Microstrueture evolution during electromigration in eutectic SnPb solder bumps

摘要

A technique has been developed to facilitate analysis of the microstructural evolution of solder bumps after current stressing. Eutectic SnPb solders were connected to under-bump metallization (UBM) of Ti/Cr-Cu/Cu and pad metallization of Cu/Ni/Au. It was found that the Cu_6Sn_5 compounds on the cathode/chip side dissolved after the current stressing by 5 X 10~3 A/cm~2 at 150 deg C for 218 h. However, on the anode/chip side, they were transformed into (Ni_x, Cu_(1-x))_3Sn_4 in the center region of the UBM, and they were converted into (Cu_y, Ni_(1-y))_6Sn_5 on the periphery of the UBM. For both cathode/substrate and anode/substrate ends, (Cu_y, Ni_(1-y))_6Sn_5 compounds were transformed into (Ni_x, Cu_(1-x))_3Sn_4. In addition, the bumps failed at cathode/chip end due to serious damage of the UBM and the Al pad. A failure mechanism induced by electromigration is proposed in this paper.