As lead free assembly is ramping up, wetting of lead free solder pastes is surfacing as the major paste performance tradeoff. Global efforts to significantly increase lead free wetting chemically have proven unproductive to date. The "drop in" lead free paste with respect to wetting looks to be improbable. This paper reports the findings of numerous studies using quantitative wetting gauges to measure solder paste wetting to the PCB surfaces. Wetting results with various lead free profiles, reflow atmosphere oxygen concentrations and lead free PCB surface metallizations are compared and contrasted for their contribution to maximizing lead free wetting. Although numerous lead free alloys are on the market today, this paper concentrates on the popular SAC (Sn/Ag/Cu) alloy specifically 95.5/4/0.5 in a no clean paste. In addition to wetting, solder defects and voiding are included in the comparisons to reveal the best overall lead free reflow process.
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