by Sunil Chhabra, Sergio Porcari, Steven Rocco Marongelli, Dr. Richard Ludwig, Dr. Ning-Cheng Lee
In order to achieve ultra-fine dot solder paste dispensing, both solder material and dispensing equipment have to be optimized. Dispensability of solder paste was evaluated in terms of “dispensing rate”, consistency of dispensing rate, and the stability of dispensing rate with time. Within the given conditions, threshold values for dispensability seem to exist for viscosity, powder size, and metal content. Small nozzle inner diameter is definitely needed to deliver a small dot size. Archimedes Metering Valve shows a greater flexibility in metering the volume than Positive Displacement Pump, primarily due to a greater sensitivity in dispensing volume to variation in pressure, and nozzle ID, besides being very sensitive to variation in encoder count. For success in high speed ultra-fine dot dispensing process, solder pastes with a low viscosity, small powder size, low metal content, and a high thixotropy are desired to deliver a high dispensing throughput. Controlwise, a high pressure and high encoder count may be promising. The consistency improves with increasing metal content, thixotropy, pressure, nozzle size, and encoder count. Viscosity, powder size, and delay time appear to have negligible effect on consistency. The stability increases with increasing flux activation temperature, and is expected to be poor for low thixotropy and low viscosity. Large powder size may cause immediate clogging, while small powder size may cold weld under repeated pressure cycling using pneumatic pump systems. In general, a very careful design and tight control of parameters discussed in this work has to be implemented in order to succeed in ultra-fine dot solder paste dispensing.
lead-free, pb-free, piston, Archimedes, fine dot, pump, SMT, surface mount, flux, solder paste, dispensability, dispensing
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Posted on 1 Jan 2009
