Introduction
The flux needed for the soldering process can be applied in several ways. Nowadays foam- and spray-fluxing are mostly used, while ultrasonic mist fluxers are sometimes used for low solids fluxes as well.
It is important, when open contacts are on the board, that flux contamination should be avoided on such contacts. Although foam-fluxing seems to be the safest method, it is often not the case as will be explained.
Foam-fluxing
The foam-fluxer is a relatively cheap fluxing system, which will apply an uncontrolled amount of flux on the solder side of the boards. Due to capillary action the flux will fill the holes in the board up to the topside. The foam blanket will "wash" the board surface, meaning that contaminants may come into the fluxer.
At high temperatures the foam crest will collapse, this will give an unstable process.
On the crest of the foam blanket, the flux bubbles will burst apart.
During this bursting process a lot of flux droplets are generated with high speeds in different directions.
This has been found when we studied high speed video films, with a time loop of 30x.
These relatively large droplets can come on the component side, since a foam fluxer has no exhaust system.
This is one reason why flux contamination can be found on the component side.
An even more obvious reason is the fact that flux will penetrate through the holes by capillary action, and so come on the component side, were it may penetrate inside components via the component leads. This can only be prevented if the components are mounted on so called "standoff's".
One can also prevent this when a spray-fluxing system is used.
Spray-fluxing
A spray-fluxer creates a well defined area were small flux droplets are sprayed to the board.
This system needs an exhaust to prevent over fluxing in case of large holes in the board.
The amount of flux can be very well controlled, so that very thin layers can be applied in case a single sided board has to be soldered.
The flux will normally not penetrate via the leaded holes, since the single droplets will not assist in capillary action.
Capillary action can only be found when there is a sufficient contact with a "liquid reservoir", such as by foam-fluxing.
With a spray fluxer and exhaust system, the risk of flux contamination on the topside of the board can be eliminated.
Mist-fluxers
This type of fluxers generate a very fine mist of flux with an ultrasonic generator. This mist will surround the board when it is entering the fluxing area.
It is therefore unavoidable that parts of the flux will also cover the component side.
The profile of the "flux-mist blanket" is not so well defined as the profile of the standard spray-fluxer.
This means that it is also difficult to define the flux deposition on the board.
ITW EAE Support Center