| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 100 === Because of the property reinforcement of soldering joints, minor element alloying has attracted great attentions in Pb-free solders development. Among the alloying elements, Co is reported effective in reducing the undercooling of the solders, and is likely caused by small Co solubility in Sn. This thesis investigates effects of minor Co additions upon the interfacial reactions between the pure Sn and Ag, Cu, ands Ni substrates. The phase equilibria of the Sn-Co-Ag ternary system at 250oC are determined as well. The phase equilibria information is crucial to the analysis of the reaction path at soldering joints.
No ternary phase is found at the 250oC isothermal section of the Sn-Co-Ag ternary system. Seven three-phase regions, ten two-phase regions, and nine single-phase regions are determined. There is no noticeable ternary solubility in most equilibrated phases except the Ag4Sn and FCC-Ag phases. The maximum Co solubility in the Ag4Sn and FCC-Ag phases are 7 and 2.5at%, respectively.
Only the Ag3Sn phase is observed in Sn-Co/Ag couples. With higher Co additions in the pure Sn, the Ag3Sn growth rate is slower, and the activation energy is lower down. The effect of minor Co additions to the reaction rates with different substrates is Cu>Ni>Ag. Co addition also refines the grains of the reaction phases. The Co effect upon the grain size of the reaction phase is Cu6Sn5>Ni3Sn4> Ag3Sn.
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