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, the 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 micrometers (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 experiments described show that 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 gas flow rates of 2.5 L/min and 5.0 L/min, respectively. The numerical analysis in this study models the gas flow effects for a wide range of chip sizes, solder geometry, and gas flow direction. In the numerical analysis, fluid computation and solder force calculation are conducted to study the gas flow effects on chip displacement that is the distance away from the well aligned position along the gas flow direction. The results show that the gas flow effects are related to many factors including chip size, gas flow rate, solder height, and flow direction. For a one-dimensional laser array, these 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 micron-level precision alignment.