Water Soluble Flux Process Control and Cleaning Guide

A water soluble flux is formulated so that its residue can be removed in water, which allows a more active chemistry to be used than a no clean product would permit. The trade is explicit: the flux solders more reliably on a difficult surface, and the residue must be completely removed because it is designed to dissolve rather than to be left behind. Water soluble flux is therefore a process choice that carries a mandatory cleaning step with it, and the two should be specified together.

What Makes a Flux Water Soluble

The active ingredients are chosen so that they and their reaction products dissolve in water rather than in a solvent. Organic acids and their salts are typical, together with a small amount of a surfactant that helps the water to wet the residue. The choice of acid determines the activity and the corrosion risk, which is why the formulation should be matched to the surfaces being soldered.

The formulation is therefore a deliberate compromise. The activity that makes it effective also makes the residue conductive and corrosive if any of it remains, so the process is only safe when the cleaning is complete. A partially cleaned board is worse than an uncleaned one in some cases, because the residue is spread over a larger area.

Why Cleaning Is Mandatory

The residue of a water soluble flux is not benign. It is hygroscopic, it ionises in the presence of moisture, and it attacks the copper and the finish over time. A board that is not cleaned will fail an insulation resistance test or a humidity test sooner or later.

That is why the process is sometimes called water clean rather than water soluble. The cleaning step is not optional and it is not a quality improvement, it is part of the soldering process itself. Treating the cleaning step as an optional extra is the most common mistake made with this chemistry. Our solder defects guide describes the residue related failures that appear when it is incomplete. Where a humidity test produces failures, the cleaning process should be the first thing examined.

Residue Chemistry and Its Risks

The main risk is ionic contamination, because dissolved salts provide a conduction path between adjacent conductors. On a fine pitch board with a small spacing, even a small residue can produce a measurable leakage current.

The second risk is corrosion. An acid residue in the presence of moisture attacks the copper, the tin plating and the finish, and the attack is faster at a joint that is already stressed. A stressed joint is also a place where a small amount of corrosion produces a large change in resistance. Both risks increase with humidity and with temperature.

<img src="https://www.gopcba.com/wp-content/uploads/2020/12/curve_overlay_big.png" alt="Conductivity meter checking the final rinse water quality” />

Cleaning Equipment Requirements

An aqueous cleaner for a water soluble process needs a wash stage with a controlled temperature, one or more rinse stages with a cascade or spray arrangement, and a drying stage. Some processes add a saponifier, although a water soluble flux normally does not need one.

The spray coverage matters more than the chemistry, because the residue has to be reached. Where the spray is adequate, the wash temperature and the dwell usually have margin; where it is not, no amount of chemistry will compensate. A board with tall components creates shadows that a poorly aimed nozzle will not clean, and the residue survives in exactly the place where it is hardest to see. A coverage test with a pattern card is the quickest way to confirm that the spray reaches the whole board.

Rinse Water Quality

The rinse water has to be cleaner than the wash water, because its job is to carry away what the wash loosened. Deionised water with a monitored conductivity is the usual specification for the final rinse. The rinse stages should be arranged in a cascade so that the cleanest water meets the cleanest board.

Conductivity is the practical control because it responds immediately to a change in the water. A rising reading on the final rinse tells you that the wash stage is carrying more residue forward or that the water supply has changed. Recording the reading per run turns a hidden variable into a trend that can be acted on. Our quality documentation describes how these results are recorded at gopcb.

Drying After Cleaning

Water left on the board after cleaning causes the same corrosion problems as the residue it was meant to remove. It hides under components, inside connectors and in vias, where a visual check will not find it.

The drying zone has to be set from the heaviest assembly on the line, and the conveyor speed has to be matched to it. Air knives before the heated zone remove the bulk of the water and let the dryer finish the job with less energy. A dryer that is adequate for a simple board will leave water in a heavy assembly without any visible sign.

Verification of Cleanliness

The ionic contamination test on a cleaned assembly is the standard measure, and the result is expressed as an equivalent salt weight per unit area. The test should be performed on a sample from each production run rather than once a month.

A surface insulation resistance test on a test pattern gives a more functional check, because it measures the property the residue actually degrades. Both tests require the sample to be handled carefully so that contamination is not introduced after cleaning. Our component guide covers the storage and handling rules that keep the cleaned board in condition. The interval between cleaning and coating should also be controlled, because a cleaned surface attracts contamination quickly.

Common Problems

The common problems are residue left under a package, water trapped in a connector, corrosion appearing after a humidity test and a leakage current that appears only in the field. Each of them points at a different part of the cleaning process.

Corrosion that appears after a humidity test but not before it is a strong indication that residue was left in a place that the cleaning did not reach. A section or a disassembly of the failed unit usually shows where. Where the residue is consistently in the same place, the spray pattern or the component layout is the cause.

Process Control Points

The controls are the flux type and its application rate, the wash temperature and chemistry, the spray coverage, the rinse water conductivity, the dryer temperature and dwell, the conveyor speed and the contamination test result.

Every one of those is measurable and each should be recorded per run. Our production flow guide places the cleaning step in the wider assembly sequence, where it sits between the soldering operation and any coating or test that follows. Water soluble flux is therefore a reliable choice when the cleaning process is designed with the same care as the soldering process.

FAQ

Can water soluble flux residue be left on the board? No. The residue is conductive and corrosive when moisture is present, so the cleaning step is mandatory rather than optional.

Does a water soluble process need a saponifier? Usually not, because the residue is designed to dissolve in water. A saponifier is used for rosin residues that would otherwise not dissolve.

How is cleanliness verified? The ionic contamination test on a sample is the standard measure, supported by a surface insulation resistance test on a test pattern and a visual check under a low angle light.