すずおよびすず-鉛めっき皮膜におけるウィスカ発生•抑制機構

Generation and growth of whiskers and nodules from electroplated tin and tin-lead films on copper or nickel substrate were studied by scanning electron microscopy and X-ray diffraction. In the case of copper substrate, whiskers generated in 0.3 Ms on the electroplated tin film whose thickness was 1 μm. On the other hand, electroplated tin-lead film on copper substrate showed only nodules even after 13 Ms. Residual stress of electroplated 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 substrate of copper. 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 electroplated film on copper substrate. When the electroplated 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 electroplated tin film on 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.