Reliability and failure mechanism of copper pillar joints under current stressing

The electromigration (EM) lifetime of copper pillars were investigated by orthogonal tests. According to the Black’s mean-time-to-failure equation, the activation energy and exponent of current density were calculated to be 0.88 eV and 1.64, respectively. The microstructure evolution of the joints under current stressing was observed. It was found that the Sn solder was usually depleted before the joint failed, which means the joint was only composed of Cu₆Sn₅ and Cu₃Sn phases as a Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu structure after a period of EM test. Three failure modes were observed: failure along the Cu/Cu₃Sn interface at the cathode side, failure along the Cu/Cu₃Sn interface at the anode side and brittle fracture through the IMCs. The percentages of these three failure modes are 55, 24 and 21 %, respectively. The formation of Kirkendall voids was suggested to be the key factor for the EM failure of the Cu pillar joints. Before the Sn solder was depleted, voids were mainly formed at the Cu₆Sn₅/Sn interface at the cathode, which is dominated by the Cu flux induced by current; while after the Sn solder was depleted, voids formation is dominated by the chemical diffusion.