The Research On Oxidation Resistance And Wettability Of Sn-Zn System Lead-free Solder

Sn and Zn in Sn-Zn lead-free solder are abundant, with low price, nontoxic and the solder has good adhension property, creep resistance and heat fatigue property, so it has become the focus of the lead-free solder. Compared with other solders, the eutectic temperture is close to Sn-Pb, has no damage to the electric apparatus, the equipment can be used well as before, the exploiture value is prominence. So the Sn-Zn system solder is considered to be one of the most expected solder to replace the Sn-Pb solder.The solder is easily to be oxidated and has poor wettability because of Zn existing, it can not be widely used so far. The solder is instantaneously oxidated durning the welding process, and the oxidation of Zn leads flux to be drossed; Futhermore, durning the welding process, the solder is easily to be oxidated, so it exists the problem of welding and reduce the credibility. The poor oxidation resistance becomes one foctor of the development restrict of the Sn-Zn system lead-free solder. So the paper aims at Sn-Zn alloy system, and research on oxidation resistance and wettability and the influence of adding other alloy elements. Adding Ag, Bi, RE and so on, analyse the influences of the elements on oxidation resistance by the increasing oxidated mass, study the influences of the elements on wettability by measuring the wetting strength and the wetting area; use the caustic flux and do the wetting experiment in the vacuum condition, analyse the reason of the poor wettability of the solder.The results show that: the Sn-Zn lead-free solder has oxidation problem, it affects the wettability of the solder. By adding Ag, Bi and RE can improve oxidation resistance, by the addition of Ag, Ag-Zn compounds increase and the oxidation of Zn was restrained, so the oxidation resistance and the wettability were both improved in the content 1.5%, they show the same exhibition; the element of Bi concentrate at the surface, leading to the decent of Zn concentration, however, the inside stress of oxide film is big, the oxide film inclines to crack, so the protection is invalidated; RE can refine the crystal particles, the solder's organization becomes uniform, the oxide of RE form an oxidation barrier. Because of the good adhesion, although the oxidation resistance is improved, the wettability is not significantly amended. By using the caustic flux and do the wetting experiment in the vacuum condition, the wettability of Sn-37Pb and Sn-4Ag-0.5Cu can be greatly improved, but the Sn-8Zn system lead-free solder are not increased more, in the vacuum condition, the solder even can not dilate upon the Cu. It indicates that oxidation is not the factor leads to the poor wettability of the solder, the factor of the poor wettability is that the chemical appetency between the elements is very big, the solder is hard for wetting.For the purpose of evaluate oxidation resistance and wettability at the same time, set the percent of alloy elements as factors and oxidation resistance and wettability as targets, then conduct quadratic regression combination design with multi-factor and multi-target. The regression equations related oxidation and wettability of solder to elements were established. After considering synthetically the effects of solder's ingredients to melting temperature, pasty range, elongation, oughness, fusion, element losing for burning etc. the contribution of Sn-Zn-Ag-Bi lead-free solder has been optimized, the obtained performances are: Ag:1.45%, Bi:3.34%, Zn:8.36% and Sn:86.85%.