Advanced Characterization of Pb-Free Interconnects

JOM, Nov 2016

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Advanced Characterization of Pb-Free Interconnects

Advanced Characterization of Pb-Free Interconnects RICHARD COYLE 0 BABAK ARFAEI 0 0 1.-Nokia Bell Labs Reliability Engineering , Murray Hill, NJ 07974 , USA. 2.-Research and Advanced Engineering, Ford Motor Company , Dearborn, MI 48124 , USA. 3.-Physics Department, Binghamton University , Binghamton, NY 13902, USA. 4.- - Pb-free solder alloy development continues to evolve in response to complex new manufacturing and reliability requirements, particularly those demanded by increasingly aggressive operating environments. Just as second generation alloys with lower Ag content were introduced to address the shortcomings of the near-eutectic tin–silver– copper (Sn–Ag–Cu; SAC) (such as poor mechanical shock performance and higher cost), third generation SAC alloys are being developed to facilitate the dramatic increase in the number of electronic devices operating under harsh environmental conditions. In automotive and down-hole applications, control modules, sensors, and components experience high operating temperatures, rapid thermal and power cycling, all in combination with vibration and mechanical shock. More stable and more resilient Pb-free microstructures are needed to serve these needs. Additionally, these alloys are under consideration as candidates for the eventual Pb-free transition in high reliability, mission-critical avionics, military, and defense applications.1–3 The metallurgy of the expanding family of Snbased Pb-free solders is quite complex compared to traditional SnPb eutectic solder. Microstructural analysis of SnPb solder joints can typically be accomplished with lower magnification optical metallography, complemented by scanning electron microscopy to study interfacial intermetallics (IMCs). In the case of SAC solder joints, a complex relationship between alloying elements, undercooling and growth behavior results in a microstructure with significant challenges for analysis after assembly, or after thermomechanical tests. Numerous publications have demonstrated that size and distribution of precipitates in the Sn matrix, Sn dendrite arm size and spacing, Sn grain numbers and orientations, and intermetallic compounds at Babak Arfaei is the JOM advisor for the Electronic Packaging & Interconnection Materials Committee of the TMS Functional Materials Division, and guest editor for the topic Progress with Lead-Free Solders in this issue. interfaces, all affect the reliability of Pb-free interconnects.1–4 Analyzing these aspects of microstructure of Pb-free interconnects becomes more challenging for the third generation of Pb-free solders, which contain alloying additions that promote solid-solution hardening to supplement the precipitate strengthening of basic alloys. Commonly utilized solid-solution alloying elements such as Bi, Sb, and In result in Sn-based alloys with off-eutectic solidification, often with substantial melting ranges. Meaningful analysis of the as-solidified microstructures of such Pb-free solders, and their evolution in response to strain and temperature, often requires sophisticated analytical methods such as electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and x-ray tomography. Three papers have been chosen in this issue to illustrate the effectiveness of some advanced analytical techniques to better investigate the microstructure of Pb-free interconnects during solidification, after reflow and its evolution after various thermomechanical tests. The authors showed that, by using scanning electron microscopy/electron backscatter diffraction (SEM/EBSD), focused ion beam (FIB) and in situ TEM, it is possible to understand details regarding grain boundaries, geometries and misorientation angle distributions, IMC formation, stress relaxation behavior and phase transformations that were not well understood by doing conventional metallography and microstructural analysis. The following papers being published under the topic of Progress with Lead-Free Solders provide great detail and research on the subject. To download any of the papers, follow the url http://link. to the table of contents page for the November 2016 issue (vol. 68 no. 11). ‘‘Three Dimensional Characterization of Tin Crystallography and Cu6Sn5 Intermetallics in Solder Joints by Multiscale Tomography’’ A. Kirubanandham, I. Lujan-Regalado, R. Vallabhaneni, and N. Chawla We look forward to another exciting TMS meeting and also to the 13th Annual Lead-Free Solders and Interconnect Technology Workshop that will be held in conjunction with the TMS 2017 Annual Meeting & Exhibition, February 26–March 2, 2017 in San Diego, California. 1. Proceedings of Surface Mount Technology Association International Conference (Edina, MN: SMTA , 2015 ). ISBN: 978-1-5108-1371-7. 2. Proceedings of 66th Electronic Components and Technology Conference (ECTC) (Piscataway, NJ: IEEE , 2016 ). 3. Proceedings of SAE World Congress 2016 (Warrendale, PA: Society of Automotive Engineers , 2016 ). 4. Proceedings of 48th International Microelectronics Assembly and Packaging Society (IMAPS) (Research Triangle Park, NC: International Microelectronics Assembly and Packaging Society , 2015 ).

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Advanced Characterization of Pb-Free Interconnects, JOM, 2016, DOI: 10.1007/s11837-016-2108-6