Microstructure evolution during electromigration in eutectic SnPb solder bumps

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 Cu6Sn5 compounds on the cathode/chip side dissolved after the current stressing by 5 x 10(3) A/cm(2) at 150 degreesC for 218 h. However, on the anode/chip side, they were transformed into (NixCu1-x)(3)Sn-4 in the center region of the UBM, and they were converted into (Cu-y,Ni1-y)(6)Sn-5 on the periphery of the UBM. For both cathode/substrate and anode/substrate ends, (Cu-y,Ni1-y)(6)Sn-5 compounds were transformed into (Ni-x,Cu1-x)(3)Sn-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.