Electromigration in flip chip solder joints has become a reliability issue in packaging technology because it can occur with a low current density, as low as 1×10
. The cathode and anode interfaces of a solder joint are the places of vacancy flux divergence and the sites of failure due to void formation. Current crowding occurs at the cathode interface where electrons flow into the solder bump, and it induces a unique failure mode of pancake-type void growth across the contact interface. The use of thick Cu UBM has removed the issue of current crowding, but the joint has only a very thin layer of solder which can be converted completely into intermetallic compound. In a very thin solder joint, the effect of electromigration induced back stress is overwhelmed by the chemical reaction at the interfaces.