Microstructures and Properties of Sn-Zn-Bi Solder Alloys
低熔点Sn-Zn-Bi无铅钎料的组织和性能

The microstructures, phase transformations and wetting behaviors of the Sn-Zn-Bi solders were investigated. The results show that adding 2%-10% Bi(mass fraction) to Sn-9Zn binary alloy, the primary Zn rich phase appears from the melts firstly on cooling, which results an increase of the pasty ranges. The crystallizing points decrease with addition of Bi, indicating a decrease of the melting points. Fixing Bi concentration, reduction of Zn decreases the pasty ranges, and the melting points keep stable. Addition of Bi improves the wettabilities, because the wetting areas on Cu substrates of the alloys are increased, and the wetting times are shorten. The diffusion of Zn into Cu at the solder/Cu interface is accelerated by increaseing Zn concentration, which decreases the solder/Cu interfacial energy. With addition of Zn, the wetting areas on Cu substrates of the Sn-Zn-Bi solders increase and the wetting forces enhance. But the diffusion needs a period of time, which means a prolonged wetting time. Therefore, Zn concentration must be controlled in order to obtain a good matching between the spreading area and the wetting time.