and 6.6 µm for the same temperatures. Comparing the results it is seen that with an oxidant atmosphere the obtained IMC thickness is always lower, because the initial oxide layer present. The difference decreases with the maximum temperature increase. That could be explained by the time needed to destroy the superficial layer of oxides present in the tests under an oxidant atmosphere. This time is lower for higher maximum temperatures. Additionally, the time to start the ÎMC formation (initial time necessary for surface oxides dissolution) compared with the full contact time between liquid solder and substrate diminish with the increase in the maximum temperature. This study was also performed for SAC405. Therefore, the Figure 4.12 presents the results of the IMC thickness for the same conditions used in the study of SAC Inert Atmosphere Oxidant atmosphere 12 IMC Thickness ( m) Temperature (ºC) Figure 4.12 Average of IMC thickness for SAC405 as a function of maximum temperature. Under inert atmosphere, the results showed a thickness of 7.63, and 7.7 µm for a temperature of 240, 250 and 260 C, respectively. However, under oxidant atmosphere the IMC thickness was 8.0, 10.8 and 11.1 µm for the same temperatures. The results showed that for 240 and 250ºC the IMC thickness under different atmospheres is similar, except for the temperature of 260ºC. This is an expected results because the presence of the flux in the solder paste allow a 54

75 more fast and efficient surface oxide removal. This way, the time for the IMC formation and growth is equivalent on both atmospheres. The IMC thickness at 260 º C, for the test with the nitrogen atmosphere, has an unexpected decrease. This results should be confirmed in a future work. The slower increase of the IMC could happen if the IMC thickness has achieved the maximum. The process tolerance usually ranges from 7 to 11 µm, so it is not possible to obtain a greater thickness. Thus, if the other tests were conducted at higher temperatures, the thickness of the IMC would vary within the range described above. In this way, considering this IMC thickness, with longer soldering times or higher temperatures the IMC will tend to dissolve into the solder. By observation of the IMC thickness results for SAC305 and SAC405, the thicknesses were higher for SAC405. These results can be explained by the difference of solders. While the SAC305 was supplied in wire form, the SAC405 was supplied in solder paste (it includes a flux). The presence of flux in solder has the advantage of reducing a number of oxides formed and this way the solder melts earlier, with the same quantity of heat, and stay over a clean non-oxided surface. Therefore, the IMC thickness for SAC405 is higher than IMC thickness of SAC

76 4.2. Oxidant atmosphere study in a production line The electronic industry has contributed to the society s development and the search for low-cost production is continuously increasing. On the other hand, it is important to maintain the performance, reliability and miniaturization. Thus, due to industry requirements, it is important to study the soldering process without nitrogen. Therefore, the project performed at Bosch Car Multimedia had as main objective the study of the oxidant atmosphere (air) influence in the production line. This sub-section presents the results of the study on the effect of oxidant atmosphere on intermetallic compound layer formation and solder joints reliability. The sub-section also presents the results extracted from the assembly process, such as SPI and AOI, circuit test, visual inspection, optical and electronic microscopy of the sectioned samples Solder paste inspection The aim of the solder paste inspection was to evaluate if there was good solder paste printing in the reflow soldering process. This evaluation was performed in all pads of the PCB, but in this work are only presented the results regarding the critical components in the application of the solder. The choice of these components was based in the pseudo-error number detected in automatic optical inspections system. So, with this system, it was possible to analyze the spreading of the solder paste on the pads and validate if the results were according to Bosch Car multimedia requirements. Figure 4.14 shows the results of the components, in an inert and oxidant atmosphere, with more pseudo-error numbers where are presented the data about the height. The data about the area, volume and offset X and Y are represented in Annex I. 56

77 Figure 4.13 Results about SPI (height) for different components. 57

78 Figure 4.14 Results about SPI (height) for different components (continued). The results concerning SPI showed no evident problems in the solder paste printing. This way, the results obtained are according with Bosch Car multimedia requirements. Although detecting pseudo-errors in the automatic optical inspection, the solder paste printing did not affect the components correct soldering under the tested conditions. 58

