Electromigration in Pb-free solder bumps with Cu column as flip chip joints

The electromigration in flip chip joints of Cu column bump with two different kinds of solder bumps; eutectic SnPb or eutectic SnAg was studied. The Cu columns were 50 /spl mu/m thick and 50 /spl mu/m diameter for eutectic SnPb solders and 80 /spl mu/m thick and 85 /spl mu/m diameter for eutectic SnAg solders. We observed that these flip chip joints did not fail after 1 month at 100 /spl deg/C with a current density of 1/spl times/10/sup 4/ A/cm/sup 2/. In these flip chip joints of Cu column bumps with solder bumps, the current distribution in the solder region was uniform and they had strong resistance against electromigration induced failure. The effect of Cu column bump on current distribution in the solder joint has been confirmed by simulation. When we compared the difference in microstructural change between the Cu column with eutectic SnPb and the Cu column with eutectic SnAg, we found that electromigration enhanced the formation of Kirkendall voids at the Cu/Cu/sub 3/Sn interface in the eutectic SnPb case due to t he smaller amount of Sn in the solder bump. Therefore, the flip chip joints of Cu column bumps with eutectic SnPb solder bumps would be weakened at the interface after high current stressing.