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Crystallographic Texture and Whisker Growth in Electroplated Sn and Sn-Alloy Films
Keywords: whisker, texture, XRD
Tin whisker growth poses a continued reliability concern for lead-free electronic systems. Currently whisker growth risk of Sn films is commonly assessed with a set of test conditions as recommended by JEDEC. However these tests are time-consuming and much effort is needed for sample preparation and whisker measurements. Additionally, whisker growth data from the JEDEC testing lacks information concerning film properties and thus does not provide directions for the improvement of plating processes. One aspect of the microstructure information on Sn and Sn alloy films, the crystallographic orientation mix of grains, known as crystallographic texture, has been studied with X-ray diffraction. Published data suggest that variations in plating process parameters can produce Sn-based films with clearly different crystallographic textures, and these films can display different rates of whisker growth in accelerations tests. These data suggest that while the optimal crystallographic texture is not yet known, it may be possible to use X-ray texture data as a guideline for controlling and monitoring plating processes to reduce the tendency of films to form whiskers. In this work, a range of electrolytes were used to electroplate Sn and Sn alloy films with different compositions and microstructures on phosphor bronze and pure copper substrates. Pole figures created using x-ray diffraction were used to determine crystallographic texture of the films as a function of processing parameters, such as current density. The tendency for these films to form whiskers or other surface defects in ambient or accelerated aging conditions was categorized by surface defect density and whisker growth rate as a function of time. The relationship between film texture, film composition and whisker formation will be discussed.
Aaron Pedigo, Ph.D. Candidate
Purdue University
West Lafayette, IN

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