79 Automated optical inspection The automatic optical inspection is a technique used to evaluate all the PCBAs and solder joints after the soldering processes. In this work, were performed two main inspections, after reflow soldering and after wave soldering. In the production of the Solar A was used the standard Bosch profile but with a different atmospheres. In the reflow soldering process it was used two atmospheres, inert and oxidant while in the wave soldering it was only used inert atmosphere. The aim of the automatic optical inspection was to detect the existence of failures in the solder joints and PCBAs. So, with this technique it was possible to analyze the influence of the atmosphere during the soldering process. Figure 4.14 and Table 4.2 show the results about the average number of the pseudo-errors and defects per PCBA obtained after the reflow and wave AOI. It is important to refer that the program used in the reflow soldering is different compared with the program of the wave soldering. Pseudo-Error number Reflow (N2) Reflow (Air) Wave (N2-N2) Wave (Air-N2) Figure 4.15 Number of pseudo-errors detected in AOI. 59

80 Table 4.2 Number of pseudo-errors after reflow and wave soldering with an inert and oxidant atmosphere. Reflow (N2) Reflow (Air) Wave (N2-N2) Wave (Air-N2) Pseudo-errors 3.66± ± ± ±3.89 The inspection results showed the all the PCBAs did not present defects but on the other hand, pseudo error presence was detected. The pseudo error can be evaluated as a defect or false error which is controlled by an operator. In the reflow soldering, the results showed that using the oxidant atmosphere, the pseudo error number per PCBA increase. This increase happens because the solder oxidizes and the solder appearance change compared with the soldering standard process. Regarding the wave soldering, the inspection results are approximately the same. The wave soldering is a dynamic process and the fluxing stage has the ability to remove the oxides present in the PCB. So, this explains that the oxidant atmosphere used in the reflow soldering do not influence the wave soldering because the oxides are removed before the soldering step. On the other hand, in the reflow soldering, the flux are contained in the solder paste. Therefore, the flux action is different because the flux will have a better performance only on the side of the PCB, and not on the component. The pseudo error numbers per PCBA increase in the wave soldering process when compared to the reflow soldering. This was due to the fact that the AOI program is not optimized. Therefore, before the use of an air atmosphere it will be necessary to adjust the inspection program for avoiding in the future a wrong analysis In circuit test In circuit test was used in order to classify and accept the circuit of all PCBAs analyzed. In addition, with circuit test is possible to detect some types of defects that were not detected previously by inspection methods, such as, reversed and short component. The circuit test was performed in 284 PCBAs and the results were classified as pass Visual Inspection The inspection and characterization were made through an optical microscope. The visual inspection was performed according to the Car Multimedia acceptance criteria and the objective of this inspection was to identify defects which cannot be detected through the others inspection. 60

81 The PCBAs analyzed were performed in 40 panels as described in Figure 3.2. Figure 4.16 shows the images obtained in the visual inspection in the PCBAs with an inert and oxidant atmosphere. Figure 4.16 Visual inspection of PCBAs after reflow and wave in an inert and oxidant atmosphere. 61

82 Visual inspection results have revealed no evident defects in the analyzed PCBAs. It was observed that PCBAs had a similar visual appearance, by comparing the results obtained after the reflow soldering process in an inert atmosphere with and with an oxidant atmosphere. It should be noted that the flux residue is darker in the PCBAs produced with an oxidant atmosphere. However, this aspect is not significant because no errors were detected in the AOI. Regarding the results obtained after the wave soldering these present also similar appearance. Therefore, it is concluded that the oxidant atmosphere did not influence significantly the product reliability Characterization of solder joints The characterization of the solder joints was evaluated by optical and electronic microscope. As described in Chapter 3, the solder joints from four components were analyzed in order to study the composition, microstructure, intermetallic layer and wetting to the component and PCB Evaluation of the solder joints The aim of the present evaluation was to inspect, using 36 cross sections the four components (described above) that have been submitted to different atmospheres during reflow soldering. The solder joints evaluated was carried out according to IPC-A-610E classe2 [33]. In Figure 4.17, 4.17, 4.18 and 4.19 are presented the solder joints of the components selected. Figure 4.17 Evaluation of solder joints in capacitor component, under different atmospheres and after reflow and wave soldering. 62

