Development of lead free nanocomposite solders for electronic packaging

DEVELOPMENT OF LEAD-FREE NANOCOMPOSITE SOLDERS FOR ELECTRONIC PACKAGING PAYMAN BABAGHORBANI (B.Sc., University of Tehran) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 Name: Payman Babaghorbani Degree: Master of Engineering (M.Eng.) Department: Mechanical Engineering (Materials Science Division) Thesis Title: Development of Lead-Free Electronic Packaging Nanocomposite Solders for Abstract To synthesize new lead-free nanocomposite solders non-coarsening reinforcements (Cu and SnO2) were successfully incorporated into Sn96.5- Ag 3.5 Characterization results convincingly established that composite technology in electronic solders can lead to simultaneous improvement in thermal performance (in terms of lower coefficient of thermal expansion) and mechanical performance (in terms of better microhardess and tensile properties) A threshold addition of reinforcements was observed to aid in optimizing the properties of the composite solder Composite solders reinforced with nano-size copper particulates yielded the best overall properties These advanced interconnect materials have the potential to benefit the microelectronics packaging and assembly industry Keywords: Lead-free solder; composite solder; nano copper particulates; mechanical properties PREFACE This thesis is submitted for the degree of Master of Engineering in Mechanical Engineering at the National University of Singapore The research described herein was conducted under the supervision of Associate Professor Manoj Gupta from the Materials Science Division, Department of Mechanical Engineering, National University of Singapore (NUS), between August 2006 and July 2008 This work is to the best of my knowledge original, except where acknowledgements and references are made to previous work Neither this, nor any substantially similar thesis has been or is being submitted for any other degrees or other qualification at any other university This thesis contains no more than 40,000 words Development of Lead-Free Nanocomposite Solders for Electronic Packaging i ACKNOWLEDGEMENTS I would like to take this opportunity to express my heartiest gratitude to the following people for their invaluable help rendered during my candidature as a M.Eng student at the Department of Mechanical Engineering, National University of Singapore First of all, I would like to express my best and sincere thanks to my supervisor, Associate Professor Manoj Gupta for his invaluable advice, encouragement and patience throughout this research work I am deeply indebted to the Agency for Science, Technology and Research (A*STAR) for the award of IGS research scholarship I would also like to express my appreciation to Mr Thomas Tan Bah Chee, Mdm Zhong Xiang Li, Mr Maung Aye Thein, Mr Ng Hong Wei, Mr Abdul Khalim Bin Abdul and Mr Juraimi Bin Madon from the Materials Science Laboratory, for their advice and help rendered Many thanks also to my friends and fellow course mates, especially Dr Nai Mui Ling Sharon and Dr Eugene Wong for their friendship and advice Most importantly, I am eternally grateful to my parents and sister for their continuous support and encouragement throughout my candidature Payman Babaghorbani July 2008 Development of Lead-Free Nanocomposite Solders for Electronic Packaging ii To my mother, father and sister Who always give me their unconditional support Development of Lead-Free Nanocomposite Solders for Electronic Packaging iii TABLE OF CONTENTS PREFACE i ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iv SUMMARY viii LIST OF FIGURES x LIST OF TABLES xiii PUBLICATIONS xiv CHAPTER INTRODUCTION 1.1 Organization of Thesis References CHAPTER LITERATURE SURVEY 2.1 Introduction 2.2 Lead-Bearing Solders 2.3.2 2.3 Health and Environmental Concerns Lead-Free Solders 2.3.2 Sn-Ag Lead-Free Solder 9 2.4 Key Properties of Solders 10 2.5 Composite Solders 10 2.6 Powder Metallurgy Technique 11 2.6.1 Introduction to Powder Metallurgy Development of Lead-Free Nanocomposite Solders for Electronic Packaging 11 iv 2.6.2 Reasons for Using Powder Metallurgy 12 2.6.3 The Future of Powder Metallurgy 14 2.6.4 Microwave vs Conventional Sintering 15 2.6.4.1 Penetrating Radiation 16 2.6.4.2 Rapid Heating 17 2.6.4.3 Controllable Field Distribution 18 2.6.4.4 