すずめっき皮膜からのウィスカ発生・成長機構

抄録

  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 oxide film 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 the substrate of copper, that of the specimen whose thickness was 10 μm showed no meaningful change and no whiskers were found. The orientation of a whisker was shown to be identical throughout the crystal even in a kinked one by electron backscattering diffraction. 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 its resulting increase in volume of the grain which received the tin atoms. Energy change in the growth of whisker was modelled and calculated, and the main factor which determines the growth was shown to be the formation enthalpy of a vacancy and the entropy of configuration.