83 Figure 4.18 Evaluation of solder joints in resistor component, under different atmospheres and after reflow and wave soldering. Figure 4.19 Evaluation of solder joints in QFP component, under different atmospheres and after reflow and wave soldering. Figure 4.20 Evaluation of solder joints in the electrolytic component, under different atmospheres and after reflow and wave soldering. 63

84 The Figure 4.16, 4.17 and 4.18 showed that the SMD s results presented a good joint appearance for all conditions. The same was found for the TH component, Figure 4.19, which presented a good joint, complete fill, and good wettability. On the other hand, all solder joints presented some voids but their presence did not influence the soldering. Thus, the results showed that solder joints after reflow in different atmospheres were well formed for all the components studied. The IPC-A-610E shows a number of points where are supported the acceptability criteria corresponding to the components in the study. The obtained results for capacitor and resistor components were supported by points and For QFP component, the points that supported the results are and Regarding the TH component (electrolytic), the point that validate the results is Through the cross-sectional and by comparing these with the IPC- A-610E was proved that all components presented are according to the standard acceptance criteria [33] Analysis of the solder joints chemical composition To determine the elemental composition of the solder joints and PCB, it was used the EDS analysis. This study was performed in order to analyze differences in the solder joints composition for both types of production. Figure 4.21 shows SEM image and EDS element mapping of the solder joints capacitor after reflow with an inert atmosphere. The EDS element mapping is presented in more detail for determining the distribution of the elements in the solder joints. The results of the other components are showed in Annex II. Figure 4.21 SEM micrograph of capacitor solder joint a) and EDS element mapping corresponding Sn b), Cu for c), Ag d), Ni e) and Al f). 64

85 Figure 4.21b, c, d, e, and f show the following evidence that can be observed: Sn-matrix with Ag small particles; the Cu layers of the PCB side and component side; some Cu into the Snmatrix from the dissolution of the Cu layer; Ni finish coating of the component side; and Al presented in the PCB. To study the influence of the atmosphere in the reflow soldering and to understand the behavior after the two soldering processes were compared EDS general mappings of capacitor component with different conditions, as shown in Figure Figure 4.22 EDS general mappings of capacitor solder joint with different conditions: a) Reflow (inert atmosphere), b) Reflow (oxidant atmosphere), c) Wave (after reflow with an inert atmosphere) and d) Wave (after reflow with an oxidant atmosphere). The solder joints represented in Figure 4.21, present general mappings of Sn, Cu, Ag, Ni, and Al elements distribution. The results presented show basically the same composition for the solder joints after reflow and wave with different atmospheres. Therefore, it can be concluded that the use of the different atmospheres in the reflow did not have influenced the composition of the solder joints Impact evaluation of the soldering processes in intermetallic layer In order to determine the effect of the oxidant atmosphere in IMC, it is important to analyze the growth of IMC. So, the thickness of the intermetallic layer was monitored as a function of the 65

86 soldering processes. Scanning electron microscopy technique was employed in the analysis of the intermetallic layer formation. Firstly, it was studying the IMC layer formed on the PCB side. In Figure 4.23, are displayed the results concerning the IMC thickness of each SMD component in the study. 4,5 4,0 IMC Thickness ( m) 3,5 3,0 2,5 Resistor Capacitor QFP 2,0 0 Reflow (N2) Wave (N2-N2) Reflow (Air) Wave (Air-N2) Figure 4.23 Average of IMC/PCB thickness in solder joints for SMD components as a function of soldering processes. The average IMC thickness for different components (SMD), different atmospheres and different soldering processes, are shown in Table 4.3. Table 4.3 Average IMC/PCB thickness for the capacitor, resistor and QFP components with different conditions. Atmosphere IMC thickness (Capacitor) IMC thickness (Resistor) IMC thickness (QFP) Reflow (N2) 3.02± ± ±0.43 Wave (N2-N2) 2.95± ± ±0.51 Reflow (Air) 3.32± ± ±0.65 Wave (Air-N2) 3.23± ± ±0.43 In the case with an inert atmosphere, the IMC thickness results after reflow of the resistor, capacitor and QFP were around 2.68, 3.02 and 3.16 µm respectively. Regarding the results after wave soldering for the same components (resistor, capacitor, and QFP), the IMC thickness was 2.65, 2.95 and 3.13 µm. These results show that the average thickness of IMC layer after reflow 66

