Electromigration and thermomigration of Pb-free microbumps in three-dimensional integrated circuits packaging

Packaging technology is currently transitioning from flip chip technology toward three-dimensional integrated circuits (3D IC) packaging. In 3D ICs, smaller dimensions of the microbumps, approximately 10 μm, are adopted. In such small size of the microbumps, the current densities applied in the microbumps will dramatically increase. Furthermore, due to intense joule heating and current crowding, current stressing may have caused a non-uniform temperature distribution in the microbumps, and thereby a thermal gradient can be established across them. The thermal gradient is expected to be larger in the 3D IC packages, because smaller dimension of Pb-free alloys are used. Thus, addressing the electromigration (EM) and thermomigration (TM) problems of the microbumps are critical for this newly developed technology. In this study, the EM of Pb-free microbumps in 3D IC packaging under electrical current stressing from 1×10⁴ A/cm² to 1×10⁵ A/cm² at 150 °C was investigated. The resistance evolution of the samples during current stressing was found to be correlated with microstructural observation of the microbumps. In addition, EM-induced damage was observed when current density was higher than 8×10⁴ A/cm² whereas no EM-induced damage on the microbumps was found after 14416 h when the current density was below 1.5×10⁴ A/cm². We proposed that the different behaviors of EM on the microbumps were mainly due to the effect of back stress. The theoretical critical current density to trigger electromigration of Sn in this study was calculated as 7.5×10⁴ A/cm². Furthermore, when a temperature gradient was across the microbumps, TM induced failure in the 3D IC sample was highly affected by dissolution of Ni UBM at hot end, resulting in the net flux of Ni toward the cold end accompanying with enhanced growth of Ni_{3<- sub>Sn4 IMCs at lower temperature region.}