In a flip chip solder joint, the cross-section of the solder bump is one to two orders of magnitude bigger than that of an interconnect wire. At the contact interface between the bump and the wire, a very large current crowding occurs and it causes a unique and fast electromigration failure in the bump. Simulation of the current crowding phenomenon in a flip chip solder bump is reported here. Experimental results of real flip chip solder bumps show that void formation begins near the current crowding region of the contact, and after it is nucleated, it spreads quickly across the contact area. By designing the solder bump to achieve a uniform current distribution, we can improve its electromigration resistance and increase its current carrying capacity.