Conformal Coating Process Guide

A conformal coating is a thin polymer film that is applied over a populated board to keep moisture, dust and chemicals away from the circuits. The film is a few tens of microns thick, it follows the shape of every component, and it is the last process step that can ruin a working assembly. Coating is easy to apply badly and hard to inspect, so the design and the process have to be planned together.

Why Coat a Board

The main reason is moisture. A film of water on a board with a bias voltage creates an electrochemical path between conductors, and the result is leakage, corrosion and eventually an open circuit where a trace has been eaten away.

The second reason is contamination from the environment: salt spray, sulphur, oils and conductive dust. Each of them attacks the board in a different way, and a coating that resists one may be permeable to another.

The third reason is mechanical. A coating holds small parts and solder joints against vibration and it reduces the movement of a wire that is anchored to the board.

The protection is described in the coating protection guide, and the process that applies it is a separate discipline with its own design rules.

Coating Types

Acrylic coatings are easy to apply, easy to rework and cheap, and they are the usual choice for a general purpose product. Their weakness is a modest resistance to solvents.

Silicone coatings tolerate a wide temperature range and stay flexible at low temperature, which makes them suitable for an automotive or an outdoor product. They are harder to rework than an acrylic and they are more expensive.

Polyurethane coatings give the best resistance to solvents and to chemical attack, and they are used in harsh environments. They are difficult to remove, so a board that needs rework after coating is a problem.

Parylene is applied as a vapour and polymerises on the surface, which gives an even film over every edge and underneath components. It offers the best barrier and the highest cost, and it cannot be applied selectively.

Masking and Keep Out Areas

Every area that must not be coated has to be masked before the process. Connectors, test points, switches, relays, sensors and any surface that has to make electrical contact with something else are the usual candidates.

The mask is a tape, a boot or a dispensed gel, and it has to survive the coating and be removable without leaving a residue. A mask that leaves adhesive on a connector contact is worse than no coating at all.

The keep out areas should be defined on the assembly drawing, because the mask is applied by hand on most lines and an operator without a drawing will guess. A connector that is coated by mistake can sometimes be cleaned and often cannot.

The masking cost often exceeds the coating cost, so the layout should group the areas that need masking. A board with masked features scattered over the surface is expensive to process, while a board that keeps them at one edge is cheap.

Application Methods

Spraying gives an even film over a large area and it is the fastest method. The viscosity of the material, the spray pattern and the distance all affect the thickness, and the edges of the board tend to receive more material than the centre.

Dipping immerses the board and coats it completely, including the underside of components. The viscosity and the withdrawal speed set the thickness, and a slow withdrawal gives a thick film that can bridge between pins.

Selective coating dispenses the material from a nozzle that follows a programmed path, which allows the coating to be placed exactly where it is wanted. The method is slow but it removes most of the masking, and it is the usual choice for a mixed technology board.

A fourth method is to use a coating film rather than a liquid. The film is laminated over the board under heat and pressure, which gives a uniform thickness and no solvent to evaporate. It needs a flat board and it does not reach underneath a component, so it suits a simple assembly rather than a dense one.

Brushing is used for a repair or for a small area. It gives a film that is uneven and it is not a production method, although it is often the only way to recoat a repaired joint.

Cure and Inspection

The cure converts the liquid into a solid film, and it is usually a thermal or a ultraviolet process, or both. An incomplete cure leaves a tacky surface that collects dust and that never develops its full barrier properties.

The coating also changes the thermal behaviour of the board, because a polymer film is a thermal insulator. A component that was already close to its limit may need a lower coating thickness or a gap in the coating over the hot part, and the thermal design should be checked after the coating is chosen.

The thickness and the coverage decide the performance, and both are affected by the profile of the board. A sharp edge, a tall component and a confined space between two parts all receive a different thickness from the same process.

Inspection is visual under ultraviolet light for a coating that contains a tracer, and by sectioning for the thickness. A common defect is dewetting, where the coating pulls away from a surface and leaves a bare patch that looks coated in white light.

The surface has to be clean before the coating is applied, because a coating over flux residue or over a release agent will dewet or delaminate. The cleaning step is part of the coating process, not a separate one.

Design Review

The review should list the areas to be masked and the areas to be coated, and it should check that the two lists do not overlap. A connector pin that appears in both lists is a documentation error that becomes a field failure.

The second item is the material and the thickness, chosen from the environment that the product will see. A coating that is specified only as conformal coating gives the process engineer no information.

The third item is the rework path, because a board that cannot be repaired is a board that has to be scrapped. The material choice should be reviewed with the service team as well as with production.

The last item is the record. The coating type, the thickness and the cure profile belong in the fabrication notes, and the quality check should include the coverage on a sample of the batch.

FAQ

Does every board need a conformal coating? Only where the environment demands it. A board in a dry indoor enclosure gains little, and the coating adds cost and removes the ability to rework easily.

Why does a coating pull away from a surface? Usually because the surface is contaminated or because the coating was applied over a residue. Cleaning before coating is the fix, not a thicker film.

Can a coated board be repaired? It can with an acrylic coating, which dissolves in a suitable solvent. A polyurethane or a parylene coating makes the repair much harder.