Long-term Electromigration Study of Lead-Free Flip-Chips with Solder Bumps with 50 μm or 60 μm Diameter Employing ENIG Surface Finish on Both Chip and Substrate Side

Abstract

Electromigration behaviour of Pb-free solder joints in flip-chip interconnects is usually studied in highly accelerated, short-term experiments using high current density and temperature. Failures typically occur in bumps which are in cathode contact at the chip side. There are only a few published studies in which Electroless Ni-P/Immersion Au (ENIG) surface finish was used as under-bump-metallization (UBM) structure, e.g. [5]. This paper deals with the long-term electromigration behaviour of Pb-free SAC305 flip-chip solder joints with a pitch of 100 μm and solder bump diameters of 50 μm or 60 μm, respectively. The ENIG surface finish was used on both the substrate and chip side. Test specimens were subjected to several levels of temperature and current density and tested up to 16,000 hours. The life time data is summarized using Weibull and lognormal distribution. The microstructure changes of interconnects in failed samples were subsequently investigated by SEM and EDX. Interconnects had failed due to consumption of Nickel, voids caused by electromigration, and Kirkendall void formation in the Ni-P-layer. The damage was asymmetric in respect to the current flow direction through the solder bumps and was most pronounced at the cathode side. Unexpectedly, however, the most severe damage occurred at the substrate and not at the chip side. We could show that - allowing for a few guidelines - lead-free flip-chip solder joints with 50 μm or 60 μm diameter have a sufficient electromigration life time for most applications.