Mechanism of Generation and Suppression of Tin Whiskers on Tin and Tin-Lead Plated Films

Abstract

Generation and growth of whiskers and nodules from electroplated tin and tin-lead films on copper or nickel substrates were studied by scanning electron microscopy and X-ray diffraction. In the case of copper substrates, whiskers were formed in 0.3 Ms on the tin film whose thickness was 1 μm. On the other hand, tin-lead films on copper substrates showed only nodules even after 13 Ms. Residual stress of the tin film (1 μm) and the number of whiskers increased with the amount of copper-tin intermetallic compounds (Cu₆Sn₅) which developed between the plated film and the copper substrate. Although residual stress and the amount of Cu₆Sn₅ also increased in the tin-lead system, the morphology of the layer of Cu₆Sn₅ was more uniform compared with the case of tin films on copper substrates. When the tin-lead film was subjected to the compression testing by a ball of zirconium oxide (1 mm-diameter, 2.94 N–0.605 Ms), the edge of the indentation did not show any whiskers but diffusion of lead was observed from right under the zirconium oxide ball to the fringe, as well as Ostwald growth of lead. In the case of the tin film on the nickel substrate, whose residual stress was weakly tensional, nickel-tin intermetallic compounds (Ni₃Sn, Ni₃Sn₂, Ni₃Sn₄) uniformly developed immediately after electroplating, and no whiskers were observed even after the compression testing. While growth of whiskers is considered to be due to diffusion of tin atoms induced by inhomogeneous strain field in the electroplated film, lead atoms in tin-lead system is considered to diffuse rapidly toward the free surface to release residual stress and to generate many nodules.