Air Knife Flux Control in Wave Soldering: 5 Rules

An air knife is a slot that blows a curtain of air across the board after fluxing, and its job is to remove the excess flux and leave a controlled film. The film thickness decides how much activator reaches the joint and how much residue is left behind.

Where the fluxer applies more than the process needs, the air knife is the only control on the quantity that actually reaches the wave. It is a simple device, and it is frequently set once and then ignored until a defect appears.

What the Air Knife Does

The knife converts a supply of compressed air into a thin, high-velocity curtain. As the board passes, the curtain drags the surplus flux off the surface and back into the reservoir, leaving a film that is thin and even.

The film that remains is what the preheat then dries and activates. Without the knife, the film is thick and uneven, and the preheat cannot drive off the solvent before the board reaches the wave. The film also decides how much residue is left after the wave, and residue that is heavier than the specification allows is a defect even when every joint is sound. That is why the knife setting is part of the residue control rather than only a soldering control.

Pressure and the Flux Film

Pressure is the primary setting. Too little leaves a heavy film that floods the preheat and produces a wet board at the wave, while too much blows the flux out of the holes and starves the top side of the joint.

Pressure has to be measured at the knife rather than at the regulator, because losses in the hose, the filter and the fittings change the value that actually reaches the slot. A gauge at the machine panel measures the supply, not the knife. The gauge is checked against a reference instrument at a defined interval, because a gauge that reads low leads the operator to raise the pressure until the film becomes too thin at the holes.

Angle, Distance and Coverage

Angle sets the direction in which the flux is pushed. A knife set perpendicular to the board drives flux straight back, while one set at an angle pushes it towards one edge of the board and leaves the other side wet.

Distance from the board changes the velocity that reaches the surface. As the gap opens, the curtain slows and spreads, so the film becomes thicker at the centre and thinner at the edges. The gap is set from the board thickness and checked with a gauge. The distance is set with a gauge that matches the board thickness, and the same gauge is used on every shift so that the setting does not drift with the operator. The check takes a minute and it prevents a shift of drift.

Maintenance and Blockage

The slot is a narrow opening, and flux residue, dust and dried solids collect in it. A partially blocked slot produces a curtain that is strong in one area and weak in another, and the resulting film follows the blockage.

Maintenance is therefore a cleaning task on a defined interval, with the slot cleaned and the curtain checked against a piece of paper or a test board. Blocked air supply lines and a saturated filter have the same effect as a blocked slot, so both are checked together. The foam flux notes describe the fluxer side of the same process. The curtain is checked with a sheet of paper held across the slot, because a weak area shows as a section that does not deflect the paper, and that area corresponds to a thick patch of flux on the board.

Interaction With Preheat

The air knife and the preheat are set together. A thin film activates quickly and needs less preheat, while a heavy film needs more, and if the two are adjusted independently the process oscillates between a wet board and an over-dried one.

The correct sequence is to set the flux film first, then the preheat, and to confirm the result by looking at the residue after the wave. A dry, pale residue indicates the balance is right, while a sticky or dark residue indicates it is not. The preheat setting is recorded with the knife setting so that the pair can be repeated, and a change to one without the other is treated as an incomplete adjustment.

Residue, Cleaning and Masking

The quantity of flux that reaches the wave determines the residue left on the board. Where the process is no-clean, a lighter film is usually required, and the no clean residue review explains what the residue has to look like.

Masking and fixtures interact with the curtain as well. A pallet that stands above the board surface deflects the air and leaves the flux film thick in the shadow of the pallet, which is a common cause of localised residue and poor fill. Where a pallet is used, the curtain is set with the pallet in place rather than with a bare board, because the pallet changes the airflow and therefore the film that reaches the product.

Measurement and Inspection

The film is measured by weight gain on a test board, or by a simple visual comparison against a reference board kept for the purpose. Both give a trend that is more useful than a single absolute value.

The flux solids content of the bath changes the film for the same knife setting, so the two measurements are always read together. The flux solids guide explains how that parameter is tracked, and the bath contamination notes describe what else changes the deposit. The weight gain is measured on the same size of test board each time, because the film per unit area is what matters and a larger board collects more flux for the same thickness.

Records and Change Control

Records should include the knife pressure, the gap, the angle and the film measurement, together with the flux batch and the preheat settings. Those values explain a change in residue long before the soldering defects appear.

Changing the flux grade, the conveyor speed or the board thickness changes the film, so the knife is re-set whenever one of them changes rather than left at the previous setting. A change of conveyor speed is a change to the film as well, since the board spends less time under the curtain and the surplus is removed less completely. The speed is therefore recorded with the knife settings in the same log entry.

FAQ

Should the air knife remove all the flux? No. It removes the excess and leaves a controlled film. A board that leaves the knife dry will not be protected at the wave, and the joints will show it.

Why does one edge of the board have more residue? Because the knife is set at an angle, or the gap is not even across the width of the machine. Both are checked with a gauge and a test board rather than by eye.

Does the knife need adjusting when the board thickness changes? Yes. The gap between the slot and the surface sets the curtain velocity, so a thicker board reduces it and the film becomes heavier unless the gap is reset.