Experimental and modeling study of high-viscosity silicone jet dispensing process for LED packaging

Silicone jet dispensing is the next generation dispensing method for LED packaging. However, as the jet dispensing is a high speed but micro dimensional transient process, the jet dispensing process is difficult to observe and measure, which brings many difficulties in dispenser designing and parameters optimization. This paper experimental studied the silicone jet dispensing process using a high-speed camera and numerically modeled it. The experiment demonstrates that 1) silicone jet dispensing process can be divided into five stages: jetted out, bridging, thinning, breaks up and recoils; 2) hundreds of milliseconds are needed for a whole dispensing process with high viscosity silicone; 3) several tenths of the energy stored in the spring is consumed by the friction. Reducing the friction, such as coating the needle, may be helpful for high viscosity silicone dispensing. The numerical model is verified by the experiment, and needle motions, forces on the needle, pressure at nozzle tip, and silicone jetting velocity during jet dispensing process are investigated. Simulation demonstrates that the needle motion can be divided into three stages: moving stage, oscillating stage and stop stage. The needle oscillation may the reason for causing satellite droplets. This study may provide useful insights for dispenser designing and optimization.