On the failure mechanism in lead-free flip-chip interconnects comprising ENIG finish during electromigration

The aim of this research was to investigate the failure mechanism of lead-free (SAC305) flip-chip solder connections subjected to long-term electromigration. Interconnects with a nominal diameter of 60 m or 50 m were assembled in flip-chip organic packages with a pitch of 100 m. The under bump metallization (UBM) and surface finish on PCBs comprised of a 5 m thick electroless nickel immersion gold (ENIG) layer directly deposited on AlCu0.5 or Cu traces, respectively. Test vehicles were subjected to electromigration tests for over 30,000 hours at constant current densities of 8 kA/cm² or 5 kA/cm², respectively and nominal temperatures of 125 °C, 100 °C, or 28 °C until failure. Lifetime data was evaluated using Weibull statistics (as well as lognormal distribution, not shown here) and the mean times to failure (MTTF) calculated. The results were subsequently used to make an estimation of activation energy for electromigration. The evaluation yielded a value of E_a = 1.13±0.18 eV. The relatively high value of Ea points to a good robustness of the bump metallization investigated in our study conducive to a long life time under moderate operating conditions. Microstructure changes of interconnects in failed samples were subsequently thoroughly investigated by SEM and EDX. Electromigration damage was asymmetric in respect to the current flow direction through the solder bumps and occurred predominantly at the cathode contacts of the solder joints. However, contrary to findings reported in the literature, the most severe damage occurred at the PCB and not at the chip side. It was established that interconnects failed through dissolution and thinning of Ni-P layer, and eventually crack formation between Cu trace and solder. The failure was facilitated by electromigration-favored diffusion of Cu through Ni-P. ENIG deposited onto Al traces was more resilient to electromigration damage than ENIG applied to Cu.