Investigation On Low Temperature Sn-Zn Base Lead-Free Solder Alloys

Tin-lead solders have been widely used in electronics manufacturing for many decades. However, there are environmental and health issues concerning the toxicity of Lead(Pb) present in these Pb-Sn solder alloys. These concerns have inspired a great deal of research to find suitable Pb-free replacement alloys. In Europe, the move towards Lead-free solder alternatives has accelerated because of legislation within the European Union. The European Union has approved the directives on waste from electrical and electronic equipment (WEEE) and restriction of hazardous substances (RoHS), which states that Lead will be banned in electronic products in the European Union from July 1, 2006. Sn-Zn base alloys are widely recommended because of the melting point of the Sn-Zn eutectic alloy is near that of the Sn-Pb eutectic alloy. However, its inferior wettability and easy oxidization are the two main disadvantages.Recently, Sn-Zn-Bi solders with low melting point have come into use in some cases. However, their application is limited by their unsatisfactory wettability and oxidation resistance. In the present paper, it is the main purpose to improve wettability of Sn-Zn-Bi solders. Firstly, the effects of Bi and Zn on wettability, melting behavior and mechanical property of Sn-Zn-Bi alloys have been investigated to find out the optimization Sn-Zn-Bi solder. At the following stage, some trace element has been added into the Sn-Zn-Bi solder to improve oxidation resistance and wettability of the alloy. At last, selected Sn-Zn-Bi-X quaternary alloy have been reflowed to achieve soldering joints. The effect of solder/Cu reactive layer growth on shear strength of aged joints has been investigated.According to the results of surface tension test, surface tension of Sn-Zn alloy melts can be reduced with addition of Bi. The results of surface tension calculation suggest a decrease in the surface tension of Sn-Zn-Bi alloys as a result of the enrichment of the surface with Bi atoms. Wetting force(Fw) and wetting time(tw) on Cu substrate of Sn-Zn-Bi solders have been measured by wetting balance test. With addition of Bi, Fw increase and tw decrease, which indicates wettability of Sn-Zn-Bi alloy has been improved.The addition of Zn increases the surface tension of Sn-Zn-Bi melts. But, because Zn atoms diffuse into Cu, solder/Cu interface tension has been decreased, which helps the solders spread out at Cu substrate. The reactive wetting procedure accompany with diffusion usually needs long time to achieve equilibrium, so wetting time has been prolonged.Nd, La and In are the effective additive elements on increasing wetting area of Sn-Zn-Bi solders. The most effective additive was Nd at 0.10%, because the wetting area is closed to that of Sn-40Pb, and the wetting time has been shortened to 0.45S.Zn atoms have been prevented from oxidation, which results from Zn concentration at the surface of solder melt has been deduced with addition of Nd. Nd atoms have been oxidated instead of Zn. But, the formation of Nd oxidation obviously slower than that of Zn oxidation, which indicates that the diffusion of O atoms into the solder has been impeded. La atoms have been oxidated instead of Zn also. A large mounts of La oxidation has formed, which shows there is no...