Effects of microstructure of deposited Sn films and orientation index of Cu foils on Sn whisker formation using substitutionally-deposited Sn films

Abstract

The effects of the orientation indexes of Cu foils, used as a copper pattern for flexible printed circuits (FPC), and the grain size of substitutionally-deposited crystalline Sn films on Sn whisker formation were investigated. In particular, the relationship between the grain size of substitutionally-deposited Sn films and the structure 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∼1.0μm in size while other had pillar-shaped grains about 5.0μm in size. We called the former "with the gelatin additive" and the latter "with the C1- ion additive" since we used gelatin and Cl- ions for the fabrication of Cu foils. In addition, single crystal Cu (100), (110), (111) samples were also used as substrates. Two types of the Sn deposition bath were used in this study, a hydrofluoroboric acid bath and an organic acid bath. The structures of the deposited Sn films and Cu foils were investigated using TEM and SEM. The number of whiskers formed on the Sn-deposited film increased with aging. The number of whiskers formed on the substitutionally-deposited Sn films using the hydrofluoroboric acid bath was larger than that formed using the organic acid bath. The number of whiskers formed on the deposited Sn films on the Cu single crystal (111) was larger than that formed on the deposited Sn films on the other single crystals. The mechanism of inhibiting whisker formation by heat treatment was also investigated. No whiskers were seen on the substitutionally-deposited Sn films with heat treatment. Analysis of TEM selected-area diffraction patterns obtained from the sample subjected to heat treatment indicated the presence of Cu 6Sn5 and Cu3Sn intermetallic compound deposits near the interface between the deposited Sn films and the Cu foils. The results suggest that the microstructures of the substrates strongly affect the Sn whisker formation on substitutionally-deposited Sn films. © 2009 The Japan Institute of Metals.