A Study On Sn-Based Lead-Free Packaging Materials

With the development in microelectronic and srrface mount technology, the Sn-Pb packaging materials commonly used in electronic industry will be restricted because of the lead's toxicity. The study on lead-free solders is becoming a new focus. In this dissertation, Sn-3Ag-0.5Cu-6In-xZn(x=0,1,3,5) and Sn-3Ag-0.5Cu-6In-xBi(x=0,1,3,5,8) packaging materials were prepared by casting method under vacuum condition. The effect of Zn and Bi elements on the properties of Sn-3Ag-0.5Cu-6In alloy was systematically studied, including melting temperatures, electrical conductivities, thermal expansibilities, wettabilities, corrosion resistivities and mechanical properties. The microstructures of the two serial alloys were also investigated.Results show that proper Zn in Sn-3Ag-0.5Cu-6In alloy decreases the melting temperature and improve the electrical conduction obviously. The melting temperature is 208.7℃while Zn content is 3%, and the electric resistivity reduces about 15% while Zn content is 5%. With the addition of Zn element, the thermal expansion coefficients has no prominent change. The values are near that of the Sn-37Pb solder alloy. With the increase of the Zn content, the wettability and corrosion resistivity decrease.Bi is an effective alloying element for the melting temperatures and wettabilities in Sn-3Ag-0.5Cu-6In alloy, When Bi content is 8%, the melting temperature is 185.5℃, near the level of the Sn-37Pb eutectic alloy. The electrical conductivity and thermal expansion are increased when Bi content is proper, for instance, the electrical resistivity of the alloys can reduce about 10% and 20%, while the Bi content is 3% and 8%. The alloy with 5%Bi addition has the lower thermal expansion coefficients, and its thermal expansion is better than conventional Sn-37Pb alloy. The values of the alloy 3%Bi is close to that of the Sn-37Pb alloy. Through the experiment of immersion corrosion in 5%NaCl solution and the polarization curves analysis, we can conclude that the more Bi the alloys have, the poor corrosion resistant property it has. But the alloys which Bi content is lower than 3% exhibit better corrosion resistivity than the Sn-37Pb solder alloy.