Mechanism of generation and growth of whiskers on tin electroplating

Abstract

Generation and growth of whiskers on tin electroplating was studied by scanning electron microscopy and X-ray diffraction. When the surface of the plating, whose thickness was 1 μm, was etched by argon ion beam to remove native oxide scale and kept in a vacuum, whiskers were rarely observed but many nodules were formed. While residual stress of tin plating (1 μm) and the number of whiskers increased with the amount of Cu6Sn5 formed between the tin plating and copper substrate, the 10 um thickness specimen showed no meaningful change and no whiskers were found. The orientation of whiskers was shown to be identical throughout a crystal by electron backscattering diffraction even when kinked. The mechanism of generation and growth of whiskers is considered to be due to diffusion of tin atoms induced by inhomogeneous strain field, formation of vacancies, and the resulting increase in volume of the grain receiving the tin atoms. Energy change in the growth of whiskers was modelled and calculated, and the main factor determining the growth was found to be the formation enthalpy of a vacancy and the entropy of configuration. © 2008 The Japan Institute of Metals.