Recent advances on kinetic analysis of electromigration enhanced intermetallic growth and damage formation in Pb-free solder joints

A comprehensive kinetic analysis was established to investigate the electromigration (EM) enhanced intermetallic compound (IMC) growth and void formation for Sn-based Pb-free solder joints with Cu under bump metallization (UBM). The kinetic model takes into account Cu-Sn interdiffusion and current stressing. Derivation of the diffusion coefficients and the effective charge numbers for the intermetallic compounds is an essential but challenging task for the study of this multi-phase multi-component intermetallic system. A new approach was developed to simultaneously derive atomic diffusivities and effective charge numbers based on simulated annealing (SA) in conjunction with the kinetic model. A consistent set of parameters were obtained, which provided important insight into the diffusion behaviors driving the IMC growth. The parameters were used in a finite difference model to numerically solve the IMC growth problem and the result accurately correlated with the experiment. EM reliability test revealed that the ultimate failure of the solder joints was caused by extensive void formation and subsequent crack propagation at the intermetallic interface. This damage formation mechanism was analyzed by first considering vacancy transport under current stressing. This was followed by a finite element analysis on the crack driving force induced by void formation. This paper is concluded with a future perspective on applying the kinetic analysis and damage mechanism developed to investigate the structural reliability of the through-Si-via in 3D interconnects.