A comparison study of electromigration performance of Pb-free flip chip solder bumps

The electromigration of flip chip solder joints is an ongoing reliability concern for manufacturers of integrated circuit (IC) components and electronic systems. As power levels of ICs continue to increase, current densities within individual solder bumps often increase, along with the operation temperatures of the die surface. Both of these factors have detrimental impact on the electromigration (EM) performance of the solder bumps, as they provide increased driving forces for the diffusion and dissolution of the under bump metallization (UBM) into the solder bumps. Additionally, these changes are occurring concurrently with the transition to Pb-free solder bumps for the next technology node. Compared to commonly used high-Pb bumps, Pb-free bumps typically have reduced lifetime with similar under bump metallization. In this study, a specially designed test vehicle was used to assess the EM performance of Pb-free solder bumps. Electroplated Sn2.6Ag solder bumps were tested with a variety of current density and temperature combinations. Lifetime data were collected for each of the conditions and a prediction model is established using Black's equation. The results were also compared with previous work done on high-Pb and eutectic SnPb bumps using the same test vehicle. From this work, Pb-free solder bumps are shown to have slightly decreased lifetime compared to high-Pb bumps, but still significantly higher than eutectic SnPb solder bumps. With the correct design strategies, including optimized via opening sizes and under bump metallization materials, SnAg Pb-free bumps can provide sufficient reliability for a majority of application conditions.