MECHANICAL CHARACTERISTICS OF 96.5SN/3.5AG SOLDER IN MICROBONDING

This paper describes the mechanical characteristics of 96.5Sn/3.5Ag solder, including elongation, microstructure, and thermal fatigue life. A controlled-collapse chip joint was used as a vehicle to evaluate thermal fatigue life. First, 96.5Sn/3.5Ag solder with the addition of several mass % of gold was examined for tensile properties and microstructure. 63Sn/37Pb solder, which is conventionally applied to electronic equipment, was also examined for comparison. Although 63Sn/37Pb solder with more than 3 mass % of gold has a poor ultimate elongation, 96.5Sn/3.5Ag solder extends well. In regard to the microstructure, the grain size of intermetallic compounds formed by 96.5Sn/3.5Ag and gold is smaller than that of 63Sn/37Pb and gold. Next, the influence of the thermal fatigue of microbonding is discussed. After a temperature cycling test, the striations traced by the thermal fatigue were observed on the fractured surface. Their intervals were as fine as 0.04–0.2 µm. The crack propagation rate of the solder joint was calculated by the change in the intervals of the striations. In this way, the life of each solder joint was estimated. As the result of this examination, the thermal fatigue life of 96.5Sn/3.5Ag solder was estimated to be about twice as long as that of 63Sn/37Pb solder. Finally, the life estimated by the striations was related to the strain calculated by the finite element method (FEM) to obtain an experimental equation estimating the life of the controlled-collapse chip joint made with 96.5Sn/3.5Ag solder. In this study, 96.5Sn/3.5Ag solder was proven to have excellent characteristics for microbonding. © 1990 IEEE