Microstructural evolution of Sn-Zn based lead free solders after temperature and humid atmosphere exposure

The Sn-Zn based lead free solders appear to be an attractive alternative with a melting temperature that is relatively close to eutectic Sn-Pb. The addition of bismuth improves wettability of Sn-Zn alloy and aluminum is found to improve the oxidation resistance of the eutectic Sn-Zn solder. The objective of this study is to explore the degradation of Sn-Zn system solders after temperature and humid atmosphere exposure. Three kinds of Sn-Zn system solders (Sn-9Zn, Sn-8Zn-3Bi and Sn-7Zn-Al(30ppm)) were soldered on copper pad with a layer of Au/Ni and the specimens were stored in 120 degC/100% relative humidity for 96 hours or 192 hours in a pressure cooker. Zn diffused to the surface and interface while coarsening at the same time. The coarser Zn had weak interface with p-Sn matrix and oxidation was formed. The mechanical fatigue test was carried in a displacement-controlled mode and the results show that high temperature humidity exposure decreases the mean life of Sn-Zn system solder joints when they are subjected to 120degC and 100% relative humidity for 192 hours after the pressure cooker test