Effect of Temperature Gradient on Interfacial Reactions in Cu/Sn-9Zn/Ni Solder Joints during Aging

The interfacial reactions in Cu/Sn-9Zn/Ni micro solder joints during aging with and without a temperature gradient (TG) were studied. For isothermal aging, a laminar Cu5Zn8 IMC formed on the Cu substrate and a bilayer structure, namely Cu5Zn8 and (Ni,Cu)(3)(Sn,Zn)(4), was detected on the Ni substrate. When Cu was the hot-end during aging under a TG, the diffusion of Cu atoms was enhanced towards the Ni cold-end. However, there existed a positive chemical potential gradient of Zn atoms from the Ni cold-end to the Cu hot-end, which drove Zn diffusing towards the Cu hot-end even against the TG, and thus laminar Cu5Zn8 IMC predominated the interfacial reaction at the Sn-9Zn/Cu interface. With the condition that Cu was the hot-end during aging under a TG, the diffusion of Cu atoms was remarkably restrained, which led to a slower interfacial reaction rate in the whole micro solder joint. With increasing aging duration, some large diffusion pits formed in the Cu substrate adjacent to the interface, while the Ni substrate was dissolved slightly whether it acted as the hot-end or the cold-end.