Dynamic surface tension measurements of molten Sn/Pb solder using oscillating slender elliptical jets

The present study investigates the dynamic surface tension of molten 63%Sn–37%Pb eutectic solder at timescales 1–3 ms applicable to solder jetting applications in microelectronics manufacturing. The oscillating jet method is employed to measure the surface tension of the melt when ejected into nitrogen atmospheres containing controlled amounts of oxygen from 15 ppm to 21% (mol). Typical jet ejection conditions correspond to We=0.16, Re=49, and Fr=165. The molten solder is steadily forced through an elliptical capillary (aspect ratio of ∼2), thus forming a stationary oscillating jet. The shape evolution of the jet is affected by the liquid/gas interface properties and is used, along with an existing theoretical model, to determine the surface tension of the liquid metal as a function of distance (likewise, surface age) from the orifice. The experimental data reveals dynamic surface tension behavior at 3700 ppm O2 concentration, and indicates a characteristic time below 1 ms for the molten-metal surface to be saturated by adsorbed oxygen at ambient O2 concentrations of 1% or higher. Mass diffusion in the gas phase is found to be the rate limiting process controlling oxygen adsorption on the solder melt surface. The results provide crucial information needed to reduce solder surface degradation, which hampers the successful implementation of solder jetting technology.