Study On Properties Of Sn-Zn-Bi Based Lead-free Solders For Soldering Al And Cu

Connection between dissimilar metals of Al and Cu was a key problem in refrigeration industry, power electronics and telecommunication equipments. In order to solve the problem of connection between Al and Cu, Sn-Zn-Bi based lead-free solders were used in this paper to achieve reliable connection between the dissimilar metals of aluminum and copper by soldering methods. Two kinds of lead-free solders, which were Sn-Zn-Bi-Ga and Sn-Zn-Bi-Ag(Ni) solders, were designed by theoretical model of "[cluster]+[glue atoms]" The microstructure of bulk solder, melting behavior at heating and cooling period, wetting ability on Al and Cu substrates and interfacial microstructure of Al/Cu solder joints as well as the effect of interfacial reaction on shear strength of solder joints were investigated. The main results were as follows:1. The properties of Sn-Zn-Bi-Ga bulk solders were investigated. The microstructure of the bulk Sn-Zn-Bi-Ga solders mainly consisted of Sn-Zn-Bi ternary microstructure, Sn-Zn binary microstructure and needle Zn-rich phase. Element Ga dissolved into Zn phase. There were two peaks on the DSC heating curve of all solders, corresponding to the melting points of the Sn-Zn-Bi ternary nonequilibrium microstructure and the Sn-Zn binary nonequilibrium microstructure. In the cooling curve, however, there were three exothermic peaks in the curve, corresponding to the melting points of the primary Zn phase, the Sn-Zn binary eutectic and the Sn-Zn-Bi ternary eutectic, respectively. The wetting area of solders on Al substrate increased with increasing the Ga content of the solder.2. The interfacial microstructure and shear strength of Al/Sn-Zn-Bi-Ga/Cu solder joints were investigated. It was found that solders and Al substrates were bonded together through forming the rich-Al(Zn) and rich-Zn(Al) solid solutions at the interface. While solders and Cu substrates were bonded together through forming different types intermatellic compounds (IMCs). The shear strength of the Al/Sn-Zn-Bi-Ga/Cu solder joints initially increased with increasing Ga content but decreased with further increasing Ga content.3. The properties of Sn-Zn-Bi-Ag(Ni) bulk solders were investigated. Element Ag was completely dissolved into Zn phase when adding a small amount of Ag. While further adding Ag content, AgZn3 was formed in the microstructure of bulk solders causing Zn phase becoming shorter. And there was exothermic peak of formation of AgZn3 IMC in the DSC cooling curves. Element Ni was completely dissolved into Zn phase when adding a small amount of Ni. While further adding Ni content, Ni3Zn14 IMC was formed in the microstructure of bulk solders.4. The interfacial microstructure of Al/Sn-Zn-Bi-Ag(Ni)/Cu solder joints were investigated. It was found that solders and Al substrates were bonded together through forming the rich-Al(Zn) and rich-Zn(Al) solid solutions at the interface. While solders and Cu substrates were bonded together through forming IMCs.