Copper dendrite formation on laser fuse structures of flip chip die

Copper dendrites on a flip chip die was analysed by SEM/EDX, FIB and TEM. The dendrites were found to consist of two layers. The upper layer visible under optical and SEM inspection consist of a carbon rich copper compound. TEM analysis revealed another layer under this carbon rich layer that consist of copper, tin and lead compounds. A possible mechanism of dendrite formation due to an electrochemical cell consisting of C4 bumps, copper at laser fuse structures and a moisture or flux electrolyte is proposed.