Evaluation of Electromigration Behaviors of Pb-Free Microbumps in Three-Dimensional Integrated Circuit Packaging
This study investigated electromigration (EM) behaviors of Pb-free microbumps in three-dimensional integrated circuit (3D IC) packaging under electrical current stressing from 1 × 104 A/cm2 to 1 × 105 A/cm2 at ambient temperature of 150°C. EM-induced fast under bump metallization consumption at the cathode of the microbumps was observed when the current density was higher than 8 × 104 A/cm2, whereas no EM-induced damage of the microbumps was found after 14,416 h when the current density was below 1.5 × 104 A/cm2. We propose that the different EM behaviors of the microbumps were mainly due to the effect of back stress. The critical microbump height to trigger EM for different current densities is discussed, and the resistance evolution of samples during current stressing was found to be correlated with the microstructure of the samples. When the resistance was stable through the whole test period, microscopic inspection of the 3D IC samples indicated that the whole microbumps were transformed to intermetallic compounds without significant EM-induced damage. However, the resistance evolution of some misaligned microbumps exhibited a feature of an early spike along with a huge resistance fluctuation during current stressing. When the resistance abruptly increased after lengthy stressing, EM-induced void formation was observed at the cathode side of the Al trace.