Effect of Ni Addition on the Comprehensive Properties of Bi–2.6Ag– <i>x</i> Ni/Cu Solder Joints

This study investigated the effects of Ni addition on the comprehensive properties of Bi–2.6Ag– x Ni/Cu solder joints, including the melting characteristics and wettability of Bi–2.6Ag– x Ni solder, as well as the interfacial microstructure and shear properties of Bi–2.6Ag– x Ni/Cu joints aged for different durations. The results indicated that Ni addition has a negligible effect on the melting characteristics of Bi–2.6Ag solder but enhances its wettability. The Bi–2.6Ag– x Ni/Cu joint exhibits a solid‐solution‐dominated interface, and the addition of Ni increases the shear strength and shear displacement of the joint by deepening the groove at the joint interface. During shear, the fracture mode remained brittle throughout. However, Ni addition shifted the fracture location from theinterface toward the interior of the solder. Notably, when Ni content reached ≥0.3 wt%, coarse NiBi 3 primary phases form within the solder, reducing wettability. At the same time, the soft and brittle nature of NiBi 3 also limits the improvement of the solder's mechanical properties. Considering the wettability of the solder joints, interfacial morphology, thermal aging characteristics, and shear properties, the optimal range for Ni addition in Bi–2.6Ag is 0.2–0.3 wt%.