Conformal Coating Coverage: Masking, Thickness and Inspection
A coating protects a circuit only where it is present and bonded, and the places where it is absent are usually the ones that matter most: the edge of a connector, the side of a component and the inside of a via. Coverage is therefore a measurement rather than an impression, and it is verified with light and with a section.
What the Coating Has to Achieve
The coating isolates conductors from moisture, from ionic residue and from airborne contamination, and it also contains anything that grows on the surface, such as a conductive filament. Each of those functions depends on the film being continuous over the area at risk rather than on the film being present on the board.
That distinction is the reason coverage is specified as an area and not as a mass. A board that has been sprayed from one direction is fully coated on the facing side and barely coated on the shadowed side, and a weighment of the whole board would not show it.
Chemistry and Thickness Ranges
Acrylic coatings are the easiest to apply and the easiest to remove, and they are normally applied at 25 to 75 micrometres. Silicone and polyurethane offer better chemical and thermal resistance, and parylene is applied by vacuum deposition at a thickness of a few micrometres to tens of micrometres with very good conformity.
The coating thickness is chosen from the environment and from the standard the product is built to, and the figure applies to the surface of the board rather than to a component lead. Measuring on a component overstates the thickness by the height of the lead, which is a common source of a result that looks too good.
Masking Strategy
The areas that must stay free of coating are the mating surfaces of connectors, test points, press-fit holes, optical devices, relay contacts and any surface that has to conduct heat. The masking is done with tape, with silicone plugs, with moulded boots and, for high volume, with a dedicated fixture.
The choice of method follows the geometry, because a plug that fits a connector shell is faster and more repeatable than taping it, and a boot that covers a whole connector can be removed in seconds. The mask has to survive the cure as well, since a tape that releases adhesive into the coating leaves a residue that is harder to remove than the coating itself.
Application Methods and Their Coverage
Spraying gives the best throughput and the poorest control of edges, dipping gives the best conformity on a simple shape and the worst control of where the coating stops, and selective application gives both control and the lowest throughput. Brushing is used for rework and for touching up an area after repair.
Whichever method is used, the coating has to reach under components and inside vias by capillary action, and that only happens where the surface energy allows it. A board with a contaminated surface will show the coating pulling away from the edges of a component instead of flowing under it, and that is an adhesion failure rather than an application failure.
Coverage Verification by Light and Section
Most coating materials contain a ultraviolet tracer, and the board is inspected under a black light where the film shows as a bright even layer and a missed area appears as a dark patch. The inspection is quick and it is the only method that can be applied to every board.
Thickness is measured with an eddy current or an ultrasonic gauge on a flat area, or by a cross section at a defined location. The section is the only way to see whether the coating has flowed under a component, and the sample is taken from the first article or from a panel that was processed alongside the production boards.
Surface Preparation and Its Effect
The coating bonds to what is on the surface, so flux residue, ionic contamination and handling oils all reduce adhesion. Cleaning before coating is therefore part of the process rather than an option, and the cleanliness is verified with the same methods used for the assembly as a whole.
Where a coating fails on one batch and not on the next, the cleaning stage is the first place to look, followed by the cure. Our assembly cleanliness notes describe the measurements that show whether the surface was ready for the film.
Cure and How It Is Confirmed
The cure may be by solvent evaporation, by moisture in the air or by heat, and each has its own failure mode. A moisture curing silicone that is applied in a dry room stays tacky, and a heat curing material that is applied too thickly cures on the outside and remains soft underneath.
The cure is confirmed by a tack test, by a solvent rub or by a hardness measurement, and the result is recorded with the batch. A coating that is not fully cured has poor chemical resistance and will not protect the circuit for the life of the product, even though it looks complete.
Common Defects and What They Mean
Bubbles and voids come from air trapped under the film or from solvent that could not escape, and they appear as isolated defects or as a line along a component edge. Dewetting and orange peel indicate a surface energy problem, and creep onto a masked contact indicates a mask that did not seal.
A thin edge is the most common defect of all, because the coating thins where it runs over a sharp geometry. The edge is inspected deliberately rather than incidentally, and the acceptance for a thin edge is written in terms of the minimum thickness that must remain rather than the appearance of the film.
Rework, Records and Process Control
Where a coating has to be removed for rework, the removal is done with the solvent recommended for the chemistry, and the area is cleaned and recoated to the same thickness. A recoated area that is thicker than the rest of the board is a handling problem, because it changes the fit of a connector.
The record carries the material and lot, the method, the mask set, the cure schedule, the measured thickness and the coverage inspection result. Our conformal coating notes, together with the board quality checks used before coating, describe the sequence that keeps a coating defect from becoming a field failure.
FAQ
Can coverage be judged by eye without a UV lamp? A clear coating on a green board looks continuous under normal light even when several areas are bare. The tracer and the lamp exist because the eye cannot make the distinction reliably.
Is a thicker coating always better? No. Beyond the specified range a coating can crack as it cures, trap solvent underneath and stress a component lead, and it also changes the fit of connectors and shields.
Why does the coating sometimes peel from a lead? Usually because the lead was contaminated or because the film was applied over a sharp edge that concentrates the stress. The section shows whether the coating flowed under the part or only bridged across the top of it.
