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74 SMT Magazine • April 2016 by Gerjan Diepstraten VITrONICS SOLTEC B.V. abstract Selective soldering is a process with more than 100 different parameters that may im- pact soldering performance. Some conditions change over time (e.g., machine temperature, humidity, contamination, wear of parts or set- tings 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, statisti- cal 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 has the tools to measure these essential process parameters and if necessary control them to maintain a robust soldering process. Apart from machine param- eters, 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 pro- cess parameters are discussed as well as methods that can be used to widen the process window. Additionally, process robustness is evaluated. introduction Selective soldering machines are available in many different configurations, primarily be- cause the assemblies are very different, not only in design, width and mass, but also in volume and in the number of solder joints. Nevertheless, they all require the same three sub-processes of fluxing, preheating and soldering. These processes can have different methods, but in the end all main parameters are identical to create a solder connection: the Achieving Repeatable, Consistent Control over the Selective Soldering Production Process fEaTu rE