Partial rebound of molten-metal droplets impacting on solid substrates

This study investigates droplet/wall impact events resulting in partial rebound, an overlooked regime between deposition and splash. Because of its relevance to the microelectronics manufacturing industry, high-purity 63%Sn-37%Pb molten solder is used. The target wall consists of a smooth, flat, unyielding silicon substrate. Droplet diameters range from 0.7 to 1.4 mm, and impact velocities from 0.7 to 1.4 m/s. These conditions correspond to Re = 0(1000), We = 0(10), Fr = 0(10-100), Bo = 0(0.1), and Oh = 0(10(-3)). High-speed imaging is performed to determine droplet lateral spread and apparent contact angles, and examine how these quantities affect the impact outcome (deposition vs. partial rebound). The results indicate that partial rebound is favored by higher impact velocities and smaller droplet diameters. A recent rebound model is evaluated as a predictive tool for the impact outcome in this transition impact regime. In addition, three existing correlations are evaluated in terms of their ability to predict the current experimental data on droplet maximum spread. (C) 2004 American Institute of Chemical Engineers.