Electromigration (EM) in solder joints under high current density has become a critical reliability issue for the future high density microelectronic packaging. This paper presents atomic density redistribution algorithm for predicting electromigration induced void nucleation and growth in solder joints of Chip Scale Package (CSP) structure. The driving force for electromigration induced failure considered here includes the electron wind force, stress gradients, temperature gradients, as well as the atomic density gradient, which were neglected in many of the existing studies on electromigration. The simulation results for void generation and time to failure (TTF) are discussed and correlated with the previous test results. EM sensitivity analysis is also performed to investigate the effect of EM parameters and mechanical properties of material on electromigration failure. The simulation results indicated that the atomic density on the activation energy is quite sensitive, and the mechanical material parameters have no impact on EM sensitivity of normalized atomic density.