Dependence of Sn whisker nucleation and growth on stress: real-time experiments and development of a predictive model

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NON-TECHNICAL SUMMARY:Because of its toxicity, the element lead (Pb) is being eliminated from electronics manufacturing. However, this produces a harmful side-effect in tin (Sn) coatings that are widely used to protect the conductive parts of electronic components. Removing the lead that was previously used in these coatings promotes the growth of tin whiskers. These whiskers are thin filaments that grow spontaneously out of the coating to lengths of millimeters and have caused failures in many critical systems including satellites, missiles and nuclear power plants. The goal of this project is to understand the causes and the mechanisms behind whisker growth in order to develop effective and reliable strategies to prevent them. The work focuses on the effect of stress as a driving force, much like squeezing a toothpaste tube makes the paste come out. However, the mechanisms in the solid tin layer leading to whisker growth are much more complicated than this. Well-controlled experiments are performed to quantify the relationship between the applied stress and resulting whisker formation rates. The experimental results are then used to develop mathematical models that can explain how whiskers grow and predict their formation under different conditions. Publications and presentations let these results be shared with the manufacturing community and collaborations let these approaches be applied to the development of new coatings to prevent whiskering. An outreach component of the project pairs undergraduate engineering students with local high schools in a weekly after-school program to promote skill-building and participation in technical fields. The research of Ph.D. and undergraduate students is supported with active recruitment of participants from under-represented groups. TECHNICAL SUMMARY:Sn whiskers are thin filaments that grow spontaneously from Pb-free Sn coatings on electronic components and can lead to failures in critical systems. Stress is proposed to be the driving force, but the relationship is complex and the mechanisms are not yet understood. To determine the fundamental materials processes underlying whisker formation, experiments are performed that control the stress in the layer while measuring the whiskers. In one set of projects, heating Sn layers on different substrates is used to create thermal expansion mismatch and the resulting stress is determined from the curvature in the substrate. The corresponding nucleation and growth kinetics are measured in real-time in the scanning electron microscope during heating. In other experiments, the stress is induced by applying pressure using a unique, miniaturized system that can be monitored in real-time inside the scanning electron microscope. These approaches are used to study the effect of stress on whisker kinetics in conjunction with the ambient environment (humidity and corrosive elements) and different alloy compositions (pure Sn, Sn-Ag-Cu, Sn-In, Sn-Bi, etc.). The data obtained from these experiments are used to develop and refine models of the whisker growth and nucleation process. A 2D finite element analysis model includes processes of power-law creep and stress-induced diffusion to the whisker base. Extending the model to 3-D enables the effects of grain boundary diffusion and polycrystallinity to be included. Kinetic parameters obtained from the modeling can be used to predict whiskering under different conditions. Understanding how conditions and materials change the whisker growth process guides the development of whisker-free coatings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.