Spray Fluxing Control: Density, Coverage and Preheat Balance

Flux has to be present in the right amount at the right place when the wave arrives, and spray fluxing is the method that places it without touching the board. The controls are the density of the liquid, the pattern the nozzle produces and the preheat that drives off the solvent without driving off the activity.

What the Fluxer Has to Deliver

The fluxer has to deposit a controlled mass of flux on the underside of the board, uniformly across the area that will see the wave, without wetting the top side or flooding the components. A fluxer that passes those tests produces a board that is neither starved nor covered in residue.

The amount is expressed as a mass per unit area rather than as a thickness, because the liquid evaporates and what remains is the solids. Spray fluxing is judged on the solids that reach the board, which is why coverage and density are measured together rather than separately.

Flux Density and Solids Content

The flux density in the tank is measured with a hydrometer or by titration, and it drifts as solvent evaporates from the bath and as flux is consumed. A density that is high leaves more residue and can leave the board tacky, while one that is low starves the joints of activator.

The flux solids figure is what actually reaches the joint, and it is quoted by the supplier for the material at its specified density. The two are read together, because bringing a dilute bath back to the right density with more flux changes the solids content rather than restoring it.

Spray Patterns and Nozzles

Air atomising nozzles produce a cone of droplets and are the commonest choice, while ultrasonic nozzles produce a finer, more uniform pattern with less overspray. The droplet size decides how well the flux penetrates between pins and how much of it bounces off the board.

The nozzle wears, and wear widens the pattern and changes the flow rate without any adjustment being made. Flow is checked at the start of a shift against a reference, and the pattern is checked by spraying onto a sheet of paper held at board height rather than by looking at the board after soldering.

Coverage Measurement

Coverage is measured by spraying onto a paper or a glass plate that has a dye indicator, or by weighing a known area before and after the spray. The result is compared against a window rather than against an ideal, because too little flux produces poor fill and too much produces residue and a tacky surface.

The measurement is repeated whenever the nozzle, the pump or the board transport speed changes, since the deposited mass depends on all three. A paper test is quick and shows the pattern at a glance, and the weighment gives the number that can be trended over a shift.

Preheat and the Evaporation Balance

The preheat has two jobs: to evaporate the solvent in the flux and to bring the board close to the temperature at which the flux activates. Both happen between the fluxer and the wave, so the conveyor speed and the preheat set points form one system rather than three settings.

A board that arrives at the wave too cool carries solvent into the solder and produces spitting and voids, while one that arrives too hot has already consumed its activator. The measurement is made on the board surface with a thermocouple, at the point where the wave meets the board rather than at the oven exit.

Flux Type and What It Leaves

A no-clean flux leaves a residue that is intended to remain, and a water soluble flux leaves one that must be removed, so the coverage window is different for the two. The same deposit mass that is correct for one leaves an unacceptable residue for the other.

The residue also affects the coating that may follow, and the ionic content matters for products that will see humidity. Our assembly cleanliness notes describe how that residue is measured after the soldering step rather than estimated from the flux specification.

Faults That Point at the Fluxer

Insufficient flux shows as incomplete fill on the joints that are hardest to reach, as icicles and as webbing between pins, and it is often worst at the edges of the board where the spray overlaps least. Excess flux shows as a tacky surface, as white residue around joints and as flux that has crept onto the top side.

A pattern that is uniform but wrong in amount points at the density or the flow, while a pattern that is patchy points at the nozzle. Separating those two is the first step in every diagnosis; our board failure notes describe how the resulting defects are catalogued.

Maintenance and Monitoring

The daily checks are the density, the flow and the pattern, and the weekly ones are the nozzle, the filters and the pump seals. The tank is cleaned on a schedule that follows the flux type, because a bath that is topped up indefinitely accumulates contamination and its solids figure drifts.

The record should carry the density at the start and the end of the shift, the flow setting, the paper pattern and the preheat temperatures measured on the board. Those four items describe the whole fluxing process, and a change to any of them is visible as a change on the board.

Records and Process Control

A change of flux type is a process change, and it brings a different density, a different preheat and possibly a different cleaning requirement with it. The first article after the change is run with the full set of measurements rather than with the settings copied from the previous material.

Our solder joint acceptance criteria describe what the joints should look like when the fluxing and the preheat are correct, and the photographs in them are the reference the operator uses on the line.

FAQ

Can density be restored by adding flux? Adding flux raises the density and the solids together, while adding solvent lowers both. The two figures are therefore tracked separately, and a bath that needs repeated correction is replaced rather than adjusted.

Why does the board feel tacky after fluxing? Because too much solids have been deposited and the preheat has not evaporated the solvent. The pattern and the weighment show which of the two has changed before any adjustment is made.

Is a finer spray always better? A finer spray penetrates better around fine pitch leads, and it also carries further and can reach places that should stay dry. The nozzle is chosen with the board’s layout in mind rather than on droplet size alone.