無電解 Sn めっき膜上に形成されるウィスカ発生に対する Sn めっき膜の結晶粒径と基板の結晶配向性の影響

Abstract

In this study, we studied how the orientation indexes of the Cu foil, which is used for a substrate, and the crystal grain size of the Sn deposited film affects tin whisker formation and the structure of the intermetallic compound deposits. In particular, we considered the relationship between the crystal grain size of Sn deposited films and the amount of intermetallic compound deposits that is formed at the interface between Sn deposited films with aging and Cu foils. We used two types of Cu foils for a substrate. One is “with gelatin additive” that has granular-shaped grains which are 0.5~1 μm in size and the other is “with Cl- ion additive” that has pillar-shaped grains and their nodules. In addition to this, we used Cu (100), (110), (111) single crystals for a substrate. We used two types of Sn plating bath for plating. One is hydrofluoroboric acid bath and another is organic acid bath. The structure of tin whiskers, tin films and Cu foils were investigated by TEM and SEM. The number of the whiskers that formed on the Sn deposited film increased after aging. And the number of the whiskers that formed on the Sn films, which were made by using hydrofluoroboric acid bath, was larger than that formed on the Sn films, which were made by using organic acid bath. We also investigated the mechanisms of heat treatment for inhibiting whiskers formation. There are no whiskers on the Sn deposited film after an aging with heat treatment. From the analysis of TEM diffraction patterns that were obtained by after-aging with heat treatment sample, there were two kinds of intermetallic compound deposits at the interface between the Sn deposited film and the Cu foils.