Analysis of electromigration for Cu pillar bump in flip chip package

A demand for small form factor in IC packaging has lead to a reduced bump size and an increased current density. The high current density accompanying with Joule heat induces an electromigration failure. In this study, we investigated the effects of under bump metallization (UBM) on the electromigration failure. Three types of UBM such as Cu 5 μm, Cu 10 μm and Cu 5 μm/Ni 2μm were compared with 60 μm high Cu pillar bump (CPB). The current density was fixed at 5.09·10⁴A/cm² and the temperature ranged from 130°C to 170°C. Mean time to failure data (MTTF) were obtained from Weibull distribution analysis. MTTF of CPB was longer than the others and the MTTF order was as follows; CPB >; Cu/Ni >; Cu 10μm >; Cu 5μm. As the temperature increased, MTTF decreased regardless of bump structures. This result implied that electromigration reliability of CPB was far better than other solder bumps and Cu/Ni UBM structure was more resistant to electromigration than single Cu UBM.