Effects of rare earth Ce on properties of Sn-9Zn lead-free solder

The influences of different Ce content on the properties of Sn-9Zn lead-free solder were investigated. The results indicate that Ce plays an important role not only in the structure and the solderability, but also in the interfacial structure of Sn-9Zn-xCe/Cu and mechanical property of soldered joint. Sn-9Zn-0.08Ce shows finer and more uniform microstructure than Sn-9Zn, and when the quantity of Ce is 0.5-1 wt%, some dark Sn-Ce compounds appear in the solder. With the addition of 0.08 wt% Ce, the solderability of solder is significantly improved because the surface tension of molten solder is decreased. Adding Ce makes the Cu5Zn8 IMCs formed at the interface of solder/Cu become much thicker than that of Sn-9Zn/Cu because much more content of Zn diffuse to the interface of solder/Cu to react with Cu. Results also indicate that adding 0.08 wt% Ce to the solder enhances mechanical property of soldered joint. When the Ce content is 0.1-0.5 wt%, some hard and brittle Cu-Zn IMCs appear in the bottom of dimples and the pull force of soldered joint decreases.