Conformal Coating Application Defects and Cures

A conformal coating fails for the same reasons a paint fails: the surface was not clean, the film was applied too thin or too thick, or the environment during cure was wrong. The coating then protects nothing, and the failure appears as corrosion under a film that looks intact.

Because the film is thin and transparent, most application defects are invisible until the board is in service. The controls that prevent them are therefore procedural, and they belong at the coating station rather than in the design office.

Cleaning Before Coating

Coating adheres to a clean surface and beads on a contaminated one. Flux residue, fingerprints, mould release agent and silicone contamination all reduce the surface energy, and a low energy surface produces the characteristic circular dewetting pattern with a crater at the centre.

The cleaning process has to be qualified with the coating rather than separately, because a solvent that leaves its own residue is as harmful as the contamination it removed. The cleaning requirement is the same one described for the cleaning process, applied with the coating as the acceptance criterion.

Dewetting and Adhesion Failure

Dewetting is the coating pulling away from the surface, and it usually begins at a contamination site and spreads. It can also follow a sharp feature, because a coating cannot bridge a stress concentration without thinning at the edge.

The remedy is diagnosis before action. A surface energy measurement on a sample, or a water break test, distinguishes a contamination problem from an application problem, and the two have completely different corrections.

Bubbles, Pinholes and Foam

Air entrained during mixing, spraying or dispensing forms bubbles that burst during cure and leave pinholes through the film. A pinhole is worse than a thin area, because it lets moisture reach the conductor through a path that the coating does not cover.

Reducing the pressure of the spray, deaerating the material before use and letting the board stand before cure all reduce the bubble population. Where the coating is applied by spraying, the distance and the angle of the gun are also variables that determine whether a film is continuous.

Coverage in Shadowed Areas

Spray coating reaches what it can see, so a component body, a tall connector or a heat sink creates a shadow where the film is thin or absent. The undersides of parts and the gaps between a lead row and a body are the places where coating is most often missing.

Where coverage under a part is required, the process needs a second pass from a different angle or a different method such as brushing or selective dispensing. Inspecting the film with ultraviolet light makes the gaps visible, and it should be done during process development rather than on shipped product.

Thickness and Measurement

The specified thickness is a range because the coating has to be thick enough to be continuous and thin enough to cure fully. A film that is too thick traps solvent, remains soft and can crack during thermal cycling.

Measurement is by an eddy current gauge on a reference area or by a cross section, and the readings should be taken at several points including the corners of the board. A single measurement in the middle of the panel hides both thin edges and thick pools.

Cure Schedule and Humidity

Most coatings cure by reacting with moisture in the air or by evaporating solvent, and both mechanisms are sensitive to the environment. A room that is too dry slows a moisture cure, while a room that is too damp traps solvent in a solvent borne film.

The cure schedule should therefore be written with the temperature, the time and the humidity, and the board should not be disturbed during the initial tack free period. Boards that are moved to a rack too early slide against one another and leave marks that are cosmetic and permanent.

Masking and Keep Out Areas

Connectors, test points, mating surfaces and anything that must conduct current later have to be masked, and the mask has to be removed cleanly. A mask material that leaves adhesive residue creates a zone that neither coats nor solders properly.

The keep out areas belong on the drawing, with their dimensions, so that the operator and the programmer work from the same information. Where a connector is masked by a plug rather than by tape, the plug should be dedicated to that connector rather than improvised.

Rework and Repair Through a Coating

Rework after coating requires the film to be removed locally without damaging the laminate or the mask, and the repaired area to be recoated. The chemistry of the removal is specific to the coating, so the two materials should be selected together rather than separately.

Where a board is expected to need rework, a coating that can be removed selectively is worth the extra cost, because the alternative is a scrapped assembly. The decision belongs in the design and the process plan, not in the repair station.

Inspection and Verification

Inspection under ultraviolet light reveals coverage and gaps, while a thickness measurement confirms the film is within its range. Neither detects poor adhesion, which is why a sample is often tested with a tape pull or a cross hatch.

The results should be recorded with the material lot and the cure parameters so that a field failure can be traced back to a batch. This is the same record keeping that supports the parameters described in board protection practice and the rest of the process evidence.

Process Qualification

Coating is qualified by demonstrating that a coated coupon survives the environment the product will see, usually humidity, thermal cycling and salt spray. The coupon should include the same component heights and the same shadowed geometry as the product.

A qualification performed on a bare board says almost nothing about a populated one, because the defects that matter occur where the geometry is complex. The coupon design is therefore part of the test rather than a detail of it.

Process Control and Verification

On a design of this kind, masking is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a board be coated without cleaning? Only where the assembly is known to be clean and the process has been qualified that way, which is rare on a board that has been handled.

What causes a crater in the film? A localised contamination site with a low surface energy, often silicone or a fingerprint, and cleaning alone may not remove it.

Is a thicker coating better protection? No. Beyond the specified range the film cures poorly and cracks, and the protection falls rather than rises.

How is an uncoated area verified? By inspection under ultraviolet light, which makes the film and the gaps equally visible.