Microstructural evolution of Sn-9Zn-3Bi and Sn-9Zn-3Bi-0.3Cr solder under high-temperature and high-humidity conditions

The microstructual stability of Sn-8Zn-3Bi/Cu and Sn-8Zn-3Bi-0.3Cr/Cu joints has been investigated under 85 °C temperature and 85% humidity conditions as a function of exposure time. The evolution of interface and near surface cross-sectional microstructures during exposure has been examined. A continuous single layer of intermetallic compound (IMC, hereafter) appears in both of the two kinds of solders. The IMCs grows during exposure at 85°C/85%RH, and the thickness of the Sn-8Zn-3Bi-0.3Cr solder joint is much thinner than that of Sn-8Zn-3Bi solder joint under the same condition. The better oxidation resistance of the Cr content solder is attributed to the Cr-rich phase which restrains the diffusion of Zn atom to the grain boundaries of Sn matrix. Near the surface and the interface of the joint, there is also Zn-depleted layer. The invalidation of the Sn-8Zn-3Bi solder is worse.