Research On The Properties And The Interfacial Reactions Of Sn-Zn-Cu(Ni) Lead-free Solders

Lead and Pb compounds have been cited by the Environmental Protection Agency (EPA) as one of the top 17 chemicals posing the greatest threat to human beings and the environment, which indicates that developing viable alternative lead-free solders for electronic assemblies and other soft soldering is a trend. In all Sn-based solders, Sn-Zn has a low cost, closed melting point to that of eutectic Sn-Pb solder and excellent mechanical properties. However, the bad wetting and easy oxidation behaviors which are caused by the active Zn element need to be resolved before the extensive use of Sn-Zn alloy in electronic packaging.Based on the results of previous experiments, the lead-free solder can get better properties by adding 2% Cu element into Sn-Zn alloy, which can reduce the oxidation of Zn in the surface of solder. So in this study, the microstructure, melting point and mechanical properties are investigated by alloying 2%Cu element and changing the Zn contents. The interfacial microstructures and wetting property of the new developed Sn-xZn-2Cu solders with Cu substrate at the temperature of 250℃ are also studied. The Ni element has the similar crystal lattice to the Cu element, and they are adjacent in the Element Cycle Table. What's more, the Ni is the common substrate film in practice, and the Ni element may diffuse into the solder during soldering and consequent using. So the Cu element in Sn-Zn-Cu alloys is displaced by Ni as the following study. The microstructure, the melting property and the wetting property of (Sn-9Zn)-xNi alloys are researched. The shear tests for (Sn-9Zn)-xNi/Cu solder joints are also implemented.The results show that:1. The intermetallic compounds (IMC) formed in all the Sn-6.5Zn-2Cu, Sn-8.8Zn-2Cu, Sn-10Zn-2Cu and Sn-12Zn-2Cu bulk alloys are Cu₅Zn₈. The melting points of Sn-6.5Zn-2Cu, Sn-8.8Zn-2Cu and Sn-10Zn-2Cu are 220.67℃, 220.33℃ and 218.84℃ respectivly, while the Sn-12Zn-2Cu alloy has only one endothermic peak which is 205.95℃.2. Both the spread ratio and the wetting area of Sn-6.5Zn-2Cu, Sn-8.8Zn-2Cu, Sn-10Zn-2Cu and Sn-12Zn-2Cu increase with the content of Zn. The shear strength Sn-xZn-2Cu/Cu desn't change a lot, and is about 20-25 Mpa for all cases. However, the shear strength decreases rapidly during the first 48 hours of aging, but then it drops slowly.3. A continuous Cu₅Zn₈IMC layer is formed at all the interfaces of Sn-8.8Zn-2Cu/Cu, Sn-10Zn-2Cu/Cu and Sn-12Zn-2Cu/Cu during soldering. The thickness of the IMC layer...