Electromigration Behavior of Sn-3.5Ag Solder Joints with (110) and (111) Nanotwinned Cu UBMs

The effect of UBM type ((111) and (110) nt-Cu, polycrystalline Cu) on electromigration behavior of solder joints with different orientations $\beta-\mathbf{S n}$ grains was investigated. The dissolution of cathode substrate and the growth of IMC at anode interface in solder joints with (111) and (110) nt-Cu UBM were slower than those with pc-Cu, and inhibitory effect of (111) nt-Cu was more significant. The electromigration lifetimes of (111) and (110) nt-Cu solder joints with medium and large angle $\theta \beta{-}$ Sn were longer than those of pc-Cu solder joints. The primary failure mode of solder joints with medium angle $\theta \beta-\mathbf{S n}$ was the excessive dissolution of cathode UBMs, and that with large angle $\theta \beta-\mathbf{S n}$ grains was the voids propagation at cathode. Therefore, nt-Cu acting as UBM could improve the electromigration reliability of solder joints.