In flip-chip solder joints, thick Cu and Ni films have been used as under bump metallization (UBM) for Pb-free solders. In addition, electromigration has become a crucial reliability concern for fine-pitch flip-chip solder joints. In this paper, the three-dimensional (3-D) finite element method was employed to simulate the current-density and temperature distributions for the eutectic SnPb solder joints with 5-μm Cu, 10-μm Cu, 25-μm Cu, and 25-μm Ni UBMs. It was found that the thicker the UBM is the lower the maximum current density inside the solder. The maximum current density is 4.37 × 10 4 A/cm 2 , 1.69 × 10 4 A/cm 2 , 7.54 × 10 3 A/cm 2 , and 1.34 × 10 4 A/cm 2 , respectively, when the solder joints with the above four UBMs are stressed by 0.567 A. The solder joints with thick UBMs can effectively relieve the current crowding effect inside the solder. In addition, the joint with the thicker Cu UBM has a lower Joule heating effect in the solder. The joint with the 25-μm Ni UBM has the highest Joule heating effect among the four models.
- Flip chip solder joint
- Under bump metallization