Effects of preexisting voids on electromigration failure of flip chip solder bumps

Abstract

For the first time, the effects of preexisting voids inside the flip chip solder bump and at the interface of bump/underbump metallization (UBM) are considered on electromigration-induced failures to provide a mechanistic understanding on solder bump electrical reliability. Local peaks of current density due to preexisting voids supported by high operating temperature and temperature gradients and gradients of mechanical stress result in inhomogeneous drift of metal atoms, which leads to the formation of hillocks and more microvoids or large voids in the bump or the bump interfaces. The voids decrease the cross-section of the solder contact, which increases local current density and local resistance. This expected positive feedback cycle can eventually lead to the so-called electromigration failure. In the meantime, the increased volume of the extruded hillocks under electromigration can lead to circuit shorts between neighboring bumps, where the distance becomes closer and closer. A stochastic approach is employed to quantify the effects of various preexisting voids. It is found that preexisting voids at the interface of solder bump/UBM is more prone to growth during electromigration and therefore, a much shorter lifetime of the bump. Voids inside the bump can also speed up the failure of the solder bump and their effects depend on their location, size and total volume fraction. The simulation results can be applied for the reliable design of flip chip solder bumps by modifying the design rule with the consideration of the effects of preexisting voids.