Effect of Substrate Composition on Whisker Growth in Sn Coatings

Abstract

Whisker growth was studied in Sn coatings deposited on three different substrates, namely pure Cu, brass (Cu-35 wt.% Zn) and pure Ni. Additionally, the effect of a Ni under-layer (electro- or sputter-deposited and placed between the Sn coating and the substrate) on whisker growth was also studied. It was observed that the substrate composition and placement of under-layers significantly affected the whisker growth in Sn coating by altering the growth rate and the morphology of the interfacial intermetallic compounds (IMC). Whisker propensity was the highest when Sn coatings were deposited directly on the brass substrate, while it was completely inhibited for at least a year when the coatings were deposited on either pure Ni or brass with a Ni under-layer. Bulk and surface stress measurements revealed that the surface of the Sn coatings on Ni, irrespective of whether it was in bulk or under-layer form, remained more compressive as compared to the bulk, throughout the observation period. Therefore, a negative out-of-plane stress gradient, which is crucial for whisker growth, could never be established in these samples. Interestingly, a phenomenon of through-thickness columnar voiding (reverse of whiskering) was observed in the Sn coatings deposited on Ni. The origin of this phenomenon is discussed.