Introduction
With a sprayfluxing system one has the advantage to apply the flux amount in a very controlled way. The distribution of the flux over all surfaces to be wetted by the solder is however often very problematic as will be explained. The fact that most joints are still well soldered, although initially poor fluxed, will be explained.
Sprayfluxing
With a sprayfluxer flux is deposited in many single small droplets that will form a liquid film on the solder side of the board when a sufficient flux amount is applied. Such a liquid film can spread over the board surface. The liquid will however recede at the boundaries of a surface. This due to the fact that the interfacial tension between the liquid and the solid board surface can not act when there is no surface available in the same plain as the force is acting. This is the case at sharp edged surface boundaries. This means that the liquid will recede at such surface boundaries. This recession also takes place at the boundaries of the through holes. This means that there is no flux 'reservoir' available, like in foam fluxing, to fill the capillary gap between the lead and the hole wall. The few droplets that may enter this small gap during fluxing are not able to fill the whole gap with a sufficient amount of flux. For a good soldering process it is however necessary to get sufficient flux on all parts that must be wetted by the solder. Since there is lack of flux on such joints, there may also be a lack of solder penetration due to lack of solderability. Since the surfaces to be wetted are not cleaned by the flux, because that flux is not present, the solder is unable to fully wet the metal parts that must be joined. This initial lack of flux is also valid for the protruding leads. Also here a complete flux film is difficult to apply on the lead surface with a sprayfluxer.
Sufficient flux in the joint areas
The reason that most processes that make use of a sprayfluxing system still give a good solder quality is due to the fact that the solderwave, as the board enters the wave, will skim part of the flux film from the board. This moving flux film is during that process partially captured between the wave front and the board. Parts of this film will as a result spread in the capillary gap that exist between the wave front and the board. So a kind of dynamic flux reservoir is created between the board and the wave. The flux reservoir in this gap makes it possible for the flux to be pushed inside the joint areas just before the solder covers these areas. In this way the flux can still do its job on all joint surfaces. Providing that the flux amount and flux activity is sufficient.