Conformal Coating for PCBs: When You Need It and Which Type to Choose

Many electronic failures in the field come from the environment the board ends up in rather than from the design: humidity in a pump house, condensation in an outdoor cabinet, dust in a factory, salt air near the coast. A conformal coating is a thin protective film, usually 25 to 75 µm for acrylic, urethane and epoxy (silicones are applied thicker, about 50–200 µm), that follows the shape of the board and its components and shields them from all of that.

When is coating worth it?

Coat the board if it will live somewhere it can see:

• high humidity or temperature swings that cause condensation;
• dust, metal particles or conductive contamination;
• salt mist (coastal and marine installations);
• vibration, where the coating also helps secure small parts;
• high voltages, where the coating raises resistance to surface leakage and arcing.

Typical examples are automotive and EV electronics, outdoor lighting and telecom equipment, solar inverters and chargers, industrial controls, agricultural electronics and medical devices.

If the board lives in a clean, dry, air-conditioned enclosure, coating may simply add cost. It is worth deciding consciously rather than by default.

The main coating types

Coatings are classified in the IPC-CC-830 standard by their chemistry:

For most industrial boards, acrylic or silicone is the starting point: acrylic when rework matters, silicone when heat and humidity are severe.

How coating is applied

• Brushing: for small batches and touch-up.
• Dipping: fast and thorough, but everything gets coated unless masked.
• Spraying: manual or automated, good coverage on medium volumes.
• Selective robotic coating: a programmed nozzle applies coating only where it is needed and keeps it off connectors and test points. It is repeatable and suits production volumes.

Parylene is the exception: it is deposited as a vapour in a vacuum chamber.

Masking: what must stay uncoated

Some areas must never be coated: connectors, switches, test points, heatsink contact surfaces, potentiometers and anything that needs to make electrical contact. These are masked with tape, caps or boots, or simply avoided by selective coating. Mark keep-out areas clearly on the assembly drawing.

How to specify coating

Put these on your drawing or purchase order:

1. Coating type (or the IPC-CC-830 type designation, such as AR or SR).
2. Target thickness range.
3. Areas to coat and areas to keep clear.
4. Inspection method. Most coatings contain a UV tracer so coverage can be checked under ultraviolet light.
5. Whether rework or repair must be possible.

A note on cleanliness

Coating seals in whatever is underneath. Flux residues or contamination trapped under the film can cause corrosion and leakage later. Clean, dry boards and good process control before coating matter as much as the coating itself.

Coating at Polyfit

Conformal coating is part of Polyfit's in-house EMS services, alongside SMT and through-hole assembly, functional testing and box build. Boards are built in an ISO 7 cleanroom. Tell us about your product and its environment and we will come back with a quote.