Comparison of the electromigration behaviors between micro-bumps and C4 solder bumps

As flip chip packages continue to migrate to Pb-free solder and micro-bumping (μbump) for 3D IC stacking, achieving high reliability with reduced dimension and concurrent increase in current density has become one of the major challenges for microelectronic packaging systems. In this study, the electromigration (EM) performance of the nominal C4 and μbump is compared. During EM tests, the resistance increase in μbumps is mainly due to intermetallic compound (IMC) growth, in contrast to the void formation and UBM/ IMC dissolution in C4 bumps. Early EM failures which were frequently observed in C4 bumps were mostly eliminated in the μ bump structures. Stress modeling was correlated with experimental observations. A proper design of the bump schemes can effectively distribute the current more uniformly, thus reducing the current crowding and local joule heating effects. Moreover, Sn grain orientation was found to critically affect the EM performance of both the μbump and the C4 bump joints.