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

The connection technology of aluminum and copper is facing reliability issues. In this study, Sn-xZn (x=5,15,25,35at%) solders were applied to solder aluminum and copper to obtain Al/Sn-xZn/Cu solder joints with excellent mechanical properties. The mechanical properties, corrosion resistance, as well as the effect of Zn content on properties and interfacial reaction of the solder joints were investigated. Sn-Zn-Ag, Sn-Zn-Al and Sn-Zn-Al-Ag solders designed by "cluster-plus-glue-atom" structural model were also used to solder aluminum and copper. The mechanical properties and the corrosion behavior of the solder joints were investigated in this study.The main conclusions were as follows:1. A continuous intermetallic layer formed at the interface between solder and copper and the type of the intermetallic layer was affected by Zn content:Al4.2Cu3.2Zno.7and CueSns formed at the interface of Sn-5Zn/Cu; AI42CU32Zno.7formed at the interface of Sn-I5Zn/Cu; CuZn5and CuZns formed at the interfaces of Sn-25Zn/Cu and Sn-35Zn/Cu. There was a Al-Zn-Sn solid solution region at the Al/solder interface as a connecting layer instead of intermetallic layers.2. According to the test results of solder joints, the shear strength of solder joints increased with increasing Zn content and the Al/Sn-35Zn/Cu joint possessed the highest shear strength under the same reflow condition; after immersed in3.5%NaCl solution for12h, the shear strength of the joints decreased as a result of electrochemical corrosion. The corrosion resistance of solder joints enhanced with increasing Zn content and the Al/Sn-35Zn/Cu joint possessed the best corrosion resistance. However, the adhesion force of Al/solder interfaces were lower than that of Cu/solder interfaces that the shear and corrosion failures both occurred at Al/solder interfaces.3. Sn-Zn-Ag、 Sn-Zn-Al、 Sn-Zn-Al-Ag eutectic solders were designed by "cluster-plus-glue-atom" structural model. The DSC curves of Sn-Zn-Ag, Sn-Zn-Al, and Sn-Zn-Al-Ag were very similar with that of eutectic Sn-Zn. Only one endothermic peak appeared on the heating curve and only one exothermic peak appeared on the cooling curve. The wetting ability of the solders on Cu substrate and Al substrate was good, and the Sn-Zn-Al solder possessed the best wetting ability both on Cu substrate and Al substrate.4. The Sn-Zn-Ag joints possessed the best corrosion resistance among these three kind of solder joints.. A thin AI42Cu3.2Zno.7layer formed at Cu/solder interface and an Al-Zn-Sn solid solution region formed at the Al/solder interface of all the solder joints. According to the test results of Sn-Zn-Ag、 Sn-Zn-Al n Sn-Zn-Al-Ag solder joints, all the solder joints have excellent shear strength with the Sn-Zn-Al solder joint possessing the best shear strength. After immersed in3.5%NaCl solution for12h, the corrosion resistance of the Sn-Zn-Ag solder joints were better than that of Sn-Zn-Al and Sn-Zn-Al-Ag solder joints.