Sn whiskers: fundamental mechanisms of nucleation and growth and applications to mitigation

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NON-TECHNICAL SUMMARY:Tin whiskers are long filaments that grow spontaneously out of coatings and solders used in electronics manufacturing. They are a critical reliability problem because they create short circuits that have led to satellite crashes, a nuclear power plant shutdown and a medical pacemaker recall, among many other failures. The risk from whiskers has become worse because the element lead (Pb) has been restricted from manufacturing for environmental reasons. In the past, addition of this element helped to suppress whisker formation. The research is focused on understanding the causes of whisker formation as well as developing and testing methods for suppressing them. The work uses a unique system that enables the surface of the coating to be monitored for whisker formation at the same time as the stress in the coating is measured. Since stress is believed to be the reason for whisker formation, this allows the connection between stress and whiskering to be directly measured. In addition to fundamental studies, there are collaborations with industrial partners to study issues related to whisker prevention. In science and technology education, a major problem has been the retention of first generation and underserved students in these fields. The principal investigator is improving this through team-based mentoring efforts to support new students and exposing them to laboratory research early in their academic careers.TECHNICAL SUMMARY:Although stress is believed to be the driving force for whisker formation, it is not understood why whiskers form from specific sites (nucleation) and what controls their volume evolution (growth). This research builds on the development of a unique system to simultaneously measure the whisker density and the stress (induced by thermal expansion when the sample is heated). Measurements show how the stress changes the nucleation rate and how much it decreases the barrier to nucleation. These studies further suggest a potential nucleation mechanism that is based on stress-induced grain boundary migration. In situ transmission and scanning electron microscope studies that monitor the grain structure under stress can capture the nucleation process to explore this mechanism further. Electron microscopy measurements of the whisker volume evolution show that the growth rate is proportional to the average stress, as long as it is above a critical threshold value. Calculations are being developed that use finite element methods to model the balance between stress relaxation due to rate-dependent plasticity and stress-driven diffusion. These model predictions can be compared directly with the growth measurements to refine the model parameters and allow prediction of whisker growth rates. In collaboration with industrial researchers, the experimental capabilities are being extended to measure the effect of corrosion/humidity on whisker nucleation and growth. A new technique for using mechanical pressure to induce whiskers is being developed to study new solder formulations to determine their ability to suppress whiskering.