Effects of Structures of Substrates on Sn Whisker Formation Using Substitutionally-Deposited Sn Films

Abstract

The effects of the microstructure of Cu foils, used as a copper pattern for flexible printed circuits (FPC), on Sn whisker formation as well as on the structure of substitutionally-deposited Sn films were investigated. In particular, the relationship between the grain size of the Cu foil and the amount of intermetallic compound deposits formed at the interface between the substitutionally-deposited Sn films and Cu foils as a function of aging was examined. Two types of Cu foils were used as substrates in this study. One had granular-shaped grains 0.5∼ 1.0 μm in size while the other had pillar-shaped grains about 5.0 μm in size. We called the former With the gelatin and the latter With the Cl - ion The grain size of the Cu foil with the gelatin additive was smaller than that of the Cu foil with the Cl - ion additive. The structures of the Sn whiskers, Sn films and Cu foils were investigated using TEM and SEM. The number of whiskers formed on the Sn-deposited film increased after aging. The number of whiskers formed on the substitutionally deposited Sn film on the Cu foil with the gelatin additive was larger than that with the Cl - ion additive. Cross-sectional structures of the Sn-deposited films and Cu foils were observed using TEM. Analysis of TEM selected-area diffraction patterns obtained from the samples after aging indicated the presence of Cu 6 Sn 5 intermetallic compound deposits at the interface between the deposited Sn films and the Cu foils. The amount of intermetallic compound deposits formed with a gelatin additive was larger than that with a Cl - ion additive. The structure of the intermetallic compound deposits was classified either as nodular-shaped or as layer-shaped. The difference between the amount of nodular-shaped intermetallic compound deposits between the Cu foils with the gelatin additive and the Cu foils with the Cl - ion additive were examined.