| The research in lead-free solder alloy has been a popular topic in recent years. However, a lot of problems in full commercial production and application of lead-free solders must be studied in detail. Based on Sn-3.5Ag-0.5Cu lead-free solder and by means of SEM,EDS,XRD,DTA and weldability test, effects of Bi, Sb elements on microstructure, wettability, melting characteristic and such physical properties were studied and the solder alloy component of favorable combination property was gotten.The results of melting characteristic experiments show that trace addition of Bi element has positive effects on depressing the melting temperature and broadening the melting range. However, the addition of Sb has little effect on melting temperature but broadens the melting range. The melting point of lead-free solder Sn-3.5Ag-0.5Cu is 216.4℃and the melting range is 6.3℃. The melting point of Sn-3.5Ag-0.5Cu-1.2Bi-0.8Sb lead-free solder reduces to 209.8℃, and the melting range increases to 13.4℃. The melting characteristics may meet the evaluation standard required by NCMS. The results of wettability experiments show that Sn-3.5Ag-0.5Cu lead-free solder has poor wettability. Adding suitable amount of Bi,Sb elements into the solder alloy may help the liquid solder spread on the surface of base metal. Compared to the specimen Sn-3.5Ag-0.5Cu, the wetting time of Sn-3.5Ag-0.5Cu-0.8Bi-1.2Sb under the condition of 240℃reduces from 1.95s to 0.78s, representing a decline of 60%. In addition, the maximum wetting force of this componential solder also reaches 0.58mN, which leads to the best wetting performance. The results of heat stability experiments show that with proper addition of Bi and Sb elements, the compact oxide film may form on the surface of the solder alloy to prevent reoxidation and melting loss. However, the adding amount of Bi should be no more than 0.8%. In addition, the results of electrical conductivity test show that trace addition of Bi(≤1.0wt%) enhances the electrical conductivity because of its purifying effect. But the addition of Sb leads to the lattice distortion so as to increase the scattering probability of electrons, and the enhancement of the chemical interaction among each component reduces effective mass of electrons. As a result, the electrical conductivity may decline obviously when adding Sb element. The results of microhardness test show that accompanied the solution strengthening and the effects of new formed dispersed Cu6(Sn, Sb)5 ternary inter-metallic compound, the addition of both Bi, Sb elements may enhance the microhardness of the solder alloy, especially obvious for the strengthening effect of Sb.By observing the microstructure of solder alloys, it can be concluded that the structure of Sn-3.5Ag-0.5Cu-yBi-zSb(0.4≤y,z≤1.2)consists of a mass ofβ-Sn primary phases and little amount of eutectic phases composed by Cu6Sn5 and Ag3Sn. It is approved that elements Bi and Sb are solid-soluted in the matrix ofβ-Sn primary phases when the addition level is less than 0.8wt% respectively. The inter-metallic Ag3Sn phases in the alloy system have been refined in a certain degree, and the strip-like IMCs turn to be acicular ones while raising Bi content. Furthermore, to increase the content of Sb, the primary phases may become coarse, and the IMCs turn to be dispersively distributed in a whorl-like formation. A lot of new particle phases are present dispersively in the matrix, and by means of EDS and XRD analysis, it can be approved that new IMCs are Cu6(Sn, Sb)5 ternary compounds.By analyzing the data gathered from the experiments, a new solder alloy was optimized for the further study. The compositions are Sn-3.5Ag-0.5Cu-0.8Bi-1.2Sb. |
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