A wave solder machine's foam fluxer is a device in which compressed air is fed through a porous tube (commonly referred to as a "stone" because this part used to be made out of unglazed porcelain) that is submerged in foaming flux. The stone is covered with a chimney which helps produce a cascading head of flux foam. A basic foaming flux schematic can be seen in the drawing.
Not all fluxes will work well in this scenario. The flux needs to have a foaming characteristic, either instituted in development or by the addition of a foaming agent. If the correct flux is used, then the flux will bubble out of the chute and create a foam head. This needs to be properly maintained to assure accurate flux deposition on the board.
There is usually an air knife or a brush following the wave head to remove any surplus from the PCB. If the printed circuit board is dripping with excess flux, the flux can drip onto the preheaters. Burned-on flux is extremely hard to remove. Also, certain fluxes are volatile and flammable. The safety risks associated with those types of fluxes are high.
The foam head is difficult to maintain; it can present a real challenge to operators. To address this, the fluxer's air supply should be adjusted slowly, if accurate head consistency is to be achieved. By introducing a large amount of air without proper buildup, we can produce large bubbles. When these bubbles reach the top the foam, they burst. The foam is very sensitive to heat. If a hot pallet, fixture, or board comes in contact with the foam, the foam head could collapse. The foam head can recover but requires time to build up again. If the operator does not notice the collapse, then there very well might be certain printed circuit board assemblies that will not have adequate, or any, flux, resulting in poor soldering.
The stone's pores are fine and can easily be clogged with dry flux; dry flux can be very difficult, if not impossible, to remove. If the machine is to be shut down (for any reason, including routine maintenance), the fluxer should be drained and cleaned extensively to avoid this problem. Also, if the compressed air in the stone is contaminated with oil or water, the flux will become compromised and not perform correctly. If a decent foam head is hard to achieve, contamination should be the first suspect.
'Til Next Time,
Next week I will be talking about a Wave Fluxer