An assessment of Sn whiskers and depleted area formation in thin Sn films using quantitative image analysis

An experimental technique was developed to determine the extent of Sn whisker growth and depleted area formation on evaporated 1 μm tin (Sn) films. Deformation of the Si substrate placed a controlled magnitude of compressive or tensile stress across the films. Quantitative image analysis was used to monitor whisker growth and size of the depleted areas. The test conditions were: stresses 10–40 MPa; temperature, 180 °C; and time durations, 1–8 weeks. The whisker length increased with compressive stress. The whiskers appeared within the first week, but then did not grow significantly with additional time. Some whiskers were located in the centers of depleted areas. The depleted areas size was not sensitive to the applied stress, but did increase with annealing time. Both Sn whiskers and depleted areas were the result of potentially similar rapid, long-range diffusion processes. However, differing trends suggested that separate driving forces and/or rate kinetics controlled the two phenomena.