Understanding the Correlation Between Intermetallic Growth, Stress Evolution, and Sn Whisker Nucleation

Stress due to intermetallic (IMC) growth is generally accepted as the driving force for Sn whisker formation, but there are still many unanswered questions regarding the development of stress and how it relates to the growth of whiskers. We have made simultaneous measurements of the evolution of stress, IMC volume, and whisker density on samples of different thicknesses to address the underlying mechanisms of whisker formation. Finite-element simulations are used to study the stress evolution due to IMC growth with various stress relaxation mechanisms: plastic deformation coupled with grain boundary diffusion is found to explain observed stress levels, even in the absence of whisker growth. A model of whisker growth suggests that the average steady-state stress is determined primarily by relaxation processes (dislocation- and diffusion-mediated) and that whisker growth is not the primary stress relaxation mechanism. Implications of our results for whisker mitigation strategies are discussed.