87 soldering present statistically the same results comparing with the average thickness of IMC layer after wave soldering. In an oxidant atmosphere after reflow soldering and after wave soldering the average thickness of IMC layer was also the same, as shown the results in Table 4.3. It is important to note that the resistor, capacitor and QFP component were inserted in the reflow soldering. However, by comparing the soldering process in an inert atmosphere results with the soldering process in an oxidant atmosphere, it was observed that the average thickness of IMC layer increase. However, this difference is not considered significant because the results found are always into the process window. Regarding the TH component, it was analyzed 3 different zones, as described above. Figure 4.24 show the results of the average thickness of IMC layer respective of TH component. 4,5 4,0 IMC Thickness ( m) 3,5 3,0 2,5 2,0 Zone 1 Zone 2 Zone 3 1,5 1,0 0,0 Reflow (N2) Wave (N2-N2) Reflow (Air) Wave (Air-N2) Figure 4.24 Average of IMC/PCB thickness in solder joints for TH component in different zones as a function of soldering processes. The average IMC thickness of electrolytic (TH) are evaluated in different zones, 1, 2 and 3 for different atmospheres and different soldering processes, are shown in Table

88 Table 4.4 Average IMC thickness of electrolytic component with different conditions. Atmosphere IMC thickness (Zone 1) The electrolytic component was inserted after reflow soldering. So, the intermetallic layer formation during reflow soldering occurred the reaction between the copper layer and surface finish (immersion tin). As expected, the average thickness of the IMC layer formed during reflow soldering was similar for different zones, as shown in Table 4.4. After the wave soldering, the average thickness of the IMC layer was different for 3 zones. The wave soldering is a dynamic process so the intermetallic layer is affected by the solder movement into the hole. The results show that in zone 1, the intermetallic layer is greater compared with zone 2 and 3, as shown in Table 4.4. This difference occurs because of the solder movement. Zone 1 and 2 are not as affected as zone 3 because the solder had more movement in this area. In addition, the solder movement removes the intermetallic layer formed. For this reason, the intermetallic layer growth is greater in zones 2 and 3. Regarding the effect of the oxidant atmosphere in the reflow soldering, it is observed that the obtained intermetallic layer thickness is similar. This proves that the solder movement removes the intermetallic layer formed in the reflow soldering. It was also studied the IMC layer formed on the side of the component. In Figure 4.25 the results about the IMC thickness of each SMD component are presented in the study. In this case, it was not studied the TH component because this was inserted after reflow. So, the component was not influenced by the oxidant atmosphere. IMC thickness (Zone 2) IMC thickness (Zone 3) Reflow (N2) 2.78± ± ±1.0 Wave (N2-N2) 1.81± ± ±0.22 Reflow (Air) 2.25± ± ±0.64 Wave (Air-N2) 1.88± ± ±

