Stress evolution and whisker growth during thermal cycling of Sn films: A comparison of analytical modeling and experiments

Sn whiskers are a re-emerging reliability issue for Pb-free manufacturing. Although stress is considered as the most likely driving force for whisker formation, there is little quantitative understanding of how stress and whisker growth are correlated. To address this issue, we have developed an analytical model which accounts for strain generation (via thermal expansion mismatch) and multiple relaxation processes (rate-dependent plasticity and diffusion-controlled whisker growth). This model is used to analyze the results of thermal cycling experiments in which the evolution of stress and average volume of whiskers/hillocks were tracked as the features grew on the Sn surface. Predictions of the model are found to be in a good agreement with the time-dependent whisker growth and stress relaxation behaviors that have been observed in the experimental measurements.

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