Effects of Minor Amounts of Zn on the Sn-Zn/Ni Interfacial Reactions and Phase Equilibria of the Ternary Sn-Zn-Ni System at 250 deg C

摘要

This study investigated the effects of minor Zn additions (less than 1 wt.percent) to pure Sn on the phase formation and microstructural evolution at the interfaces between Ni and Sn-Zn solders. When the Zn content was less than 0.3 wt.percent, the reaction product was Ni_(3)Sn_(4), which had no Zn solubility. By contrast, for reactions with more than 0.4 wt.percent Zn, one high-Zn-containing phase (about Sn-35 at.(percent)Ni-20 at.(percent)Zn) was formed, which was proven to be a ternary intermetallic compound, the tau-phase. In addition to composition analysis, x-ray diffraction (XRD) analysis revealed that the crystallographic structure of the tau-phase was different from that of Ni_(3)Sn_(4). Furthermore, a partial isothermal section in the Sn-Zn-Ni ternary system (less than 40 at.percent Ni) at 250 deg C was experimentally determined to contain 12 different Sn-Zn-Ni alloys. Three three-phase regions were identified: Ni_(3)Sn_(4) + tau + liquid, Ni_(5)Zn_(21) + tau + liquid, and Ni_(5)Zn_(21) + Zn + liquid. The equilibrium phase boundaries for liquid separately with Ni_(5)Zn_(21), tau, and Ni_(3)Sn_(4) were highly consistent with the Zn concentrations where the phase transitions occurred in the interfacial reactions.