Mutual Electrostatic Interactions between Closely Spaced Charged Solder Droplets

Emerging technologies of net-form manufacturing and electronic packaging rely on the use of precisely deposited molten metal droplets with sizes on the order of 100 μm in diameter. In many technological realizations, closely spaced droplets are electrostatically charged and deflected onto a substrate in a manner similar to ink-jet printing in order to “print” fine features onto a board for electronics applications or onto a substrate for net-form manufacturing. Unlike ink-jet printing, the aforementioned emerging technologies require the printing of large lateral dimensions onto the substrate by means of electrostatic charging and deflection (on the order of several centimeters), and hence these applications require the droplets to have significantly higher charges than in the ink-jet printing technology. The high charges of the closely spaced droplets can lead to inter-droplet electrostatic interactions that will cause significant deviations in the droplets’ trajectories. Hence, the understanding of the physics of inter-droplet electrostatic interactions is of primary importance in order to assure the fidelity of the net-formed component or the printed electronic package. In this work, we present a model that predicts the trajectories of the droplets when charged and deflected and corresponding experimental validations. Conditions for which electrostatic interactions are negligible are sought.