The electromigration damage in a flip-chip solder bump combination structure was studied after current stressing at 140C, with a density of 2.55 x 104A/cm2, for up to 20h. The under-bump interface with the Pb–3%Sn solder on the chip side was TiW/Cu/electroplated Cu. The bond-pad for the Sn-37%Pb solder on the printed circuit side was electroless Ni/Au. It was observed that failure occurred at the top of the bump, with a downward current flow, while there was no failure with the opposite current polarity. The Pb atoms were found to move in the same direction as the electron current flow. Thus, in the case of a downward electron flow, the composition of the upper solder bump changed from Pb–3Sn to Pb–17Sn and this allowed Cu6Sn5 to precipitate on the chip side. Due to the precipitation and growth of Cu6Sn5, the Cu under-bump surface was rapidly consumed and the consequent void formation led to failure.

Electromigration in Flip Chip Solder Bump of 97Pb–3Sn/37Pb–63Sn Combination Structure. J.W.Nah, J.H.Kim, H.M.Lee, K.W.Paik: Acta Materialia, 2004, 52[1], 129-36