3-D simulation on current density distribution in flip-chip solder joints with thick Cu UBM under current stressing

Abstract

In flip-chip solder joints, Cu has been used as a underbump metallization (UBM) for its excellent wettability with solders. In addition, electromigration has become an crucial reliability concerns for fine-pitch flip chip solder joints. In this paper, 3-D finite element method was employed to simulate the current density distribution for the eutectic SnPb solder joints with 5 μ, 10 μm, and 20 μm thick Cu UBM. It was found that the thicker the UBM is, the lower the maximum current density inside the solder. The maximum current density decreased from 4.37 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to 7.54 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> when the thickness of the UBM changed from 5 μm to 20 μm. Thicker Cu UBM can effectively relieve the current crowding effect inside the solder.