Gas flow effects on precision solder self-alignment

Self-aligning soldering technology is being developed for low cost, passive, precision optical alignments. To avoid contamination problems, a solder reflow process must use reacting or inert gas instead of chemical flux materials. Since the accuracy of these optical alignments should reach the range of a few μm, the gas flow may affect the aligning process. Therefore, the effects of the gas flow on the self-aligning process must be understood. The present experiment shows that the gas flow effects do exist. The top plate, 8.4 mm×8.4 mm, can be moved by the gas flow by as much as 4.5 μm and 7.8 μm at the gas flow rates of 2.5 L/min and 5.0 L/min, respectively. The numerical analysis in this study examines the gas flow effects for a wide range of chip sizes and in some particular solder shape and flow directions. In the numerical analysis, fluid computation and solder force calculation are conducted to study chip displacement caused by the gas flow. The results show that the gas flow effects are related to many factors including chip size, gas flow rate, solder height, flow directions. For a one-dimensional laser array, theses effects are negligible because the chip size is very small. However, for a chip larger than 5 mm×5 mm, the effects should be controlled for μm-level precision alignment