The evolution of IMCs' morphologies and types in SAC and SAC+ solder bumps during thermal shock process

The types and morphologies evolution of IMCs affect the reliability of SAC bumps during the process of thermal shock effectively, especially when Ni element was added to manufacture SAC+ bumps. The IMCs at the interface between the solder and the Au/Ni/Cu coating as well as the ones in the solder bulk were studied. The IMCs' morphology evolution was observed by SEM both in cross-section view and top view; the types and composition transformation were tested by EDX and line scanning; the effect of Ni was researched by comparing the difference between SAC and SAC+. As a result, the IMCs in SAC solder bumps right after soldering was mainly needle like (Cu, Ni)6Sn5 at the interface and uniform Ag3Sn net in the bulk; for the SAC+ solder, (Ni,Cu)3Sn4 blocks and needle like (Cu, Ni)6Sn5 were both found, and lots of (Ni,Cu)3Sn4 particles broke the Ag3Sn net in the bulk. During thermal shock, the IMC of SAC+ is thicker than that of SAC at beginning, but its growth rate later was less. After 1500 cycles, the (Ni, Cu)3Sn4 can be separated from the interface layer and then be turned into (Cu,Ni)6Sn5; at area without the cover of (Ni, Cu)3Sn4, (Cu,Ni)6Sn5 can grow much faster with a new (Ni, Cu)3Sn4 layer fomed under it; the thickness of the SAC+ bumps affects the thickness as well as morphology of the IMCs. Especially (Cu,Ni)6Sn5 in thinner bumps is polygon but in thicker bumps was needle type.