Cruciform Pattern of Ni5Zn21 Formed in Interfacial Reactions Between Ni and Sn–Zn Solders
A unique cruciform pattern was observed for the Ni5Zn21 phase formed in both liquid-state and solid-state interfacial reactions between rectangular Ni substrate and both Sn–9 wt.%Zn and Sn–80 wt.%Zn. In these reactions, the Ni5Zn21 layers were uniformly formed at the interfaces. However, the Ni5Zn21 layer ruptured near the edges of the rectangular Ni substrate and the reaction layers were discontinuous. The cruciform pattern was closely correlated with linear growth of the Ni5Zn21 phase and the tensile stress generated because of the edge geometry. The liquid-state reaction between Sn–9 wt.%Zn and Ni resulted in a crescent shape at the edges because Ni5Zn21 growth was enhanced, owing to its linear growth. Most importantly, cruciform pattern formation also occurred in the solid-state reactions between Sn–Zn and Ni. In the liquid-state reaction between Sn–80 wt.%Zn and Ni, a porous structure and unusually fast growth at the acute-angled corner were observed. The reactions between Cu and Sn–9 wt.%Zn and Sn–80 wt.%Zn were also studied. For these, in contrast, parabolic growth was observed. The reaction layer was uniformly formed at the corners of the Cu substrate, leading to complete coverage.