89 7 6 IMC Thickness ( m) Resistor Capacitor QFP 1 0 Reflow (N2) Wave (N2-N2) Reflow (Air) Wave (Air-N2) Figure 4.25 Average of IMC/Component thickness in solder joints for SMD components as a function of soldering processes. The average IMC thickness for different components (SMD), different atmospheres and different soldering processes, are shown in Table 4.5. Table 4.5 Average IMC/Component thickness for the capacitor, resistor and QFP components with different conditions. Atmosphere IMC thickness (Capacitor) IMC thickness (Resistor) Through the results, it is possible to observe that after reflow the intermetallic layer thickness compared with after wave, for resistor and capacitor component, was statistically equal, as shown the results in Figure 4.25 and Table 4.5. Although the solder melts again during the wave soldering, the intermetallic layer keeps approximately the same thickness. This happens due to the resistor and capacitor component have metallization on nickel. The diffusion rate of Cu in SAC is high but the diffusion rate of Ni is low in the reaction with solder. Consequently, nickel is often used as a diffusion barrier between Cu and Sn in order to limit the diffusion. So, the nickel finish have a large influence on the growth rate of the IMC layer. IMC thickness (QFP) Reflow (N2) 2.35± ± ±1.83 Wave (N2-N2) 1.96± ± ±2.01 Reflow (Air) 2.63± ± ±1.95 Wave (Air-N2) 2.66± ± ±

90 On the other hand, the results of the QFP intermetallic layer increase after wave soldering. As described above, the diffusion rate of Cu in SAC is high but the QFP had metallization on Ni. So when the solder back melts in the wave soldering, the intermetallic layer should not grow. However, this happens because the Ni metallization is irregular (discontinuance) and had a thick thickness (Figure 4.26). So the Ni was firstly dissolved during reflow soldering and after, in the wave soldering, due to the absence of nickel, copper reacts with the solder by growing the intermetallic layer. Figure 4.26 SEM micrograph of QFP solder joint a) and EDS element mapping general b), Sn for c), Cu d), Ag e) and Ni f). IMC thickness comparisons with inert and oxidant atmosphere show that the use the oxidant atmosphere in the reflow soldering do not effect in the IMC layer thickness on the side of the component Influence evaluation of the inert and oxidant atmosphere on wetting A qualitative wetting measure was done for all SMD components. The measurement angle is explained in Chapter 3. The wetting measurement results are presented in Figure It was only considered the reflow soldering results because in this soldering process were used different atmospheres. 70

91 40 Contact angle ( m) Reflow (N2) Reflow (Air) 10 0 Resistor Capacitor QFP Figure 4.27 Contact angles of the resistor, capacitor and QFP components measured as a function of atmosphere used. It was observed that the contact angles for reflow with an inert atmosphere varied between 11.4º and 17.9º. However, the contact angles when using an oxidant atmosphere varied between 14.9º and 35.9º. Therefore, the highest contact angles were obtained when used an oxidant atmosphere during reflow soldering. These results can be explained by the poor rise of the solder caused by the solder oxidation on the side of the component. Clearly, smaller contact angles were obtained when using an inert atmosphere. Figure 4.28 shows the solder height results for the QFP component. This study was only performed in this component because present gull wing leads. The results show also the solder height in gull wing when used an inert and oxidant atmosphere in the reflow soldering. 71

92 Solder height ( m) Reflow (N2) Reflow (Air) Figure 4.28 Solder height in the QFP component fabricated in inert and oxidant atmosphere. The results are indicative of greater wetting when used different atmospheres. A higher solder height implies that the wet is better. So through the results, it is observed that solder height varied between µm and µm for oxidant (24 %) and inert atmospheres during reflow, respectively. These results indicate that for reflow with an inert atmosphere, the wetting is better compared by reflow with an oxidant atmosphere. As mentioned previously this behavior is explained because the use of inert atmosphere not only prevented the solder and copper pad from oxidization but, also improve the wetting. So this result in better wetting behavior and higher solder height than used oxidant atmosphere during reflow. 72

