The failure behavior of the fine pitch SnAg bump under current stressing

As the miniaturization of electronic products leads the trend, the dimensions of the solder joints devote to scaling down. With decreasing the size of the solder bump, the current density gets higher. EM becomes a serious reliability issue. The general bump pitch of the solder bump is reduced from 270 μm to below 150 μm due to the demand of miniaturization. In this work, the influence of two Ag-contents of SnAg solder bump on EM lifetime and failure behavior in the fine pitch solder bump of flip chip package is presented. The two SnAg solder, Sn2.4Ag and Sn1.3Ag, were stressed with 4.5×10⁴A/cm² at 150°C and 160°C. It indicated the EM lifetime of Sn1.3Ag is close to Sn2.4Ag under the condition of 4.5×10⁴A/cm² and 150°C. However, Sn2.4Ag content could decrease the rate of descent of lifetime at 160°C. Accompanying accelerating failure modes led by current crowding and consumption of the UBM layer, the Ag content has no effect on EM-induced failure. The changes of electrical resistance with the current stressing time has three modes of variation relation. Voids were observed at the cathode side of solder bump that led to the eventual open circuit. The void formation in the corresponding resistance trends versus current stressing time was clarified. Most scattering Ag₃Sn particles were found in both Sn1.3Ag and Sn2.4Ag solder bumps after current stressing. This indicates Ag atom is not easy driven by current stressing.