Electromigration of flip chip solder bump on Cu/Ni(V)/Al thin film under bump metallization

The electromigration of a flip chip solder bump (eutectic SnPb) has been studied at temperatures of 100, 125 and 140/spl deg/C and current densities of 1.9 to 2.75/spl times/10/sup 4/ A/cm/sup 2/. The under-bump-metallization on the chip side is thin film Al/Ni(V)/Cu and on the board side the bond-pad is thick electroless Ni coated with 30 nm of Au. When stressed at the higher current density, the mean-time-to-failure (MTTF) was found to decrease much faster than was expected from the published Black's equation. The sequence of void propagation is observed at the cathode side. This is due to the current crowding at the contact between the solder bump and the thin UBM. This is confirmed by simulation of current distribution in the solder bump. The Cu-Sn intermetallic compounds formed during reflow are known to adhere well to the thin film UBM, but they were dissolved to the solder bump after current stressing. Therefore, the UBM itself, besides the solder bump, could be part of the reliability problem of the flip chip solder joint under electromigration.