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The interfacial reactions of Ni with Sn-Zn alloys with 1 wt.% to 9 wt.% Zn at 250°C were examined. The Zn content greatly affected the intermetallic compounds formed and microstructural evolution. A continuous Ni 5 Zn 21 layer was formed for the Sn-Zn/Ni couples with a Zn content higher than 5 wt.%. A stable reaction layer existed at the interface and grew thicker with time. When decreasing to 3 wt.% Zn, two thin reaction layers of Ni 5 Zn 21 and (Ni,Zn) 3 Sn 4 were simultaneously observed initially, and then an extremely large faceted Ni 5 Zn 21 phase was formed near the boundary between the Ni 5 Zn 21 layer and the solder. Furthermore, when the Zn content was lower than 2 wt.%, the dominant phase changed to (Ni,Zn) 3 Sn 4 . The Zn concentration of the solder gradually decreased with reaction, and thus the interfacial stability was reduced. Subsequently, a large amount of (Ni,Zn) 3 Sn 4 grains were dispersed into the molten solder, and finally the reaction product at the interface changed to Ni 3 Sn 4 .

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This study investigated the effects of minor Zn additions (less than 1 wt.%) 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.%, the reaction product was Ni 3 Sn 4 , which had no Zn solubility. By contrast, for reactions with more than 0.4 wt.% Zn, one high-Zn-containing phase (about Sn-35 at.%Ni-20 at.%Zn) was formed, which was proven to be a ternary intermetallic compound, the s-phase. In addition to composition analysis, x-ray diffraction (XRD) analysis revealed that the crystallographic structure of the s-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.% Ni) at 250°C was experimentally determined to contain 12 different Sn-Zn-Ni alloys. Three three-phase regions were identified: Ni 3 Sn 4 + s + liquid, Ni 5 Zn 21 + s + liquid, and Ni 5 Zn 21 + Zn + liquid. The equilibrium phase boundaries for liquid separately with Ni 5 Zn 21 , s, and Ni 3 Sn 4 were highly consistent with the Zn concentrations where the phase transitions occurred in the interfacial reactions.