The main types of fluxer unit are foam fluxers and spray fluxers – if you had researched this on the web, or by reading older books, you might also come across wave fluxing and brush fluxing, both of which have now virtually disappeared. You might also come across spray systems other than the sealed spray types that are now preferred.

Both foam and spray fluxers will do a competent job on ‘average’ boards. The main problem with the foam fluxer is that the flux is continually re-circulated in the atmosphere, with air bubbled through it, with the result that it will lose solvent, whether this is an organic material or water. In consequence, action has to be taken to maintain both the concentration of active ingredient and the viscosity of the flux by ‘topping up’ the solvent on a regular basis. This can be done either automatically or by manual intervention every 3–4 hours.

The advantages that a sealed spray system gives mostly relate to the fact that virgin flux is applied to the board, so there is no need for any thinning of the material, and there is also much less waste, because the spray can be confined to the board. If you want to change fluxes, then a sealed spray system is far easier to maintain.

Another advantage of a sealed spray system over foam fluxing is its improved penetration, which is helpful in ensuring that through-holes are adequately fluxed. Sealed spray systems are also most useful for low solids fluxes, that are otherwise difficult to maintain in good condition, and when soldering boards with high density SM components.

From these final comments, one can deduce that a spray system would be best for a high density SM board, the reasons being that there is better flux penetration into restricted areas, and the quantity of flux deposited is relatively well-controlled. One can argue that, for a very high density board, it might be better to use an ultrasonic spray head rather than a reciprocating air-powered nozzle, because the flux droplets are smaller and better controlled, giving a more even coverage and better penetration.

Note that flux choice depends as much as the nature of the flux being used as on the application. For example, it is not unusual for foam fluxers to be used with water-based fluxes, using as an advantage the fact that the evaporation losses are much lower than with solvents.