Conformal Coating Application Methods Compared

A conformal coating is a thin polymer film applied over an assembled board to protect it from moisture, contamination and the mechanical effects of the environment. The film itself is chosen for its chemistry, but the way it is applied determines whether it actually covers the surfaces that need protection.

This article compares the common application methods, how masking and cure are handled, and how the method is matched to the assembly.

What The Coating Has To Cover

The requirement is a continuous film over every surface that can be attacked, with no pinholes, no thin areas and no unwetted regions. The difficulty is that the surfaces are not flat. A board carries component bodies, leads, connectors, tall parts that shadow the spray, and narrow gaps between a component and the board where the coating has to penetrate. A film that covers the top of a component but not the gap under it leaves the most vulnerable area, the solder joint, unprotected.

Coverage also has to stop where it is not wanted. Connectors, test points, switches, optical surfaces and any area that must make a mechanical or electrical contact during service have to remain free of the film, which means the method has to be able to coat selectively or the board has to be masked. The chemistry of the materials and their protection mechanism is described under conformal coating as board protection.

Spray Application

Spraying is the most flexible method. A manual spray gives an operator control over the direction and the dwell, which is useful for a small volume or for a rework repair, and it depends entirely on the operator’s consistency. An automated spray uses a programmed path and a fixed flow, which gives a repeatable film thickness and a documented coverage pattern, at the cost of a programme that has to be written and validated for each product.

The characteristic weakness of spray coating is shadowing. A tall component blocks the spray from the area behind it, and the region downstream of the air flow receives a thinner film. The remedy is to approach from several directions in separate passes, which the automated systems do by programming a sequence of passes at different angles. Where the board has a very tall part, the spray may not reach behind it at all, and the design has to accept a partial coating there or the part has to be coated separately.

Selective Coating

Selective coating machines dispense a defined volume of material along a programmed path using a needle or a small nozzle. The advantage is precision: the machine can coat one area and leave the next untouched, which removes most of the masking and covers only the surfaces that need protection. It also deposits a controlled volume, so the film thickness is set by the volume and the area rather than by the spray pattern.

The limitation is speed and reach. A needle that follows a path is slower than a spray that covers a board in one pass, and it cannot coat an area it cannot reach, so a narrow gap under a large component or a region behind a connector may be left uncoated. Selective coating is best suited to a design with a defined set of coated areas and a modest number of components, and it usually needs a masking plan for the areas that must stay clear even though the machine can avoid them. The design decisions that make the coated areas accessible are part of design guidelines for manufacturability.

Dip And Brush

Dipping immerses the assembly in the coating material and then withdraws it, which covers every exposed surface including the gaps under components. It is the most complete method for coverage and the least selective: everything that is wetted is coated, so connectors and switches have to be masked thoroughly, and the material enters places it should not go. The withdrawal rate sets the film thickness, and the viscosity and the drain time have to be controlled to avoid thick pools and thin runs.

Brushing is used for repair and for touch up. It gives local control, it can reach a specific area on a board that is otherwise complete, and it depends entirely on the operator. Its place is in rework rather than in production, and the repaired region should be inspected after cure to confirm that the film is continuous and that the coating around it has not been disturbed. The related technique of encapsulating a whole assembly is described under potting and dispensing adhesives.

Masking And Its Cost

Masking is often the largest part of the cost of a coating operation. It is applied by hand, it has to be removed without leaving adhesive residue and without pulling on a delicate part, and the mask material itself has to be resistant to the coating and to the cure. Where a product has a large number of connectors and test points, the masking time can exceed the coating time.

The design measures that reduce the cost are to group the areas that must stay clear, to avoid placing a test point in the middle of an area to be coated, and to specify the coating boundary as a region rather than as a set of individual features. Where the design allows, a permanent mask such as a piece of tape that stays in place, or a mechanical cover, is cheaper and more reliable than a temporary mask that has to be applied and removed.

Cure And Inspection

The cure of a coating is controlled by temperature, humidity or ultraviolet light depending on the chemistry, and an incomplete cure leaves a film that is soft, tacky and permeable. The cure is verified by a solvent rub or a hardness test on a coupon, or by a measurement of the film’s electrical properties where the specification requires it. The thickness is measured on the board itself, usually with an eddy current gauge over a copper area or by a section, and the coverage is inspected under ultraviolet light where the coating contains a tracer.

Inspection for coverage is the step that catches the failure mode of every method: a region that looks coated at normal light but has a thin or missing film in a shadowed area. Ultraviolet inspection makes the gaps visible, and it is far more reliable than a visual check. Where the coating is critical to the product’s survival, the inspection is part of the process record rather than a final look at the finished board.

FAQ

Which method gives the best coverage? Dipping covers the most surface, including under components, and is the least selective. Spraying covers well on flat and open areas, and selective coating covers exactly what it is programmed to cover.

Does the coating have to be removed for rework? Yes. A joint cannot be reworked through a cured coating, so the coating is removed locally, the repair is made and the area is recoated. Rework plans should identify the areas where this is likely.

How thick should the coating be? The thickness comes from the material specification and from the environment the product will see. It is measured on the board rather than on a coupon, because shadowing and drainage make the thickness vary across an assembly.