Effect of Grain Size on Pressure-Induced Tin Whisker Formation

This paper discusses the effect of grain size on pressure-induced tin whisker formation. Since a pressure has to overcome the increase in surface energy involved with tin whisker growth, a threshold stress for tin whisker growth exists. Based on traditional nucleation theory, the threshold stress is inversely proportional to the radius of the whisker. For the verification of the proposed theory, nanoindentation tests were carried out on three kinds of finishes with different grain sizes; tin-copper, bright tin, and matte tin. Whiskers and nodules were formed at the contact edge as the applied load increases up to the critical value related to grain size. The finite-element analysis revealed that high pressure concentrates near the contact edge on the tin-copper finish. The value of pressure to whisker formation is up to 40 MPa. This value agrees with the estimation from the proposed theory with a whisker radius of 0.1 μ m. Based on the proposed theory, the difference between whiskers and hillocks can be explained.