Conformal Coating Application Methods For PCB

A conformal coating is a thin polymer film applied over an assembled board to protect it from moisture, dust, salt and chemical attack. It does not make a board waterproof in the way a sealed enclosure does, but it raises the surface insulation resistance and slows the corrosion that a contaminated surface would otherwise suffer.

The coating is only as good as its application. A film that is too thin, that has voids across a fine pitch row, or that has failed to cure will not protect anything, and the defect is usually invisible until the product has been in service for some time.

What The Coating Has To Do

The primary job is to prevent moisture and ionic contamination from forming a conductive path on the surface. That means the film has to cover the conductors and the flux residue completely, with no gaps at the edges of a pad or between the leads of a fine pitch device.

Secondary jobs include mechanical protection during handling, damping of vibration on heavy components, and electrical insulation where the design relies on surface separation. Each of those adds a requirement to the thickness and to the adhesion of the film.

The Main Application Methods

Spray, selective coating, dipping and brushing are the four methods in common use. Spray gives good coverage and speed for a whole board, selective coating deposits material only where it is wanted, dipping coats everything including areas that may need to stay clear, and brushing is used for repair and for small quantities.

The method is chosen from the product volume, the areas that must remain uncoated, and the components that cannot tolerate the process. Connectors, test points and adjustable parts usually have to be masked whatever method is used.

Spray Application

Spray coating is fast and gives a uniform film over a complex board. The material is atomised and directed at the assembly, and the board is normally moved through a spray chamber on a conveyor to give even coverage on both sides.

The parameters that matter are the material flow, the number of passes and the board speed. A single pass gives a thin film that may not cover a tall component, while too many passes produce runs and pools around the leads of a connector.

Selective Coating And Masking

Selective coating uses a programmable dispenser or a narrow spray head to apply the material only to defined areas. It removes most of the masking labour, which is often the largest cost in the coating process, and it protects connectors and test points without tape.

Where masking is still needed, the tape or the peelable resist must be applied before the coating and removed afterwards without leaving adhesive or lifting a component. Masking is a manual step and it is where most of the variation between operators enters the process.

Dipping And Brushing

Dipping gives excellent coverage on a board with components on one side only, because the material can flow around every lead. The problem is that it also coats the connector and the mounting holes, so those have to be masked or cleaned afterwards.

Brushing is the least controlled method and is used mainly for touch up and repair. The film thickness depends entirely on the operator, so a brushed area is seldom identical to the sprayed area around it.

Thickness And Coverage

Typical dry film thickness is 25 to 75 microns for a sprayed acrylic and 50 to 130 microns for a polyurethane. The thickness is specified after cure, since the wet film loses most of its solvent, and a measurement taken wet is not comparable.

Coverage is a separate question from thickness. A thick film with a void over one pad gives no protection at that pad, so the inspection method has to detect gaps rather than only confirm the average thickness. The pass criteria are described under PCB design quality characteristics.

Cure Schedules

Solvent based coatings cure by evaporation followed by a chemical reaction, and the schedule is usually room temperature for the solvent followed by an oven bake for the reaction. UV cured materials need light to reach the material, which is difficult under a component.

An under-cured film stays tacky, has poor adhesion, and can release solvent into a sealed enclosure. The cure should be verified by a solvent rub test or by a hardness measurement rather than by time and temperature alone.

Inspection And Defects

Inspection starts with a visual check under ultraviolet light, because most coatings contain a fluorescent tracer that makes voids and thin areas visible. The check looks for uncovered pads, bubbles, runs and areas where the film has pulled away from a lead.

The common defects are voids over a fine pitch row, bridging between the pins of a connector that should have stayed clear, and adhesion failure at the edges of a large component. Each of them has a process cause that can be corrected.

Rework And Repair

Rework on a coated board means removing the coating locally, which is done with a solvent, a hot air jet or a scraping tool. The area is then repaired and the coating is reapplied, usually by brushing, and the repaired area is inspected separately.

The repair rarely matches the original film, so a board that has been reworked several times accumulates areas of varying thickness. Where the application is critical, the repaired area should be tested rather than assumed to be equivalent.

Choosing A Method

The choice usually follows the volume and the masking requirement. A high volume product with few keepout areas suits spray, while a product with several connectors and test points suits selective coating even at lower volume.

Cost should include the masking labour, the material consumption and the cycle time, not just the price of the coating. The materials and their protection role are described under conformal coating and board protection.

Process Control and Verification

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does coating replace the need for cleaning? No. Coating over an ionic residue traps the contamination against the surface and can make the situation worse. The board should be clean and dry before coating.

Can a connector be coated? Most connector contacts must stay uncoated, because the film increases contact resistance. Those areas are masked or selectively avoided, and the requirement is written into the drawing.

How is thickness measured on a finished board? By an eddy current or ultrasonic gauge on a witness coupon coated at the same time, or by a cross section. Direct measurement on a component is unreliable.