COATER TRACK FLUX REFLOW FOR SOLDER BUMPS
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-23
During the process of electroplating solder bumps on wafers it is necessary to reflow the solder to reform the electroplated solder into a ball. The temperature of the reflow process is a function of the melting point temperature of the solder being used. For the case of Pb-Sn alloys with about 63 wt. % Sn the reflow temperature is around 240°C. For this alloy composition it is important to keep the reflow temperature as low as possible in order to minimize the amount of reaction between the Sn in the solder and the Cu portion of the bump under- neath. Furthermore, during the processing steps prior to the reflow the as-electroplated solder becomes oxidized, and this oxide prevents the solder from reflowing into the proper shape. Therefore, the reflow must take place in an environ- ment which allows for reduction of the oxides, and prevention of further oxidation. Coating the wafers with solder flux provides the required atmosphere, and allows the reflow to occur at relatively low temperatures.