Indentation-induced tin whiskers on electroplated tin coatings

The indentation-induced growth of tin whiskers/hillocks was observed for the tin coatings over copper substrate. There existed multiple tin whiskers/hillocks surrounding the indents in contrast to only a few whiskers/hillocks in the nonindented area. The number of the tin whiskers surrounding the indents increased with the increase in the indentation load and decreased with the increase in the thickness of the tin coatings. The formation and growth of the tin whiskers were due to the pressure gradient at the contact edge, which is qualitatively in accord with the analysis for the indentation of thin coatings with a power-law flow behavior.