Selective Heating of Materials 18 2.6.4.5 Self-Limiting Characterization 19 2.7 Existing Work on the Development of Composite Solders 19 2.8 Selection of Materials for Investigation 20 2.8.1 Solder Matrix Material 20 2.8.2 Reinforcement Materials 21 2.9 Applications 21 References 25 CHAPTER MATERIALS AND EXPERIMENTAL PROCEDURES 32 3.1 Introduction 32 3.2 Experimental Work Overview 32 3.3 Materials 33 3.4 Processing 33 3.4.1 Synthesis of Monolithic and Composite Solders 33 3.5 Density Measurements 37 3.6 Thermomechanical Analysis 38 3.7 Microstructure Characterization 38 Development of Lead-Free Nanocomposite Solders for Electronic Packaging v 3.8 X-Ray Diffraction Analysis 38 3.9 Mechanical Characterization 39 3.9.1 Microhardness Tests 39 3.9.2 Tensile Tests 39 3.10 Fractography 39 References 40 CHAPTER ENHANCING THE MECHANICAL RESPONSE OF A LEADFREE SOLDER USING AN ENERGY EFFICIENT MICROWAVE SINTERING ROUTE 41 4.1 Objective 41 4.2 Results and Discussion 41 4.2.1 Macrostructure 41 4.2.2 Densification Behavior 42 4.2.3 Microstructural Characterization 43 4.2.4 Tensile Properties Characterization 49 4.3 Conclusions 51 References 51 CHAPTER INTEGRATING COPPER AT THE NANOMETER LENGTH SCALE WITH Sn-3.5Ag SOLDER TO DEVELOP HIGH PERFORMANCE NANOCOMPOSITES 53 5.1 Objective 53 5.2 Results and Discussion 54 5.2.1 Synthesis of Monolithic Sn-3.5Ag and Sn-3.5Ag/Cu Composites 54 5.2.2 Density Measurements 54 Development of Lead-Free Nanocomposite Solders for Electronic Packaging vi 5.2.3 Microstructure Characterization 55 5.2.4 Coefficient of Thermal Expansion 58 5.2.5 Mechanical Behavior 58 5.2.6 Fracture Behavior 61 5.3 Conclusions 62 References 62 CHAPTER DEVELOPMENT OF LEAD-FREE Sn-3.5Ag/SnO2 NANOCOMPOSITE SOLDERS 65 6.1 Objective 65 6.2 Results and Discussion 66 6.2.1 Synthesis of Monolithic Sn-3.5Ag and Sn-3.5Ag/SnO2 Nanocomposites 66 6.2.2 Density Measurements 66 6.2.3 Microstructure Characterization 67 6.2.4 Mechanical Behavior 70 6.3 Conclusions 73 References 73 CHAPTER OVERALL CONCLUSIONS 77 CHAPTER RECOMMENDATIONS 79 APPENDIX A APPENDIX B APPENDIX C Development of Lead-Free Nanocomposite Solders for Electronic Packaging vii Summary Tin-lead (Sn-Pb) solders have been widely utilized in the electronics industry in the past few decades due to their unique properties such as low melting point, ability to wet substrate and good mechanical properties However, in recent years, increasing environmental and health concerns over the use of toxic Pb, coupled with strict legislations on the ban of Pb usage in consumer electronics has been the driving force in the development of new lead-free solder alloys The primary focus is to develop a new generation of interconnect materials that is equipped with a combination of good mechanical, electrical and thermal properties, in order to fulfill the ever-stricter service requirements In this project, a new generation of lead-free (Sn96.5-Ag3.5) composite solders was developed to address the above-mentioned issues Composite approach was used to improve the service performance of conventional solders In the first stage of this study, processing methodology was optimized Afterwards, three new lead-free composite solders were successfully synthesized using the powder metallurgy method incorporating microwave sintering route Non-coarsening reinforcements (nano-size Cu and SnO2 particulates) were intentionally incorporated into the solder matrix Characterization studies were then carried out to determine the physical, thermal, microstructural and mechanical properties of the composite solders Composite solders containing nano copper particulates were found to yield the best overall properties Characterization results in this project convincingly established that composite technology in electronic solders can lead to simultaneous improvement in thermal Development of Lead-Free Nanocomposite Solders for Electronic Packaging viii Development of Lead-Free Sn-3.5Ag/SnO2 Nanocomposite Solders and 31% higher 0.2% YS and UTS, respectively over monolithic samples and 98% and 118% higher 0.2% YS and UTS (without any change in total failure strain), respectively over published values by QualitekR (see Table 6.2) The improvement in tensile strengths in the case of Sn-3.5Ag reinforced with 0.5 and 0.7 volume percent SnO2 nanoparticulates can