Cracking failure of Cu pillar bump caused by electromigration and stress concentration under thermo-electric coupling loads

Three current density levels, $2 \times 10 ^{4}\,\mathrm{A} /cm^{2}$, $2.5 \times 10 ^{4}\,\mathrm{A} /cm^{2}$ and $3 \times 10 ^{4}\,\mathrm{A} /cm^{2}$, were selected to conduct the electromigration tests on the Cu pillar bump samples at ambient temperatures of $100 ^{\circ}\mathrm{C}, 125 ^{\circ}\mathrm{C}$ and $150 ^{\circ}\mathrm{C}$ respectively. Scanning electron microscope (SEM) was used to observe the microstructure evolution and failure mode of Cu pillar bumps after electromigration. A finite element model is established to reveal the mechanical property degradation of Cu pillar bump caused by material migration during electromigration. The results show that, higher current density and higher ambient temperature can induce a faster electromigration of Cu pillar bump, thus results in a large number of cavities generate in the solder IMC. These cavities expanded continuously and caused more and more obvious stress concentration in the IMC during the process of electromigration. This stress concentration reached above 68MPa, exceeding the fracture strength of Cu6 Sn5, which can be partly explained the fracture failure of Cu pillar bumps observed in the experiment.