Electromigration in flip chip solder bump of 97Pb–3Sn/37Pb–63Sn combination structure

Abstract

Electromigration damage in the flip chip solder bump of 97Pb–3Sn/37Pb–63Sn (numbers are all in wt% unless specified otherwise) combination structure was studied after current stressing at 140 °C with a density of 2.55 × 10 4 A/cm 2 for up to 20 h. The under bump metallurgy for the 97Pb–3Sn solder on the chip side was TiW/Cu/electroplated Cu while the bond-pad for the 37Pb–63Sn solder on the printed circuit board (PCB) side was electroless Ni/Au. We observed in the thermo-electromigration test that failure occurred at the top of the bump with a downward electrical current flow while there was no failure in the opposite current polarity. The Pb atoms were found to move in the same direction as with the electron current flow. Therefore, in the case of the downward electron flow, the composition of the upper solder bump changed from 97Pb–3Sn to 83Pb–17Sn and it enabled the Cu 6Sn 5 phase to precipitate onto the chip side. Due to the precipitation and growth of the Cu 6Sn 5 intermetallic compound, the Cu under bump metallurgy was quickly consumed and the subsequent void formation induced failure.