Selective soldering is a process with more than one hundred different parameters that may impact soldering performance. Some conditions change over time, e.g. machine temperature, humidity, contamination, wear of parts or settings after maintenance. Other factors in the process include the materials used, component wettability, solder mask surface energy, board material Tg and Td values, solder oxidation and composition.
During production, conditions may begin to drift. In order to avoid solder defects, statistical process control or SPC is the best method of identifying unexpected changes in the process. When using this statistical technique, however, it is important that the machine have the tools to measure these essential process parameters and if necessary control them to maintain a robust soldering process. Apart from machine parameters, the materials have a big impact. A robust selective soldering process should have a wide process window that is able to handle variations in material quality.
In this paper, critical process parameters are discussed as well as methods that can be used to widen the process window. Additionally, process robustness is evaluated.