be mainly attributed to: (a) Orowan strengthening mechanism ( Δσ Orowan ) [35, 36, 37] due to presence of nano-size tin oxide particulates in the solder matrix, (b) progressive increase in dislocation density due to coefficient of thermal expansion (CTE) mismatch ( Δσ CTE ) [34] between Sn-3.5Ag and SnO2 particulates (3.76 × 10-6/ ºC and 26.4 × 10-6/ ºC for SnO2 [38] and Sn-3.5Ag, respectively) and (c) elastic modulus (EM) mismatch ( Δσ EM ) [34] between matrix and reinforcement (233 GPa for SnO2 [38] compared with 10 GPa for Sn-3.5Ag) The strength of a reinforced matrix can be defined by [34]: σ my = σ mo + Δσ (6.1) Where σmy and σmo are the yield strength of the reinforced and unreinforced matrix, respectively Δσ represents the total increment in yield stress of the reinforced Sn-3.5Ag matrix and is estimated by [39]: Δσ = (Δσ Orowan )2 + (Δσ CTE )2 + (Δσ EM )2 (6.2) However, the results also showed a decline in tensile characteristics of Sn-3.5Ag when the amount of SnO2 was increased from 0.7 to volume percent and this can be attributed to the highest porosity level as well as highest pores aspect ratio compared to other materials (see Table 6.1 and Fig 6.4) The higher pores aspect ratio in Sn-3.5Ag/1 SnO2 means presence of sharp edge pores in the microstructure (see Fig 6.4) convincingly Development of Lead-Free Nanocomposite Solders for Electronic Packaging 71 Development of Lead-Free Sn-3.5Ag/SnO2 Nanocomposite Solders justifying lower tensile properties in this material [40] In a recent study, Alam et al [41] have shown that increasing the pores aspect ratio in pure tin led to decrease in strength consistent with the observation made in this study The ability of porosity to serve as a crack initiation site and to adversely affect strength has been established convincingly for both monolithic and composite materials [28, 42, 43] Moreover, it has been reported by other researchers that amount of reinforcement should not exceed the certain amount otherwise the properties will be affected adversely [29-31] (a) (b) (c) Figure 6.4 Representative FESEM micrographs showing pores morphology of: (a) Sn-3.5Ag, (b) Sn-3.5Ag/0.7 SnO2 and (c) Sn-3.5Ag/1 SnO2 Development of Lead-Free Nanocomposite Solders for Electronic Packaging 72 Development of Lead-Free Sn-3.5Ag/SnO2 Nanocomposite Solders Total failure strain (TFS) of all composite samples was found to decrease as the amount of reinforcement increased in the solder matrix (see Table 6.2) This can be attributed to the presence of harder reinforcing phase serving as crack nucleation sites, resulting in a decrease in TFS under tensile loading conditions This observation is consistent with the observation made by other researchers working on the similar and other metal based composite systems [30, 34, 44, 45] 6.3 Conclusions The following conclusions can be drawn from the experimental findings of this part of study: Sn-3.5Ag composites containing SnO2 particulates at nanometer length scale can be successfully synthesized using powder metallurgy technique incorporating hybrid microwave sintering route Microstructural characterization revealed that secondary phase (Ag3Sn) and nanosize SnO2 particulates were uniformly dispersed throughout the matrix The best overall combination in mechanical properties was achieved in Sn-3.5Ag reinforced with 0.7 volume percent of SnO2 particulates at nanometer length scale Improvement of 18% in 0.2%YS and 31% in UTS over monolithic Sn-3.5Ag was observed References R Darveaux, K L Murty, and I Turlik, J Miner., Met Mater Soc 44, 36 (1992) K L Murty and I Turlik, Proc Joint 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Packaging 76 General Conclusions Chapter Overall Conclusions Pure Sn-3.5Ag Pure Sn-3.5Ag solder can be successfully synthesized using microwave assisted powder metallurgy route Highest density, the best pore characteristics (size and roundness), minimum grain size and distribution and minimum interparticle spacing of secondary phase (Ag3Sn) can be realized in Sn-3.5Ag solder using microwave assisted powder metallurgy route Sn-3.5Ag solder samples synthesized using microwave assisted powder metallurgy route exhibited the best combination of 0.2% yield strength, ultimate tensile strength and work of fracture when compared to unsintered and conventional sintering