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Electromigration is a reliability concern of microelectronic interconnections, especially for flip chip solder bump with high current density applied. This study shows that with the line-to-bump geometry in a flip chip solder joint, the current density changes significantly between the Al trace and the bump, while the current crowding effect generates more heat between them. This large Joule heating under high current density can enhance the migration of Sn atoms at the current entrance of the solder bump, and cause the void formation at the entrance point. The present study finds two kinds of electromigration failure modes at the cathode/chip side of the solder bump: the pancake-type and the cotton-type void. The experimental finding shows that the effects of polarity and tilting are key factors to observe in the electromigration behavior of SnAg3.0Cu0.5 solder bumps. Consequently, this study has designed a 3-D numerical model and a corresponding test vehicle to verify the numerical finding. The maximum current density is simulated through the finite element method to provide a better understanding of local heat and current crowding. This study finds that the current crowding ratio is reduced linearly while the void formation is increased. Furthermore, it is concluded that there is a linear relationship between the growth of the intermetallic compound (IMC) layer and the applied current density at the anode/substrate side.