93 5. CHAPTER 5 CONCLUSIONS The focus of this Master s dissertation was the study and analysis of the intermetallic layer formation when an oxidant atmosphere is used during the soldering process. It is important to refer that the present work was divided in two projects, the first project was performed at the University of Minho and the second project was performed at Bosch Car Multimedia. The main goals of first project consisted in studying the influence of an oxidant atmosphere in the thermal properties of the solder, and it was analyzed the possibility of soldering process using different atmospheres. Firstly, it was studied the thermal properties of the SAC405 by using DTA technique under an inert and oxidant atmosphere. The results showed that under an oxidant atmosphere, the start of melting temperature increases approximately 1 ºC. This happens due to the use of an oxidant atmosphere, which promotes the formation of oxides that require more energy to be dissolved. Therefore, soldering processes using an oxidant atmosphere require a temperature increase in order to fully dissolve the oxides. The microstructural characteristics study of the interface between solder and substrate showed similarities under different conditions, for both SAC305 and SAC405 solders. In what concerns the intermetallic layer, it comprises an important part of the solder joint structure, and the different conditions can have a significant in its formation. The intermetallic layer formation is attributed to the diffusion of atoms from the substrate into the solder matrix. Therefore, it was observed that the intermetallic layer composition of the studied solders presented the same elements (Cu and Sn). The reaction between solders and substrate showed the intermetallic layers were formed for all conditions and the IMC thicknesses were increased with the rise of temperature. In summary, it was observed that using SAC305, the IMC thickness tends to be similar under different atmosphere with the rise of temperature. This can be explained because when the IMC thickness achieves the maximum, the atmosphere does not influence. The same condition was not observed in SAC405. However, as mentioned before, this difference is caused by the fact of the SAC405 being supplied in the form of a solder paste. The second project of the present study was to test in a production line, the effect of oxidant atmosphere during reflow soldering process, in a product chosen by Bosch Car Multimedia. 73

94 The influence of an oxidant atmosphere, namely the atmosphere of reflow soldering process on the solder joints was investigated. Firstly, the product Solar A was submitted to the normal process inspections during production, such as SPI, AOI after reflow and wave soldering and ICT. The results from SPI showed that the solder paste printing was according to Bosch Car Multimedia requirements. During AOI, it was detected an increase of the pseudo-error number when using an oxidant atmosphere, however, no errors were detected. Nevertheless, a few deviations were detected, but these showed to be in the safety range. All the PCBAs were soldered with success under different atmospheres, as confirmed by the inspection results. Posteriorly, the visual inspection confirmed the non-existence of any defects. In order to study of the solder joint and the intermetallic layer formation, four different components were chosen (capacitor, resistor, QFP and electrolytic). By evaluating the solder joints through the cross-sectional, and by comparing these with the IPC-A-610E it was proved that all components were according to the acceptance criteria, regardless of the atmosphere used. It was also performed an analysis of the solder joint composition, whose results concluded that the use of the different atmospheres did not have influence in the solder joints composition. Regarding the formation and growth of the intermetallic layer, two types of the IMC layers were analyzed, the reaction between the solder/substrate and solder/component. The results concerning the reaction between the solder and substrate for SMD s components showed that the average thickness of IMC layer after reflow and after wave soldering present the same statistically results. However, a comparison of the results with different atmospheres showed an increase when used an oxidant atmosphere. However, these results are not meaningful because they are within the process window. The results for TH component showed that the oxidant atmosphere does not have influence in the intermetallic layer thickness. This was proved by the solder movement into the electrolytic holes, which removes the intermetallic layer formed in the reflow soldering. The intermetallic layer formation and growth study in the reaction between solder and component was only performed with the SMD s components. These results showed that the capacitor and resistor kept the same thickness after reflow and after wave soldering. Therefore, it is possible to conclude that in this particular case, the distinct atmosphere and soldering processes, did not influenced the intermetallic layer thickness. This conclusion was explained by the presence of Ni layer of component side (metallization). On the other hand, the same was not observed for the electrolytic component, which is explained but the fact that the metallization thickness is greater in the capacitor and resistor components. 74

