Modification Research Of Sulfur Adding On Tin-based Lead-free Solder

Traditional Sn-Pb solder is of excellent performance and low price, but Pb which is toxic is harmful to human and environment, so it is imperative to develop lead-free solders. Sn-Zn lead-free solder which is of excellent mechanically properties, low-melting point and low cost is considered as the best candidate. The limit to its widely applied is the worse wettability, corrosion resistance and oxidation resistance.The article improve the performance by adding sulfur. Experiment involve a series of physical and chemical properties of wettability, corrosion resistance, oxidation resistance and electro conductibility and so on. Meanwhile, observe the microstructure and the solder joint interface structure.The wettability experiment shows that, it is good for Sn-9Zn lead-free solder wettability when sulfur adding is under 0.1% and exist a maximum value at 1% levels. It is bad for wettability when sulfur adding is more than 0.1%.It similar to Sn-6.5Zn that it is good for wettability under 0.1% sulfur adding. And its maximum value appeared in 0.005% content. It makes no difference to Sn-0.7Cu lead-free sold wettability with sulfur adding.Analysis experiments of wetting mechanism show that, sulfur adding makes no difference to melting point of Sn-9Zn lead-free solder by DSC melting point experiment. Its melting point changes within 2℃,which will not affect the superheat. Oxidability experiment shows that, Sn-Zn and Sn-Cu system oxidation resistance improved by sulfur adding.The other physical and chemical properties experiments shows that, Sn-9Zn lead-free solder corrosion resistance improved after sulfur adding. Electro conductibility and thermal conductivity also have a small increase. It is not be worsen in hardness with sulfur adding.Metallographic observation shows that, sulfur refined Sn-9Zn lead-free solder organization and thick Zn-rich phase transition tiny with no impact on the interfacial structure. There is a gap between Sn-9Zn/Cu and matrix and it will cause a problem of bond strength. Sulfur adding has no impact on Sn-0.7Cu organization and interface.Sn-0.7Cu/Cu tightly bonded to the substrate. Comparison of high temperature microstructure and normal microstructure shows that, Sn Cu will grow up at high temperature.