Reflow soldering evaluation of lead free solder alloys

A quantitative dynamic solder wettability measurement was used to evaluate the effects of reflow processing on the wettability parameters associated with the nonlead bearing solders 96.5% Sn/3.5% Ag and 58% Bi/42% Sn. An experimental design approach employing full factorial experiments was formulated to investigate the solder wetting dependence of the reflow parameters, atmosphere, peak reflow temperature, time above liquidus, and metallization. Solder wettability was determined with respect to the final degree of spread and the extent of solder wetting onto the lands of surface mount components. The solder alloy composition of 96.5% Sn/3.5% Ag was found to exhibit better wetting characteristics than the 58% Bi/42% Sn alloy. This wetting behavior was enhanced under the reflow conditions of a nitrogen atmosphere and the use of a gold metallization. The wetting of the conventional 63% Sn/37% Pb solder alloy was improved over the comparatively processed 58% Bi/42% Sn alloy. However, the 63% Sn/37% Pb solder alloy displayed a greater sensitivity to reflow atmosphere than the 96.5% Sn/3.5% Ag alloy, which generally exhibited better wetting characteristics than the Sn/Pb alloy