Improvement in the Properties of Sn-Zn Eutectic Based Pb-free Solder

摘要

The melting temperature, 183°C, of the conventional eutectic Sn-Pb solder has become an important reference point for the development of Pb-free solder. The closest eutectic temperature to the Sn-Pb solder is 198°C of Sn-9Zn system. People have expressed strong interest in the Sn-9Zn system because of its relatively low eutectic temperature comparing to several other investigated Pb-free solders. Sn-Zn solder also exhibits good mechanical strength and low cost. However, Zn suffers from easy oxidation. This character renders Sn-Zn alloy to poor wettability. It is of importance to improve oxidation resistance of the Sn-Zn solder system in order to practically apply it in electronic packaging. Al was found to reduce the oxidation resistance of the eutectic Sn-Zn solder. The Sn-9(Zn-5Al) solder exhibits eutectic behavior of which the eutectic melting temperature stays close to that of the eutectic Sn-9Zn solder. Al of the Sn-9(Zn-5Al) solder tends to form a compact IMC layer with Cu that behaves as a barrier layer blocking the interduffusion between Cu substrate and Sn of solder. The introduction of Al did not improve the wettability of the solder to any extent. Meanwhile, Ag was also investigated for its effect on the oxidation resistance and wettability of Sn-Zn solder. Wetting time of 1.5 sec was achieved with a laboratory prepared flux for the Sn-Zn-Ag solder. The wetting interaction between Cu substrate and Sn-Zn-Ag solder gives rise to AgZn{sub}3 compound at the interface. A layer of Cu-Zn compound also formed underneath the AgZn{sub}3 compound. It is obvious that the ternary Sn-Zn-Al and Sn-Zn-Ag solders needs further enhancement on oxidation resistance and wetting behavior. A five-element (5-E) solder was thus developed in the sense of improving both behaviors. The Sn-Zn-Ag-Al-Ga solder systems were thus investigated for its wettability, mechanical properties, and oxidation resistance. It was found that addition of 0.25～0.5%Ga to the Sn-Zn-Ag-Al solder gives rise to much higher tensile strength and strain than the conventional eutectic Sn-37Pb solder. A further increase in Ga% enhances the tensile strength yet significantly reduces the strain of the s.older. It is also of interest to note that the 5-E solder exhibits much lower oxidation rate than the Sn-Pb solder at 250°C. The wetting time of the solder is linearly reduced with respect the increase in Ga%. A laboratory prepared flux has been developed that gives rise to less than 1 sec of wetting time between Cu and the 5-E solder at 230°C. The 5-E solder is been cast into solder bar and drawn to wire to test the manufacturability. A DSC investigation shows that the 5-E solder exhibits eutectic behavior of single endothermal peak at 198°C.