Conformal Coating Thickness And Coverage Verification

A conformal coating is a thin polymer film that is applied after assembly to protect the board from moisture, ionic contamination, and the occasional mechanical insult. It is not a paint, and its performance depends on three properties that can each be measured: the film has to be thick enough in the right places, continuous over the surfaces it must protect, and fully cured. A film that fails any of those three tests can be worse than no coating at all, because it hides the surface it was meant to protect.

This article explains what the coating has to do, how thickness and coverage are specified and verified, which process controls keep the film sound, and how the common defects are diagnosed.

What The Coating Has To Do

The film works as a barrier. It keeps liquid water and ionic species away from the surface, it raises the surface insulation resistance under humid conditions, and it reduces the chance that a conductive filament grows between two closely spaced features. It also provides some protection against handling damage and against the corrosion that a hostile atmosphere would otherwise cause on the exposed metal.

That protection depends on adhesion and continuity. A film that does not wet the surface properly leaves gaps at the edges of pads and along the component body where moisture collects, and a film that is thick in the middle of the board but thin at a sharp edge leaves the edge exposed. Coating that creeps into a connector or onto a mating surface causes a different class of problem, because it changes the contact resistance of a connector or blocks a test point.

How Thickness Is Specified And Measured

The coating thickness is specified as a range and a measurement location, and the location matters as much as the value. A wet film thickness of twenty five to seventy five micrometres is typical for an acrylic coating, and silicone materials are often applied thicker, but the useful figure is the thickness over a pad, an edge, or a component termination rather than the average over the flat laminate. The general requirements are described under board level protection with conformal coating.

The measurement is usually taken on a witness coupon that is coated at the same time as the production board, because the same spray pattern or dip produces the same film on a flat test piece and because measuring on the board itself would damage the coating. A micrometer on a cured coupon gives a direct reading, an eddy current gauge works over a metallic substrate, and a cross section through a coated test board shows the profile over an edge and over a lead. The board itself is then checked for coverage rather than for thickness.

Inspecting Coverage And Cure

Coverage is inspected with a fluorescent tracer, which most coating materials contain, under ultraviolet light. The tracer makes the film visible and reveals the areas that were missed, the shadowed regions behind tall components, and the edges where the coating has pulled back. Visual inspection under magnification then confirms that the film is continuous and free of bubbles, and that the keep out areas such as connectors, test points, and mating surfaces are clean.

Cure is verified separately, because a film can be the right thickness and still be under cured. A solvent rub test, in which the surface is wiped with a defined solvent and inspected for softening, is the traditional check, and a differential scanning calorimeter or an infrared measurement can be used where the material supplier provides reference data. An under cured film stays tacky, collects dust, and provides much less protection than its thickness suggests.

Process Controls Before And During Coating

The surface has to be clean before the coating is applied, and this is the step that is most often skipped. Flux residue and ionic contamination that remain on the board are sealed in by the film, and the trapped chemistry then attacks the surface in the presence of moisture, which is a worse outcome than leaving the board uncoated. Cleaning, rinsing, and drying therefore come before masking, and the ionic cleanliness is verified as it would be for any other assembly.

Masking defines where the coating must not go. Connectors, press fit pins, test points, gold fingers, and any surface that will mate or conduct are covered with tape, boots, or a dispensed gel mask, and the mask has to survive the application and the cure. The application method then sets the profile: spraying gives a thin, even film with some overspray, dipping coats both sides and fills gaps but can pool, and selective dispensing places material exactly where it is wanted at the cost of a slower cycle. Viscosity, temperature, and the number of passes all change the result, and the cure schedule has to match the material rather than the line schedule.

Common Defects And Their Causes

Bubbles and pinholes come from trapped air or solvent, from a viscous material that cannot release them, or from a spray parameter that atomises the coating too coarsely. Dewetting, which appears as circular bare spots, indicates contamination on the surface or a coating whose surface tension does not match the substrate. Orange peel comes from solvent that evaporates too slowly or from a film that is applied too thickly in one pass.

Thin films at edges are a physical effect rather than a process error, because surface tension pulls the liquid away from a sharp corner, so a coating that looks complete on the flat can be very thin along the top of the board edge. Coating that wicks into a connector or under a shield is the opposite problem, and it is controlled by masking and by the volume that is dispensed. Where a component has to be reworked after coating, the repair material and the touch up procedure have to be qualified, because a hand applied patch rarely matches the properties of the original film.

Verification And Acceptance

The acceptance criteria should be written on the drawing and in the process specification, covering the thickness range, the areas that must be coated, the areas that must be kept clear, the appearance requirements, and the cure verification. Inspection then proceeds in the order of coverage first, appearance second, and thickness on the coupon, with the cure check performed on a sample from each batch or each shift.

Record keeping closes the loop, because a coating defect is often found much later, during a field failure analysis. The material batch, the application parameters, the cure schedule, and the inspection result should be traceable to the panel, which is the same discipline that applies to the rest of the assembly. The surrounding quality framework is described under PCB design quality characteristics, and the related dispensing processes under potting and dispensing adhesives.

FAQ

Should thickness be measured on the board or on a coupon? On a coupon that is coated with the production boards. Measuring on the board damages the film and cannot be done over a component, so the coupon is the practical reference while the board is checked for coverage.

Can a board be coated without cleaning? It should not be. Coating over flux residue seals the contamination against the surface, where it promotes corrosion and electrochemical migration under humidity.

How is an under cured coating detected? With a solvent rub test or with an infrared or thermal analysis that compares the film with a fully cured reference. The visual clue is a film that remains tacky and collects dust.