Effect Of Addition Of Rare Earth On Microstructure And Properties Of Sn-Ag-Cu And Sn-Bi-Cu Lead-free Solders

Sn-Pb solder with excellent properties has played an important role in the history of the development of human technology; however, it has caused great harm to human and nature. As people's awareness of environmental protection, the study of lead-free solder materials has become the focus of material connection fields. Currently, the research of solder mainly focuses on the aspect of composition, proportion and the micro-alloying, etc. In recent years, some studies have found that the thermal history of melt and the micro-alloying of solder will affect the final microstructure and properties. Therefore, it has great significance to improve the solder microstructure and mechanical properties as well as the development of new lead-free solder by further study trace elements and melt structural transition.In this paper, the effects of rare earth Cerium (x=0%,0.2%,0.3%,0.5%,1.0%,2.0%) and melt structural transition induced by temperature on the microstructure and properties of lead-free solders, Sn-3.8Ag-0.7Cu and Sn-5Bi-0.7Cu, were investigated by DC resistance method, iron-mold experiment, spreading experiment and tensile test. The main content and several conclusions can be summarized as follows:(1) The relationship of resistivity-temperature curve of two continuous heating and cooling cycles of Sn-3.8Ag-0.7Cu-xCe and Sn-5Bi-0.7Cu-xCe has been surveyed by the DC four-probe method. It is found that these solder exist temperature-induced liquid-liquid structural transition, and the transition is partially reversible. Cerium raises the melting point of the alloy and increases the melting temperature range of Sn-3.8Ag-0.7Cu alloy, but decreases the melting temperature range of Sn-5Bi-0.7Cu alloy(2) After solidification experiments of Sn-3.8Ag-0.7Cu-xCe and Sn-5Bi-0.7Cu-xCe, the results indicate that the microstructure of solder can be refined with addition a small amount of Cerium, solidification microstructure is the most small and uniform when Cerium content reaches 0.2%; with increasing Cerium content, the bulky structure becomes serious. Meanwhile, the melt structural transformation also can refine the solidification structure of the alloy and make microstructure more small and uniform.(3) Spreading experiment of solder samples which solidified by iron mold was taken on the copper plate. The result shows that the wettability and spreading area of Sn-3.8Ag-0.7Cu and Sn-5Bi-0.7Cu solders are promoted obviously by the addition of Cerium. The wettability of two kinds of solder is the best when Cerium content reaches 0.2%. In addition, the melt structural transformation can also improve wetting property of the solders.(4) The shear strength of Sn-3.8Ag-0.7Cu-xCe solders/Cu joints has been measured by tensile test. The results indicate that with increasing Cerium content, the shear strength of solders/Cu joints first increases and then decreases, the joint with 0.2% Cerium content has the highest strength. Moreover, the melt structural transformation also enhances shear strength of solder joint.