Electromigration in flip chip solder joints having a thick Cu column bump and a shallow solder interconnect

Abstract

In advanced electronic products, current crowding induced electromigration failure is one of the serious problems in fine pitch flip chip solder joints. To explore a strong resistance against current crowding induced electromigration failure, a very thick Cu column bump combined with a shallow solder interconnect at 100 μm pitch for flip chip applications has been studied in this paper. Results revealed that these interconnects do not fail after 720 h of current stressing at 100 °C with a current density of 1× 104 A cm2 based on the area of interface between Cu column bump and solder. The reduction of current crowding in the solder region by using thick Cu column bumps increased the reliability against electromigration induced failure. The current distribution in a flip chip joint of a Cu column bump combined with a shallow solder has been confirmed by simulation. However, Kirkendall void formation was found to be much serious and enhanced by electromigration at the Cu Cu3 Sn interface due to the large CuSn ratio. Since this is a system of a limited amount of Sn and an infinite supply of Cu, the Cu6 Sn5 transforms to the Cu3 Sn after all the Sn content in the solder bump is consumed and the Cu3 Sn can grow very thick; the vacancy flux that opposes the Cu flux will condense to form Kirkendall voids. The mechanism of electromigration induced Kirkendall void formation in the Cu column with the shallow solder joint is discussed. Furthermore, a very large temperature gradient exists across the shallow solder interconnects, leading to thermomigration. Electromigration accompanied by thermomigration could replace current crowding as a serious reliability issue in using Cu column based interconnects. © 2006 American Institute of Physics.