Electromigration-induced accelerated consumption of Cu pad in flip chip Sn2.6Ag solder joints

The electromigration-induced accelerate consumption of Cu pad in flip chip Sn2.6Ag solder joint was studied. The under bump metallization on the chip side was 0.5 μm Ti/0.1 μm Cu/2.0 μm Ni, and on the substrate side was 20 μm thick Cu pad. The current stressing was conducted at a current density of 1.0×10⁴ A/cm². The uneven consumption morphology of Cu pad formed after reflow results in local current crowding at the cathodic pad side. The merge of the small concavities gives rise to the large cavities and wavelike morphology of Cu pad. The effect of current crowding at the wave tip caused rapid consumption of Cu pad. The entire 20 μm Cu pad was consumed at the current crowding area after current stressing of 478 h at 1.0×10⁴ A/cm². A mechanism was proposed for the accelerate consumption of Cu pad due to current stressing.