When 10:30 AM - 11:30 AM Nov 04, 2016
Where 1571 G.G. Brown
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Heterogeneous Stress Relaxation in Tin Thin Films: Whiskers, Hillocks, and Beyond

Carol Handwerker
School of Materials Engineering, Purdue University

When tin thin films are stressed, the film microstructures can become unstable, leading to the formation of long single crystal whiskers that can create short circuits in electronic circuits and destroy MEMS devices in electronic assemblies. The conditions for whisker formation are inherently local as indicted by their frequency: there can be typically 1 whisker for every 103 -106 film grains. Stress relaxation occurs by diffusion to specific grain boundaries in the plane of the film, leading to an out-of-plane relaxation, with hillocks being formed when grain boundary migration accompanies growth out of the plane of the film and whiskers in the absence of grain boundary migration. In this seminar, I will discuss the microstructural characteristics for specific grains to form whiskers in β-Sn films in light of our recent whisker growth model establishing a relationship between grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling and cyclic bending. Our recent finite element simulations of stresses induced by room temperature aging and thermal cycling of textured microstructures also provide insight into the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites. Taken together with experiments on films with different textures and under different conditions of stress, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.

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