Flux Spitting in Wave Soldering: Causes and Controls

Flux spitting is the ejection of flux droplets from the applicator that land on areas of the board where they were not intended. The droplets appear as isolated spots, sometimes with a dark ring, and they are usually seen on the top side or on surfaces that should have stayed clean.

The defect matters for two reasons. It changes the flux distribution that the wave soldering process depends on, and it leaves residue in places that may not be cleanable, which is a particular problem for no clean processes.

What Flux Spitting Looks Like

The deposits are discrete droplets or small splashes, typically a millimetre or two across, with a darker centre where the flux has concentrated solids. They stand out against a clean surface and are usually found on the component side or on the edges of the panel.

They should not be confused with solder splash, which contains metal and appears after the wave. A flux droplet is a chemistry deposit and behaves differently; it can be removed by cleaning, whereas solder splash cannot.

Where It Comes From in the Applicator

In a spray fluxer, spitting comes from the nozzle itself. A worn or partly blocked orifice produces an irregular jet, and a nozzle that is operated above its rated pressure throws droplets sideways rather than atomising them.

In a foam fluxer, the mechanism is different: the air that creates the foam bursts at the surface and throws droplets into the air, and an overfilled tank or excessive air pressure makes that worse. Whichever type is fitted, the cause is an applicator operating outside its design condition.

Spray Fluxer Settings and Droplet Size

Droplet size is set by the nozzle, the pressure and the flux properties. A well set system produces a fine mist with droplets in the tens of micrometres, which wets the surface evenly and does not travel far.

Conditions that increase droplet size also increase throw distance. Lowering the pressure to reduce the flux quantity can paradoxically make spitting worse, because a coarse spray carries further than a fine one. The correct adjustment is to change the nozzle or the travel speed rather than to run the pump below its working range. Nozzle condition changes with use as well, since flux is abrasive to some degree and the orifice of a worn nozzle becomes oval, producing a fan shaped spray instead of a cone.

Foam Fluxer Overflow and Bubbles

Foam applicators need a foam column that is dense, uniform and stable. Air pressure that is too high produces large bubbles that burst energetically, and a foam stone that is partly blocked produces a column that surges and collapses.

The flux level matters as well, since a low level changes the relationship between the stone and the surface and produces a coarser foam. Checking the stone for blockage and keeping the level within its marked range removes most of this class of spitting. Where a foam fluxer is fitted, the exhaust duct is part of the same system, because a blocked duct raises the pressure above the foam surface and pushes the break up point toward the board.

Preheat and Solvent Flash

A board that enters the preheat zone with a heavy flux film releases a large volume of solvent quickly, and the vapour can carry droplets as it escapes. The effect is strongest when the first preheat zone is set very hot, since the surface then flashes off before the bulk warms.

A gentler rise across the first zones lets the solvent leave in a controlled way. The mass of the assembly decides how much of a problem this is, since a board with heavy copper heats more slowly and reaches the flash point later in the tunnel, where the airflow behaves differently. Measured top side board temperatures between 90 and 110 C at the wave are the usual target for a no clean flux, and the shape of the curve on the way there is as important as the value at the end.

Flux Chemistry and Water Content

Water based fluxes behave differently from solvent based ones. Water has a higher surface tension and a higher heat of vaporisation, so an insufficient preheat leaves moisture that flashes violently at the wave, and that flash throws material onto the board.

Contamination of the flux also changes its behaviour. Alcohol based fluxes absorb water from the atmosphere, and a tank that is left open will gradually accumulate moisture that makes the spray coarser and the flash more violent. Tanks should be closed when not in use and the flux replaced on a defined interval.

Effects on the Board and the Pot

Droplets that land on the top side leave residue that can be difficult to remove after the wave, particularly under components. On a no clean assembly, that residue stays on the board and becomes a cleanliness question rather than a cosmetic one.

Droplets that fall into the solder pot do something else. Flux in molten solder creates dross and smoke, and repeated additions of organic material degrade the alloy and the surface of the wave, which then shows up as poor wetting on later boards.

Detection and Cleaning

Inspection for spitting is done before cleaning, with the board examined on both sides under good light. Recording where the droplets are found is diagnostic: droplets on the top side point to the applicator, while droplets around the edges point to airflow patterns in the machine.

Removing the residue is a cleaning question, and the method depends on the flux. A no clean flux that is not designed to be washed may still be removable with a saponifier, while a rosin based flux responds to a solvent process. The cleaning verification should confirm the result by measurement rather than by appearance.

Controls and Maintenance

Fixing spitting is a matter of restoring the applicator to its design condition. Replace worn nozzles, clean or replace foam stones, check pressures against the machine specification and confirm that airflow inside the machine is not directed at the flux film.

A short maintenance interval on the applicator prevents most cases, and the checks are quick: nozzle pattern on a paper card, foam uniformity in the tank and a clean flux filter. Baking the boards or reducing the flux film is not a substitute for any of them, though the same rules that govern spray fluxer setup apply directly to keeping the deposit where it belongs.

FAQ

What causes flux spitting in wave soldering? An applicator operating outside its design condition. Worn spray nozzles, over pressurised foam fluxers and excessive solvent flash at preheat are the common sources.

Is flux spitting the same as solder splash? No. Flux spitting deposits chemistry, while solder splash deposits metal. The two look different under magnification and require different corrective actions. In practice they often appear together, because both indicate a machine condition that needs attention, and the inspection routine should look for metal and chemistry separately on the same board.

Can spitting be removed by cleaning? Generally yes, if the cleaning chemistry suits the flux. Reducing the amount of flux that lands on the board is preferable, because residue under components is difficult to remove consistently.

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