The effect of under-bump-metallization (UBM) on electromigration was investigated at temperatures ranging from 135 o C to 165 o C. 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 Cu 6 Sn 5 intermetallic compounds are responsible for the electromigration failure in the joint. Yet, the failure mode changes to void formation in the interface of Ni 3 Sn 4 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.

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This paper uses an interesting specific case study to highlight the non-destructive fault isolation demonstration of 3DIC stacked dies applied the optical beam induced resistance change (OBIRCH) approach.