PCB Cleaning: Aqueous And Solvent Processes

Cleaning a printed circuit assembly is a process decision rather than a cosmetic one, and it is made for three reasons: to remove a flux residue that would otherwise be chemically active, to remove ionic contamination that would otherwise promote leakage and corrosion, and to prepare the surface for a coating that has to adhere to it. The two families of process, water based and solvent based, achieve the same result by different means and carry different costs.

This article explains what has to be removed, how each family works, how the process is chosen and controlled, and how the result is verified.

What Has To Be Removed

The residue from a soldering process contains the flux vehicle, its activators and their reaction products, and any ionic material that was on the board before assembly. Some of it is water soluble, some is soluble in organic solvents, and some is a polymer that neither dissolves readily and has to be lifted mechanically. Particulate contamination, which may be dust, fibre from a depaneled edge, or solder spheres, is a separate problem that the same equipment usually addresses.

The difficult region is always the same: under a component with a low standoff. A cleaning fluid has to reach the gap, dissolve or displace what is there, and carry it away, and the narrower the gap the harder that becomes. A chip resistor with a small standoff is easy, while a large ball grid array with a narrow gap under its centre is a region that no spray will reach directly, which is why the cleaning of a densely populated board is a question of soak and displacement rather than of pressure.

Water Based Cleaning

Water based processes use deionised water with a detergent or a saponifier, applied at an elevated temperature, followed by rinse stages in pure water and a drying step. The detergent raises the pH and emulsifies the flux, and the mechanical energy of the spray removes the loosened material. The medium is cheap, non flammable, and effective on ionic contamination, and the process is the default for a water soluble flux.

Its difficulties are all about water. The rinse water has to be pure enough that it does not leave its own residue, which means monitoring the resistivity, and the drying step has to remove water from every place it can hide, including vias, connector cavities, and the gaps under components. Water left on the board after cleaning causes corrosion, stains, and blisters when the board is heated, and a coating applied over a damp surface will fail. Some finishes also change appearance or solderability after a water based clean, which is worth confirming before the process is fixed.

Solvent Cleaning

Solvent processes dissolve the residue rather than saponifying it. The traditional method was vapour degreasing with a chlorinated solvent, which was effective and is now largely restricted, and the modern equipment uses hydrocarbon or modified alcohol solvents, often as part of a co-solvent process in which the solvent is displaced by a water rinse. The advantage of a solvent is its low surface tension, which lets it penetrate under a component more easily than water, and its rapid evaporation, which removes the drying problem.

The disadvantages are cost, regulation, and compatibility. A solvent process consumes more expensive chemistry, it is subject to volatile organic compound limits, and many solvents attack plastics, labels, and certain coatings, so the materials on the assembly have to be checked before the process is chosen. Flammability is the other engineering constraint, and it drives the design of the equipment as much as the chemistry.

Choosing The Process

The flux chemistry is the first input. A water soluble flux must be cleaned, and the choice is then simply which water based process. A no clean flux does not require cleaning for its own sake, and the decision is made from the circuit requirement, the environment, and whether a coating will be applied. A rosin based flux falls in between and is usually cleaned with a solvent or with a saponified water process.

The geometry is the second input. A board with a dense population and low standoffs, or with components that trap fluid such as a connector with a cavity underneath, is harder to clean and harder to dry, and the process window narrows accordingly. Where the requirement is not absolute, a no clean process with qualified residue is often the more reliable choice, because a cleaning process that is not fully controlled can introduce defects of its own.

Equipment And Parameters

The equipment may be a batch machine or an in line system, and the energy may be applied by spray, by immersion, or by ultrasonic agitation. Each has a range: spray is effective on the surfaces it reaches and weak in the shadowed areas, immersion relies on chemistry and time, and ultrasonic agitation reaches narrow gaps but can damage fragile components and wire bonds. The parameters that matter are the chemical concentration, the temperature, the contact time, the spray pressure, and the purity of the rinse water.

Those parameters are controlled rather than set once. The concentration is titrated, the rinse water resistivity is monitored continuously because it is the direct measure of whether residue is being carried away, and the conveyor speed or the cycle time is fixed so that the exposure is repeatable. A cleaning line that is not monitored drifts, and the drift appears as a coating adhesion failure or as a leakage problem much later in the process.

Cost And Verification

The cost of cleaning is not only the chemistry. Water based equipment consumes energy for heating and drying, the waste water has to be treated, and the floor space is significant. Solvent equipment carries capital cost, regulation, and consumable cost, with less floor space. The comparison is usually made over the whole process, including the cost of the defects that each option prevents.

Verification uses the same tools as the process itself. The ionic contamination test measures the conductivity of a solvent extract of the board and reports the result against a limit, ion chromatography identifies which ions remain when the result is marginal, and a surface insulation resistance test measures the property that the cleaning is meant to protect. The cleaning decision and the coating decision are linked, since the coating has to be applied to a surface the process has prepared, which is described under board level protection. The fabrication context is described under PCB design and fabrication, and the wider framework under PCB design quality characteristics.

FAQ

Is cleaning always better than not cleaning? No. A cleaning process that is not controlled can leave its own residue, trap water under a component, or damage a finish. Where a no clean process is qualified and the circuit is not sensitive, leaving the residue can be the more reliable choice.

Why is drying so important in a water based process? Because water that remains in a via or under a component causes corrosion, stains, and blisters, and it ruins the adhesion of a coating applied afterwards.

Can a board be cleaned after a conformal coating? No. The coating has to be applied to a clean, dry surface. Cleaning after coating only removes the coating, and cleaning before coating is the step that makes the coating work.