Early failure induced by phase transformation in microjoints for chip-level integration

The undercut issue of the CuNiSn2.5Ag microbump has been solved by reducing the Cu seed layer thickness and dry etching, but the bump height uniformity was degraded. Therefore, microjoints with different standoff heights were acquired by both thermocompressive bonding and reflow soldering. The numerical analyses indicated that the microjoints with a lower standoff sustained a higher fatigue stress under temperature cycling and the growth of Ni ₃Sn ₄ layer was more rapid because of a shorter diffusion route, which hastened the formation of micropores in the interconnection zone and was the root-cause of early failure.