Mechanical characteristics of 96.5 Sn/3.5 Ag solder in micro-bonding

The influence of gold on the tensile properties and microstructure of 96.5 Sn/3.5 Ag solder and the thermal fatigue life of a 96.5 Sn/3.5 Ag-solder joint applied to a controlled-collapse chip joint was investigated and compared with the conventional solder, 63 Sn/37 Pb. A 96.5 Sn/3.5 Ag solder with several mass% of gold added is examined in terms of tensile properties and microstructure. Although 63 Sn/37 Pb with more than 3 mass% of gold has a poor ultimate elongation, 96.5 Sn/3.5 Ag extends well. Regarding the microstructure, intermetallic compounds alloyed by 96.5 Sn/3.5 Ag and gold are smaller than those of 63 Sn/37 Pb and gold. The influence of the thermal fatigue of microbonding is discussed. After the temperature cycling test, striations formed by the thermal fatigue are observed on the fractured surface. Their intervals are as fine as 0.04-0.2 μm. The crack-propagation rate of the solder joint is calculated according to the alternating intervals of the striations. In this way, the life of each solder joint is estimated. The thermal fatigue life of 96.5 Sn/3.5 Ag solder is found to be about twice that of 63 Sn/37 Pb. The life estimated according to the striations is 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 of 96.5 Sn/3.5 Ag. The 96.5 Sn/3.5 Ag solder is proven to have excellent characteristics for microbonding