95 The wetting in the SMD s components was another study performed in this project. The contact angle between two surfaces is a general indication of wetting. So, a small contact angle means a good wetting and a strong bond. A higher contact angles indicates a decreased ability of the solder to wet and flow over the surface. The results showed that the oxidant atmosphere used caused an increase of the contact angle. This happens because the absence of the inert atmosphere affected the solder and component (oxidation). The same results were found in the study of the solder height. In summary, the results showed that the use of oxidant atmosphere did not have a significant influence in the formation and growth of the intermetallic layer. On the other hand, was observed that the use of an oxidant atmosphere had influence of the wetting of the component. 75

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97 6. CHAPTER 6 FUTURE WORK Following the Bosch Car Multimedia requirements, below are presented some proposals for future works that must be considered for understanding the influence of an oxidant atmosphere during soldering processes: Perform the reliability tests of the product Solar A after to soldering under an oxidant atmosphere and compare with the standard process; Analyse the influence of the oxidant atmosphere in all surface finish types, such as, immersion tin, nickel-gold, OSP and immersion Ag and to study the optimal conditions to ensure the reliability of the solder joints; Perform a study of all costs associated with production under an oxidant atmosphere: the reduction of nitrogen consumption versus increase of power consumption. 77

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99 REFERENCES [1] Tao Y., Ding D., Li T. (2014) Reliability of 1206 capacitor/sac305 solder joint reflowed in protective atmosphere. 15th International Conference on Eletronic Packaging Techonology, [2] Zeng G., Xue S., Zhang L., Gao L., Dai W., Luo J. (2010) A review on the interfacial intermetallic compounds between Sn Ag Cu based solders and substrates. College of Materials Science and Technology, [3] Dong C. C., Schwarz A., Roth V. D. (2010) Effects of atmosphere composition on soldering performance of lead-free alternatives. Air Products and Chemicals. [4] Khandpur S. R. (2006) Printed Circuit Boards Design, Fabrication and Assembly. McGraw- Hill Electronic Engeneering. [5] Judd M. and Brindley K. (1999) Soldering in Electronics Assembly. Newnes Publications. Second edition. [6] Pinto R. (2015) Avaliação da camada intermetálica em placas de circuito impresso com acabamento superficial HASL com diferentes processos de soldagem. Dissertação de Mestrado em Engenharia de Materiais, Universidade do Minho. [7] Boggs W. D. (2010) Introducing the OSP process as an alternative to HASL. Senior Process Enginner. [8] Pan J., Wang J., Shaddock M. D. (2005) Lead-free Solder Joint Reliability State of the Art and Perspectives. International Microelectronics And Packaging Society, [9] Coombs F. C. (2008) Printed Circuits Handbook. Sixth Edition. McGraw-Hill Handbooks. [10] Bath S. J. (2007) Lead-Free Soldering. Solectron Corporation. Springer Science+Business Media. [11] Ma H. and Suhling C. J.(2009) A review of mechanical properties of lead-free solders for electronic packaging. Springer Science+Business Media,

100 [12] Schueller R., Ables W., Fitch J., Dell I. (2008) A Case Study for Transitioning Class a Server Motherboards to Lead-free. Proceedings of SMTA International. [13] IPC Designers Council. Available from Acceded at 21 July [14] Chang D., Bai F. Wang P. Y., Hsiao S. C. (2004) The Study of OSP as Reliable Surface Finish of BGA Substrate. Electronics Packaging Technology Conference, [15] EPEC Build to print electronics. Available from Acedded at 6 June [16] Lee C. N. (2002) Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies. Newnes. [17] Prasad P. R. (1997) Surface Mount Technology. Springer Science +Business Media. [18] QFN Mounting Manual. Available from Acceded at 25 june [19] Strauss R. (1998) SMT Soldering Handbook. Newnes. [20] Plumbridge J. W., Matela J. R., Westwater A. (2004) Structural Integrity and Reliability in Electronics. Springer ScienceBusiness Media. [21] Alves D. (2014) CM Soldering catalogue. Reflow, Wave and Selective Solderging. Process Rules for Production. Bosch Car Multimedia, S.A. [22] Theriault M. and Blostein P. (2000) Nitrogen and soldering: reviewing the issue of inerting. The Magazine for Electronics Assembly. SMT 101. [23] Oliver K. and Puskás L. (2011) Investigating the Effect of Nitrogen Atmosphere on Lead-free Solder Wetting Angle. 17th International Symposium for Design and Technology in Electronic Packaging, [24] Kantarcioglu A. (2012) Development of new lead-free solders for electronics industry. The Degree of Master of Science in Metallurgical and Materials engineering. Middle East Technical University. 80