routes Sn-3.5Ag/Copper Nanocomposite System Powder metallurgy technique incorporating rapid microwave sintering can be successfully used to synthesize Sn-3.5Ag composites containing nano-size copper particulates A marginal reduction in the average CTE values of the Sn-3.5Ag matrix with increasing volume fraction of reinforcement was realized The amount of nano-size copper particles should exceed volume percent to realize an improvement in microhardness, 0.2%YS and UTS Development of Lead-Free Nanocomposite Solders for Electronic Packaging 77 General Conclusions Uniform distribution of nano-size copper particles in the lead-free solder matrix can be achieved through judicious selection of primary and secondary processing parameters Sn-3.5Ag lead-free solder reinforced with 2.5 volume percent copper particles at the nanometer length scale exhibited best overall hardness and strength characteristics Sn-3.5Ag/SnO2 Nanocomposite System Sn-3.5Ag composites containing SnO2 particulates at nanometer length scale can be successfully synthesized using powder metallurgy technique incorporating hybrid microwave sintering route Secondary phase (Ag3Sn) and nano-size SnO2 particulates were uniformly dispersed throughout the matrix The best overall combination in mechanical properties was achieved in Sn-3.5Ag reinforced with 0.7 volume percent of SnO2 particulates at nanometer length scale Development of Lead-Free Nanocomposite Solders for Electronic Packaging 78 Recommendations Chapter Recommendations This research work has showed the potential of using composite technology in electronic solders Although the technology to produce such composite solders is established, in order to use these solders in large-scale manufacturing environment, further studies as suggested below can be conducted Fatigue tests and drop tests can be conducted to further assess the performance of the composite solders Although composite solders powder mixture can be mechanically mixed with flux to form the composite solder paste, atomization techniques can be developed to convert the composite solders directly into a powder form to make the paste Methodology for TEM sample preparation can be developed to study the microstructure of such composite solders The most recent bonding techniques (solid-state bonding) such as thermal compression, surface activated, thermosonic and ultrasonic bonding can be performed using the developed monolithic and composite materials to investigate the properties and microstructure of the joints made of composite solders Development of Lead-Free Nanocomposite Solders for Electronic Packaging 79 Appendix A Appendix A Copper (Cu) Purity: 99+% APS: 50 nm Morphology: Spherical True Density: 8.93 g/cm3 Melting Point: 1083 °C Boiling Point: 2595 °C Mohs Hardness @ 20 °C 3.0 Crystal Structure Cubic, Face Centered Development of Lead-Free Nanocomposite Solders for Electronic Packaging A-1 Appendix B Appendix B Tin Oxide (SnO2) Purity: 99.5% APS: 61 nm Color: White Morphology: Faceted Bulk Density: 0.95 g/cm3 True Density: 6.95 g/cm3 Melting Point: 1630 °C Price: $78/100g $175/500g $240/1kg Development of Lead-Free Nanocomposite Solders for Electronic Packaging B-1 Appendix C Appendix C t-Test After density measurements, it seems that the density of microwave-sintered, conventionally sintered and unsintered samples are almost the same and not differ Hence, two-sample t-test (statistical test) at the 95% confidence level was conducted After running t-test in Microsoft Office Excel, some values will be given Among these values, p-value is the most important to determine that these groups not differ or they If the calculated p-value is less than the threshold chosen for statistical significance (0.05 in our study), then the null hypothesis which usually states that the two groups not differ is rejected in favor of an alternative hypothesis, which typically states that the groups differ According to the t-test results shown in Tables 1, and 3, the p-value calculated is below the threshold chosen for statistical significance, the 0.05 level Therefore, the null hypothesis that the two groups not differ is rejected and the groups differ Table Two-Sample Assuming Unequal Variances to compare MSint & USint Mean Variance Observations Hypothesized Mean Difference df t Stat P(T