Abstract
Electromigration damage was examined under a current density magnitude higher than 105 A/cm2, which led to the void in the cathode and the hillock in the anode and consequently, the failure of the aluminum conductor. Electromigration has also become one of the failure mechanisms when the flip chip solder joint interconnects shrink to 35 μm, in which the current density magnitude will increase up to 104 A/cm2. With these, the reactions and effects of flip chip solder joint electromigration have become to be of great interest. Thus, this study investigates the electromigration of SnAg3.0Cu0.5 flip chip bumps by adopting Black’s equation to investigate the mean-time-to-failure prediction. Maximum current density was simulated using the finite element method, thereby providing a better understanding of local heat as well as current crowding. The current crowding phenomenon enhances the void formation at the entry points of the cathode side of the solder bumps. In the aspect of electromigration-induced void phenomenon, in addition to the pancake-type void failure mode proposed in the literature, a cotton-type void failure mode was also found in this study. Cross-sectional scanning electro microscopy images showed that the cotton-type void formation may due to the bump with a crack in the cathode/chip side before or during the current stressing. The cotton-type void failure mode intermetallic compounds layer at the anode/substrate side was thicker than that of the pancake-type void failure mode. In the aspect of electromigration effects on interfacial reaction, in addition to the polarity effect proposed in the literature, a more obvious tilting effect was found at the anode/chip side along the electron flow path under a higher current stressing condition. The unpassivated aluminum thin film test specimens and the four-point-bend equipment have been successfully fabricated to investigate external mechanical stress impact during the electromigration test. This study also found that once the thin films reach equilibrium after the electromigration test, the aluminum atomic concentration, originally at 74%, decreases to about 60% at the cathode and increases up to about 80% at the anode. In addition, this study revealed that applying external mechanical stress has a small but finite effect on critical length. Under the external mechanical stress range of –50 to +50 MPa tensile stress, the higher the tensile stress applied, the shorter the critical length becomes.