101 [25] Puttlitz J. K. and Stalter A. K. (2004) Handbook of Lead-Free Solder Technology for Microelectronic Assemblies. Marcel Dekker. [26] Bernasko K. P. (2012) Study of Intermetallic Compound Layer Formation, Growth and Evaluation of Shear Strength of Lead-Free Solder Joints. The Degree of Doctor of Philosophy. University of Greenwich, UK. [27] Liang J., Dariavach N., Callahan P., Shangguan D. (2006) Metallurgy and Kinetics of Liquid Solid Interfacial Reaction during Lead-Free Soldering. Materials Transactions, [28] Kim S. K., Huh H. S., Suganuma K. (2003) Effects of intermetallic compounds on properties of Sn-Ag-Cu lead-free soldered joints. Journal of Alloys and Compounds, [29] Gain K. A., Fouzder T., Chan C. Y., Yung C. K. W. (2011) Microstructure, kinetic analysis and hardness of Sn Ag Cu 1 wt% nano-zro2 composite solder on OSP-Cu pads. Journal of Alloys and Compounds, [30] Syed A., Kim S. T., Cho M. Y., Kim W. C., Yoo M. (2006) Alloying effect of Ni, Co, and Sb in SAC solder for improved drop performance of chip scale packages with Cu OSP pad finish. Electronics Packaging Technology Conference, [31] Zheng Y. (2005) Effect of surface finishes and intermetallics on the reliability of SnAgCu interconnects. The Degree of Doctor of Philosophy. University of Maryland. [32] Subramanian K. N. (2012) Lead-free Solders: Materials Reliability for Electronics. John Wiley & Sons. [33] IPC-A-610E (2010) Acceptability of Electronic Assemblies. IPC Association Connecting Electronics Industries. [34] Solder world. Available from Acedded at 29 May [35] General Information on solder paste. Available from Acedded at 29 May [36] Haines P. J. (1995) Thermal Methods of Analysis: Principles, Applications and Problems. Springer Science+Business Media. 81

102 [37] Reimer L. (1998) Sacnning Electron Microscopy Physics of Image Formation and Microanalysis. Springer. [38] Ribeiro M. (2013) Solder Paste Inspection-SPI. Process Rules for Production. Bosch Car Multimedia S.A. [39] Ribeiro M. (2014) Automatic Optical Inspection-AOI Pos-reflow, Wave and Selective Soldering. Process Rules for Production. Bosch Car Multimedia S.A. [40] Efzan E. and Singh A. (2014) Review on the effect of alloying element and nanoparticle additions on the properties of Sn-Ag-Cu solder alloys. Soldering & Surface Mount Technology, [41] Arenas F. M. and Acoff L. V. (2004) Contact Angle Measurements of Sn-Ag and Sn-Cu Lead- Free Solders on Copper Substrates. Journal of Electronic Materials,

103 ANNEX I Figure A.1 Results about SPI (area) for different components. Figure A.2 Results about SPI (volume) for different components. 83

104 Figure A.3 Results about SPI (offset X) for different components. Figure A.4 Results about SPI (offset Y) for different components. 84

105 ANNEX II Figure A.5 SEM micrograph of resistor solder joint a) and EDS element mapping corresponding Sn b), Cu for c), Ag d), Ni e) and Al f). Figure A.6 SEM micrograph of QFP solder joint a) and EDS element mapping corresponding Sn b), Cu for c), Ag d), Ni e) and Al f). 85

106 Figure A.7 SEM micrograph of QFP solder joint a) and EDS element mapping corresponding Sn b), Cu for c), Ag d), Ni e) and Al f). 86