The impact of different under bump metallurgies and redistribution layers on the electromigration of solder balls for wafer-level packaging

Electromigration performance has been characterized for lead-free solder balls in wafer-level packaging for different solder metallurgy, under bump metallurgy thickness, and redistribution layer thickness and composition. The electromigration lifetimes in this study were found to strongly correlate with the thickness of under bump metallurgy as well as redistribution layer, spanning more than an order-of-magnitude in median time to failure. Also, a redistribution layer comprising of a Ni/Cu bilayer led to a significant lifetime improvement over its Cu-only counterpart, while a change in solder composition did not affect lifetime. Through extensive failure analysis, the differences in lifetimes can be linked to the amount CuSn formation as determined by the under bump metallurgy thickness as well as the location of the CuSn formation as determined by the redistribution layer thickness. Finally, activation energy has been characterized for a process leg with Cu redistribution layer and under bump metallurgy to be 1.34eV, and a current density exponent to be 3.8.