The eutectic Sn-3.5wt.%Ag solder alloy is expected to be one of the candidates for Pb-free solder bump materials. But it is difficult for the conventional Sn-Ag alloy plating method to obtain eutectic composition with good uniformity within wafer, and to control the composition of plating solution for long term product run. Therefore, we have developed the 2-step electro-plating bump process using separate plating reactors for Ag and Sn. The eutectic Sn-Ag alloy bumps were easily obtained by annealing the Ag/Sn metal stacks sequentially electroplated. It is easy to control the Sn-Ag alloy composition by changing the thickness ratio of Ag layer to Sn layer in an Ag/Sn metal stack. Furthermore, the 2-step electroplating bump process does not need to control the content ratio of Ag to Sn in a plating solution. The eutectic Sn-Ag solder bump process described here is confirmed to be easily applied to mass production.
