Enhancing Sn-Ag Solder Joints Electromigration Lifetime via the Under-Bump-Metallization Structure Design

The authors demonstrate a Ni/Cu bilayer structure for the under-bump-metallization (UBM) in Sn-Ag flip chip solder joints to enhance the electronmigration lifetime. The Ni layer provides two main contributions for the lifetime improvement. First, since Ni is a good diffusion barrier for Cu, it reduces the electromigration failure by UBM dissolution. Furthermore, the Ni layer limits the current crowding effect by the current redistribution which has a good agreement with our three-dimensional simulation result. By applying infrared microscope (IR), the thermal distribution inside solder joints can be obtained as well. The hot spot temperature near the entrance point was 129°C inside Cu UBM system, while it was only 122°C in CuNi UBM system. In summery, the addition of higher resistivity Ni layer successfully redistributed the temperature and current route.