Effect of Nano-Cu6Sn5 upon development mechanism of Cu–Ni–Sn compounds and mechanical characteristics for mixed solder joints

This investigation aimed to examine the development mechanism of Cu–Ni–Sn compounds and analyze the mechanical properties of mixed solder (MS) joints. Results show that an intermetallic compound (IMC) layer was formed in an SAC305 solder joint prepared herein comprising (Ni, Cu)3Sn4 and (Cu, Ni)6Sn5 phases following the reflow process. The morphology of the (Cu, Ni)6Sn5 grains was identified to be dodecahedral. Meanwhile, the IMC layer of the prepared MS joints contained only the (Cu, Ni)6Sn5 phase, which exhibited a hexagonal prism morphology. The lattice characteristics of Cu6Sn5 changed after Ni dissolution, forming (Cu, Ni)6Sn5 crystals with an X-shaped morphology in the MS joints that exhibited a high Cu content during the aging process. After reflow, the MS joints exhibited slightly higher shear strength than the SAC305 solder joint. After aging at 165 °C for 1000 h, the shear strength of the SAC305 solder joint reduced by 21.72% compared to its value after reflow owing to the annexation of Sn grains. However, the number of Sn grains in the MS joints did not considerably decrease after aging. Further, the grain boundaries of β-Sn and (Cu, Ni)6Sn5 particles contributed to the strengthening of the MS joints. The shear strength of the MS joints decreased by 11.06%–18.35% after aging.