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 substitutionallydeposited 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 mm in size while other had pillar-shaped grains about 5.0 mm in size. We called the former ''with the gelatin additive'' and the latter ''with the Cl À ion additive'' since we used gelatin and Cl À 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 6 Sn 5 and Cu 3 Sn 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.

Risk of Tin Whiskers in Medical Devices

“…J Biomed Eng Res 2018 | Vol 2: 101 grain size and orientation (i.e., film texture) [19][20][21]; film microstructure [22][23][24][25][26][27]; film composition [24,25,[28][29][30]; film thickness [14,19,20,22,31,32]; and thermal cycling, corrosion, and mechanical deformation [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] can enhance the formation of stress gradients in the films and contribute to tin whisker growth.…”

Numerical modeling of bronze coating formation using concentration-dependent diffusivity

“…The η and ε intermetallic phases can form at the lower ranges of temperatures of this system, thus making them present in applications such as soldering of electronic devices. With the recent call to eliminate lead (Pb)-based solder [8]- [11], these intermetallics have thus been the focus of many researchers.…”