Electromigration-induced UBM consumption and the resulting failure mechanisms in flip-chip solder joints

Eutectic PbSn flip chip solder joint was subjected to 5 X 10(3) A/cm(2) current stressing at 150 degrees C and 3.5 X 10(4) A/cm(2) current stressing at 30 degrees C. The under bump metallurgy (UBM) on the chip was sputtered Ni/Cu, and the substrate side was a thick Cu trace. It was shown through in-situ observation that the local temperature near the entrance of electrons from the Al interconnect to the solder became higher than the rest of the joint. The accelerated local Ni UBM consumption near the entrance was also observed. Once the Ni was consumed at a location, a porous structure formed, and the flow of the electrons was blocked there. It was found that the formation of the void and the formation of the porous structure were competing with each other. If the porous structure formed first, then the void would not be able to nucleate there. On the other hand, if the void could nucleate before the UBM above lost its conductivity, then the joint would fail by the void formation-and-propagation mechanism.