Corrosion performance of Pb-free Sn-Zn solders in salt spray

Environmental regulations around the world have been targeted to eliminate the usage of leaded solders in electronic assemblies. Sn-Zn based alloys are considered to be a very promising candidate for its substantially same melting point as Sn-Pb solder. Fujitsu Limited has developed several lead-free solders with low melting point such as Sn-7Zn-30 ppmAl and Sn-9Zn-30ppmAl. Those solders have already launched application to commercial products since 2002. A reliable solder material requires good corrosion resistance in various ambient conditions. In this research, the salt spray corrosion resistances of Sn-7Zn, Sn-9Zn, Sn-7Zn-30ppmAl and Sn-9Zn-30 ppmAl alloys were investigated by Salt Spray Tester according to the national standard GB/T2423.17-93. The surface microstructure features before and after corrosion was observed by optical microscopy and scanning electron microscopy (SEM) with energy dispersive analysis of X-ray (EDX) respectively. The results show that comparing with Sn-7Zn-XAl (X = 0, 30 ppm), the increase of zinc content in Sn-9Zn-XAl (X =0, 30 ppm) induces a decrease of corrosion resistance. The main corrosion mode of all solder alloys is intercrystalline corrosion. It was found that 30 ppm Al additive to the Sn-7Zn solder improves the corrosion behavior in the salt spray with dissolution rate decreasing. But the addition of 30 ppm Al has not too much influence on Sn-9Zn performance of corrosion resistance.