Whisker Formation in Sn Coatings on Cu: Fundamental Mechanisms and Approaches to Mitigation

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This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Sn-based alloy coatings have been used for many years in the manufacture of electronic components to enhance solderability and prevent corrosion. Recent environmental legislation has forced the elimination of Pb in these parts, which has enabled the re-emergence of tin whiskers, i.e. thin filaments that grow from the surface and can ultimately cause short circuits. Whisker-induced failures have been documented in numerous systems (satellites, aviation systems, medical devices, etc.) and represent a significant risk for manufacturers of high reliability systems. Despite significant research, many fundamental aspects of whisker formation are still not understood. Part of the difficulty is that multiple materials processes interact to create the whiskers (e.g. interdiffusion, phase transformations, stress generation and relaxation, etc.) so that it is difficult to identify the underlying mechanisms. Moreover, many processing variables (film thickness, grain size, plating conditions, etc.) have been shown to play a role in whisker formation. This makes it difficult to compare the results of different studies done under different conditions. The proposed research program is designed to identify the fundamental mechanisms that govern whisker formation by focusing on the several questions: 1. Which processes control stress in the Sn layer? 2. How is the stress evolution related to whisker formation? 3. What causes whiskers to nucleate at particular sites? 4. How does the addition of Pb prevent whisker formation? 5) How can we use our understanding of whisker formation to prevent their occurrence? Real-time thin film diagnostics will be used to quantify the simultaneous temporal evolution of several critical parameters such as film stress, intermetallic phase formation and whisker density, using carefully controlled samples. These kinetic studies will be complemented by electron microscopy studies to determine the underlying atomic-scale mechanisms behind these processes as well as whisker nucleation. Microstructure, film thickness, and Sn content in Pb-Sn alloys will be varied systematically to isolate and identify different kinetic processes that contribute to whisker formation. Analytical models will be used to interpret the measurements and to develop predictive capability for assessing reliability.NON-TECHNICAL SUMMARY:The formation of Sn whiskers, i.e. thin filaments of pure tin that grow from the surface of Sn-rich lead-free alloys, is a serious concern in Pb-free electronics manufacturing because they can ultimately cause short circuits in electronic components. The proposed research program is designed to identify the fundamental mechanisms that govern whisker formation by identifying the processes that control stress in Sn-rich layers in Sn-containing alloys, learning how the stress evolves and relates to whisker formation, finding out what promotes the nucleation of whiskers and why the addition of Pb prevents whisker formation. An ultimate objective is to discover how the acquired understanding of whisker formation can be used to prevent their occurrence so that new coatings and Sn-rich alloys can be prepared with predictable reliability. An informal collaboration with the EMC Corporation will broaden the knowledge of students as well as the industrial participants. These participants will expose the students to problem solving in a real-world environment, where issues such as economics can be as important as technical issues. This work will involve the education of graduate and undergraduate students in experiments and modeling of a fundamental materials problem. Involvement of under-represented minorities and women will be enhanced by recruiting and a collaboration-student exchange with the Florida State University-Florida A&M University (an HBCU) association. K-12 educational outreach will be achieved by working with teachers to develop modules on thin film stress to be used in school visit program at Brown University. These outreach activities will be leveraged through coordination with the Brown MRSEC educational programs.