Superior reliability of Fe-45Ni UBM in electromigration

Fe-Ni film with compositions of 45wt.% Ni, which is also denoted as Fe-45Ni, was used as under bump metallization (UBM) to evaluate the interfacial reliability of SnAgCu/Fe-Ni solder joint under electromigration (EM) test. For comparison, commercial Cu UBM was also adopted in the reliability test. The microstructural evolutions of these two UBM solder joints were observed by scanning electron microscope (SEM). For SAC/Cu solder joints, the competition between the consumption of Cu cathode and the solder voiding led the ultimate failure, while the SAC/Fe-45Ni solder joints kept at a perfect condition without any signs of failure after 1000 hours EM test. The characteristic lifetime calculated by Weibull analysis for Fe-45Ni UBM was 2340 hours, which is three times longer than that of Cu UBM (698 hours), demonstrating an outstanding performance for Fe-45Ni UBM in electro-migration resistance. Accordingly, the superior EM resistance can be attributed to the reasons below. Firstly, Fe-45Ni UBM has an excellent barrier effect for Cu diffusion, due to the low diffusivity and high activation energy of interfacial intermetallic compounds (IMCs). Secondly, Fe flux originating from EM effect transports in the opposite direction to the diffusion flux under thermal effect at the cathode, thus it partly counteracted the diffusion flux and slow down the dissolution process of Fe-45Ni cathode.