A Predictive Model for Whisker Formation Based on Local Microstructure and Grain Boundary Properties

Abstract

Whisker and hillock formation in thin films is well known as a highly local mechanism for stress relaxation, where in many cases, only a few whiskers form out of thousands of grains in a film. In this article, the microstructural characteristics for specific grains to form whiskers in β-Sn films are discussed in light of our recent whisker growth model, establishing a relationship among grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling. Through our recent finite-element simulations of stresses induced by room-temperature aging and thermal cycling of textured microstructures, the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites and the general propensity of a film to form whiskers have been proposed for a range of β-Sn film textures. Taken together, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, then whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.