Conformal Coating: Coverage and Process
Why Coating Is Applied
A conformal coating is a thin polymer film applied over an assembled board to protect it from moisture, condensation, salt spray, chemical exposure and, to a lesser extent, mechanical abrasion and tin whiskers. Its value is greatest where the product will see humidity, temperature cycling or contamination, which is why it is standard in automotive, industrial, medical and outdoor equipment. It is not a substitute for a good design: a coating slows the ingress of moisture, but it does not seal a board, and a coating applied over ionic contamination traps that contamination against the surface rather than protecting the circuit from it.
Material Types
The common chemistries are acrylic, silicone, polyurethane, epoxy and parylene. Acrylic is cheap and easy to apply and to rework, with moderate protection. Silicone has the widest temperature range and good flexibility, which suits thermal cycling, but it is more difficult to remove. Polyurethane offers good chemical resistance and hardness. Epoxy is the hardest and the most resistant, and the least reworkable. Parylene is applied by vapour deposition rather than spraying, which gives a uniform, pinhole-free film even on sharp edges, at a much higher cost and with a much longer process. The choice follows from the environment, the temperature range and whether rework is expected.
Masking and Keep-Outs
Coating that lands where it should not causes defects that are expensive to find. Connectors and their contacts must be masked, because a coating on a contact surface causes intermittent connections. Test points and probe pads should be masked so that they remain accessible. Areas that will be soldered later, such as a press-fit connector or a shield frame that will be soldered in place, must be kept clear. Boards that will be reworked should have the coating kept away from the areas likely to need repair. Getting this right is a drawing exercise: the keep-out areas should be defined as polygons and, ideally, shown on an assembly drawing rather than described in a note.
Application Methods
Spraying gives good coverage on a complex board and can be automated with a programmed path, though overspray has to be managed. Selective coating with a dispensing robot deposits material precisely where it is wanted, which reduces masking but is slower. Dipping gives a uniform coating over the whole board and is fast, but it coats everything that is not masked, including the underside, and it can fill connectors. Brush application is used for repair and for small batches. The method has to match the masking strategy: a process that masks heavily suits dipping, while a selective dispenser can do with much less.
Thickness and Coverage
The protection comes from the film being continuous rather than from it being thick. A typical specification calls for a thickness in the region of 25 to 75 micrometres on the flat surfaces, with the requirement that the film covers the tops and sides of components and reaches the board surface between them. The thin film that lands on a sharp edge is the weak point, because surface tension pulls the coating away and leaves a breach; this is one of the advantages of parylene, which is deposited rather than flowed. The coverage, rather than the thickness, is what should be verified: a nominally thick film with a pinhole at a solder joint gives almost no protection at that joint.
Curing
Most coatings cure by solvent evaporation, by moisture, by heat or by ultraviolet light, and many use a combination. An ultraviolet cure alone leaves shadowed areas under components uncured, which is why a secondary moisture or heat cure is often applied. The cure schedule matters for the same reason the coverage does: an incompletely cured coating is softer, absorbs solvent and can release corrosive species. Where the process uses UV, the coating is usually formulated with a fluorescent tracer, which makes inspection possible under ultraviolet light and reveals the shadowed areas that received no light.
Inspection
Inspection is normally visual under white light and ultraviolet. The tracer shows where coating is present, and the absence of fluorescence in a shadowed area is a signal that the film there is thin or missing. Where the specification is critical, cross sections and thickness measurements are used on samples, and adhesion is checked by a tape test or a cross-hatch test. The most useful inspection, however, is the one done before the coating is applied: verifying that the masking is complete and that the board is clean and dry is simpler than finding a defect afterwards.
Design and Process Rules
Define the keep-out areas as drawings, not as notes. Keep the coating away from connectors, test points and anything to be soldered later. Provide enough space around components for the dispenser or the spray to reach the board between them, and avoid tall components that shadow the surface behind them. Ensure the board is clean, dry and free of flux residue before coating, because the coating will lock in whatever is there. Finally, decide the rework strategy before choosing the material, since a coating that cannot be removed makes a repair into a board replacement.
FAQ
What does conformal coating protect against? Moisture, condensation, salt spray, chemical exposure and some forms of mechanical contamination. It is not a hermetic seal.
Which material should I choose? It follows from the environment and from whether rework is expected. Silicone for wide temperature range, polyurethane or epoxy for chemical resistance, acrylic for easy rework.
Does the coating have to be thick? No. Continuity matters more than thickness. Typical films are 25 to 75 micrometres on flat surfaces.
Why mask test points? So that they remain probeable. Coating on a test point causes unreliable contact and false failures.
How is coverage verified? Usually by inspection under ultraviolet light with a fluorescent tracer, supplemented by cross sections and adhesion tests on samples.
Conclusion
Coating is a process that works only when the masking, the board cleanliness and the coverage are all correct, and where the rework strategy has been decided in advance. Define the keep-outs on the drawing, choose the material from the environment, verify coverage rather than thickness, and inspect under ultraviolet light. Coating capability is described alongside conformal coating, the assembly steps that precede it are covered by PCB assembly, and the verification that follows belongs to PCBA testing. Coated assemblies are normally qualified through quality management in 2026.
