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Preheating in Selective Soldering

Preheating is a necessary step in the soldering process to enable the evaporation of the flux solvent and to prepare the flux activators. Also preheating will assist the soldering process since it reduces the thermal energy that must be supplied by the solder to create a sound joint.  In the soldering process an increased preheating level might help in getting better solder results even with lower wave temperatures.

The maximum preheater settings are commonly mentioned in the flux data sheet. It is important not to preheat over this recommended maximum, since otherwise the flux activity during soldering may be affected in a negative way.  This can then result in more solder bridging, flags or solder webbing as these defects are the result of a flux that is exhausted before the soldering process was finished.  It is important that the flux is able to be active until the process is finished, e.g. when the joints left the solder wave. Too much preheating or a too long dwell time may cause flux exhaust.

Another limitation for the maximum preheating temperature for a specific board can be the thermal limitations of the components that are used. The maximum allowable temperature profile for a specific component can be found in the components data sheet.

Another point of consideration is if the cycle time allows a longer time above the preheater in order to get a higher preheat temperature if needed. It might in some cases be necessary to increase the dwell time over the preheat section, or to use two preheater sections to get the best setting.  The solder nozzle that has to deliver the highest thermal demand during the soldering process dictates the solder temperature. The best setting depends on the nozzle size and the mass of joints that must be soldered with that nozzle. Inmost cases the smallest nozzle will dictate the necessary solder pot temperature for a good soldering results on all joints. This is because the relatively small solder mass in such nozzles will in general cool most rapidly.

If all joints are soldered well at a given setting one could try to reduce the solder pot temperature using the same dwell time. As long as the solder quality is OK it is beneficial to use this lower temperature setting. Lower temperature settings will create less dross and solder contamination. Also it will give less thermal load to the soldered joints and components.

If the board needs a MultiWave and a SelectWave soldering treatment, then the MultiWave soldering should be the first process. In this way the board temperature is kept as equal as possible. This is important in view of the thermal expansion of the board in relation to the nozzle plate, since they must have a perfect match during the soldering process.

For soldering with the SelectWave, which is often a more time consuming process than MultiWave soldering, the preheating and the general board temperature are less critical for a good soldering process as long as the flux was preheated well.

Jeff is the author of this solution article.

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