Effect of under-bump-metallization structure on electromigration of Sn-Ag solder joints

The effect of under-bump-metallization (UBM) on electromigration was investigated at temperatures ranging from 135C to 165C. The UBM structures were examined: 5-μm-Cu/3-μm-Ni and 5 μm Cu. Experimental results show that the solder joint with the Cu/Ni UBM has a longer electromigration lifetime than the solder joint with the Cu UBM. Three important parameters were analyzed to explain the difference in failure time, including maximum current density, hot-spot temperature, and electromigration activation energy. The simulation and experimental results illustrate that the addition 3-μm-Ni layer is able to reduce the maximum current density and hot-spot temperature in solder, resulting in a longer electromigration lifetime. In addition, the Ni layer changes the electromigration failure mode. With the 5 μm Cu UBM, dissolution of Cu layer and formation of Cu6Sn5 intermetallic compounds are responsible for the electromigration failure in the joint. Yet, the failure mode changes to void formation in the interface of Ni3Sn4 and the solder for the joint with the Cu/Ni UBM. The measured activation energy is 0.85 eV and 1.06 eV for the joint with the Cu/Ni and the Cu UBM, respectively.