Effect of underfill on electromigration lifetime in flip chip joints

Electromigration phenomena in Sn–3Ag–0.5Cu and In solders in flip chip joints were investigated. An electromigration lifetime increased with underfill, and significant solder joint deformation toward the cathode occurred without underfill. Thus, a larger internal stress was induced in the solder joint by the underfill, and the internal stress was relieved by the solder deformation. A greater Young's modulus of an underfill created a greater back stress, resulting in a longer electromigration lifetime. However, the effect of the underfill disappeared at high current density.