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The micro-droplet behavior of a molten lead-free solder in an inkjet printing process

An experimental investigation on the droplet formation of molten Sn3.0 wt%Ag0.5 wt%Cu alloy by an inkjet printing process was conducted. The printing process used a piezoelectric print head with a nozzle orifice diameter of 50 µm. Micro-droplets of a molten lead-free solder were ejected at 230 °C. The print head was driven by a bipolar pulse 40 V in amplitude. The major variables for this study were two pulse times; trise/tfinalrise and tfall, as well as N2 back-pressure in the molten solder reservoir. Under various printing conditions, extrusion of the liquid column, contraction of liquid thread and pinch-off of liquid thread at nozzle exit were observed by monitoring the dynamics of the molten solder droplet ejection process. The droplet formation was found to be insensitive to trise and tfinalrise in the range of 250–1000 µs. The behavior of droplet formation was, however, significantly affected by the transfer rate, tfall, in the range of 30–60 µs and tfall of 50 µs yielded the most desirable condition of single droplet formation. The N2 back-pressure was also found to be critical, where a back pressure between 10 and 21 kPa could give the desirable single-droplet formation condition.