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In Situ Electromigration and Reliability of Pb-Free Solders at Extremely In situ Electromigration and Reliability of Pb-Free Solders At Extremely Small Length Scales by Antony Kirubanandham A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Approved November 2016 by the Graduate Supervisory Committee: Nikhilesh Chawla, Chair Yang Jiao Minhua Lu Jagannathan Rajagopalan ARIZONA STATE UNIVERSITY December 2016 ABSTRACT Over the past several years, the density of integrated circuits has been increasing at a very fast rate, following Moore’s law. The advent of three dimensional (3D) packaging technologies enable the increase in density of integrated circuits without necessarily shrinking the dimensions of the device. Under such constraints, the solder volume necessary to join the various layers of the package is also extremely small. At smaller length scales, the local cooling rates are higher, so the microstructures are much finer than that obtained in larger joints (BGA, C4). The fraction of intermetallic compounds (IMCs) present in solder joints in these volumes will be larger. The Cu6Sn5 precipitate size and spacing, and Sn grain structure and crystallography will be different at very small volumes. These factors will most certainly affect the performance of the solder. Examining the mechanical behavior and reliability of Pb-free solders is difficult, primarily because a methodology to characterize the microstructure and the mechanics of deformation at these extremely small length scales has yet to be developed. In this study, Sn grain orientation and Cu6Sn5 IMC fraction, size, and morphology are characterized in 3D, in pure Sn based solder joints. The obtained results show differences in morphology of Sn grains and IMC precipitates as a function of location within the solder joint indicating influence of local cooling rate differences. Ex situ and in situ electromigration tests done on 250 m and 500 m pure Sn solder joints elucidate the evolution of microstructure, specifically Sn grain growth, IMC segregation and surface degradation. This research implements 3D quantification of microstructural features over micro and nano-scales, thereby enabling a multi-scale / multi-characterization approach. i DEDICATION To my parents, Simon Kirubanandham and Josephine Flora Mary, who have inspired and encouraged me to dream big throughout my life. ii ACKNOWLEDGMENTS I would like to first acknowledge my advisor, Dr. Nikhilesh Chawla, for providing me the opportunity to work on this project. His constant guidance, support, and encouragement throughout the past few years has trained me to do high quality research and given me the tools to achieve my Ph.D. I would like express my gratitude toward my committee members, Dr. Jagannathan Rajagopalan, Dr. Yang Jiao, and Dr. Minhua Lu for taking their time in evaluating my doctoral research. I would like to acknowledge the financial support from the Semiconductor Research Corporation (SRC) through tasks #1292.084/.084, research direction from the SRC review panel, and technical guidance from the Task’s industrial liaisons, including Minhua Lu (IBM), Fay Hua (Intel), and Brett Wilkerson (Freescale Semiconductor). I would like to especially thank Research Scientist Dr. Jason Williams, who has been of great help with technical challenges in the lab and in my research. I would also like to also acknowledge my colleagues and seniors in Professor Chawla’s Research Group, especially Sudhanshu Singh and James Mertens for all the technical discussions and collaborations. And finally, I would like to thank my family, my girlfriend Irene, and my friends for believing in me and constantly motivating me towards achieving bigger goals in life. iii TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................................. vii LIST OF FIGURES .............................................................................................................. viii CHAPTER 1. INTRODUCTION ........................................................................................................ 1 2. LITERATURE REVIEW ........................................................................................... 6 Solder Joints Types in Microelectronics Packages ............................................... 7 Microstructural Characterization of Pure Sn Lead-Free Solder Joint .................. 8 Volume Effect On Microstructure of a Pure Sn Lead-Free Solder Joint .......... 12 Electromigration in Lead-Free Solder Joints ....................................................... 13 Fundamentals of Electromigration ...................................................................... 15 Thermal Effects of Electromigration ................................................................... 16 Influence of Thermal Cycling .............................................................................. 17 3. RESEARCH OBJECTIVES AND APPROACH .................................................... 18 Investigation of Sn Grain Orientation in 3D ....................................................... 19 Investigation of the Cu6Sn5 IMC Distribution .................................................... 25 Study of the Volume Effect ................................................................................. 25 In Situ Electromigration Testing Jig .................................................................... 26 iv CHAPTER Page 4. 3D CHARACTERIZATION OF TIN CRYSTALLOGRAPHY AND CU6SN5 INTERMETALLICS IN SOLDER JOINTS BY MULTISCALE TOMOGRAPHY.. ...................................................................................................... 32 Introduction .......................................................................................................... 32 Materials and Experimental Procedure................................................................ 35 Results and Discussion ......................................................................................... 39 3D Sn grain orientation ........................................................................................ 39 Investigation of Cu6Sn5 IMCs …………………………………………….....48 Conclusions ......................................................................................................... 55 5. EX SITU CHARACTERIZATION OF ELECTROMIGRATION DAMAGE IN PURE SN BASED SOLDER JOINTS ...................................................................... 56 Introduction .......................................................................................................... 56 Materials and Experimental Procedure................................................................ 57 Results and Discussion ......................................................................................... 65 Summary ............................................................................................................ 78 6. AN IN SITU STUDY ON THE INFLUENCE OF TEMPERATURE ON FAILURE MECHANISMS IN PURE SN BASED SOLDER JOINTS .................. 79 Introduction .......................................................................................................... 79 Materials and Experimental Procedure................................................................ 80 Results and Discussion ......................................................................................... 83 Interrupted Aging Experiment ............................................................................. 83 Uninterrupted Aging Experiment ........................................................................ 86 v CHAPTER Page In situ Thermal Cycling Behavior ....................................................................... 89 Summary............................................................................................................... 96 7. CONCLUSIONS ........................................................................................................ 97 Summary of Research Findings ........................................................................... 97 Future Work ......................................................................................................... 98 REFERENCES ..................................................................................................................... 100 vi LIST OF TABLES Table Page 1. 2D Quantification Results of the Cu6Sn5 Nanoparticles ........................................... ...49 2. 3D Quantification Results of the Cu6Sn5 IMC Needles ............................................... 51 3. Parameters Used for Thermal Cycling Test ................................................................. 89 4. Thermal Expansion Coefficients for the Different Phases in Pure Sn Solder Joints...92 vii LIST OF FIGURES Figure Page 1. A) SEM Image of Cross-Sections of the 250M Solder Joint Used in This Study. B) High Magnification Image Showing Distribution of Fine Cu6Sn5 Precipitates Throughout the Solder Volume……………………………………………………...20 2. Silicon V-Groove Stage Used To Prepare Solder Joints A) Schematic Of Setup And B) Optical Image Of Silicon Stage Taped On To A Copper Block C) Flowchart Describing The Preparation Method For The Multi-Groove Silicon Wafer Stage…..21 3. Reflow Profile Followed For Fabricating Solder Joints For This Study………………22 4. A) Optical Images of Fiducial Marks, Which Helps Monitor the Polishing Rate. B) Plot Showing Standardized Polishing Rate as Thickness Loss Vs Polishing Cycle. C) Vickers Indent Geometry Using Which the Change In Depth is Calculated………..24
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