Effects of electromigration on microstructural evolution of eutectic SnPb flip chip solder bumps

The flip chip solder bump was investigated in situ with current stressing in an ambient temperature of 423 K. For the in situ investigation of the solder joints, a single flip chip package was examined during whole investigation time. Cross-sectional studies were conducted with the scanning electron microscope (SEM). Cu mini bumps were formed on a Si chip for the solder bumping and electroless Ni-immersion Au (ENIG) surface treatment was conducted on the Cu pads in substrate side. Cu6Sn5 and Ni3Sn4 intermetallic compound (IMC) layers were formed in the both interface, while a typical eutectic structure of Pb-rich and Sn-rich phases were formed within the solder region. After current stressing of 6 h, separation of the Pb-rich and Sn-rich phases were occurred in the left solder bump, while a void was formed under the Cu6Sn5 IMC layer in the right side solder bump. After current stressing of 7.5 h, the solder joints were catastrophically failed due to the remelting of the solder bump.