Understanding the Fundamental Processes of Whisker Nucleation and Growth in Sn-based Coatings

Pei, Fei

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Year:: 2015
Contributor:: Pei, Fei (creator); Chason, Eric (Director); Bower, Allan (Reader); Sheldon, Brian (Reader); Brown University. ENGINEERING: Materials Science (sponsor)
Genre:: theses
Subject:: tin whiskers; Pb-free manufacturing
Extent:: 20, 158 p.
DOI: https://doi.org/10.7301/Z07D2SJT

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Abstract:: Whiskers are conductive filaments that spontaneously form on Sn-based coatings. They are a critical reliability issue as they can cause short circuits in electronic systems. Although compressive stress is believed to be the primary driving force for whisker growth, there is not yet a comprehensive quantitative understanding of how and why they form in response to the stress. In this context, we have performed experiments to address two outstanding questions: What leads to the formation of whiskers at specific sites on the Sn surface? How do whisker nucleation and growth depend on the stress in Sn? Systematic studies were performed to find out what is special about the grains that form into whiskers. We used scanning electron microscopy and electron backscatter diffraction to determine which grain orientations form whiskers. The sub-surface structure of whisker grains was studied from cross-sectional measurements. We also examined the interface structure after the Sn layer was selectively removed to correlate surface deformation with underlying intermetallic particles. To understand how whisker formation depends on stress, we used the well-controlled strain induced by thermal-expansion mismatch to study the whiskering behavior of Sn electrodeposits in real-time. A custom experimental system was used to monitor the stress and the whisker density simultaneously. Analysis of the results provides a quantitative estimate of how the nucleation rate changes with the applied stress. A model based on classical nucleation theory was developed to explain the observed formation kinetics. To investigate the growth kinetics, we measured the volume evolution of individual whiskers with scanning electron microscopy after intervals of heating. The results enabled us to determine the temperature- and stress-dependent parameters that control whisker growth and to quantify the strain relaxation induced by whiskers. For a fundamental understanding of the essential features observed in the thermal experiments, a finite element model was used to investigate the connection between stress evolution and whisker growth in the Sn films subjected to thermal straining. Real-time synchrotron X-ray micro-diffraction experiments were also conducted to examine how local microstructure and strain/stress evolve before and during whisker formation.
Notes:: Thesis (Ph.D. -- Brown University (2015)

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Pei, Fei, "Understanding the Fundamental Processes of Whisker Nucleation and Growth in Sn-based Coatings" (2015). Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z07D2SJT

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Materials Science Engineering Theses and Dissertations

Theses and Dissertations for the Materials Science Engineering department.
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