There are a number of reasons for unfilled joints following wave soldering, several of which are related to fluxing. The flux is required so that the solder alloy will wet the component lead and the metallisation on the printed circuit board (PCB). The action of the flux will be impaired if:
- it is contaminated
- it is too dilute (too much thinner added)
- its specific gravity is too high
- its application is inconsistent (e.g. blocked nozzle, uneven foamer)
- the air knife is set too high (flux blown off)
- the preheat is set too high (flux activity used up).
Any of these may lead to skipped joints, assuming that the flux is suitable in the first instance for the materials being soldered.
Another possible cause is the solder wave itself being inconsistent, especially with palleted PCBs. The solder height may be too low, the wave not level, or it may have gaps caused by blockages in the flow.
If a joint is consistently skipped, then it is likely that a design issue may be the cause. Shadowing of a pad or hole can cause the solder to miss that joint despite the materials and process parameters appearing adequate. Shadowing can be caused by a component or by an inappropriately designed carrier.