How to choose a conformal coating for your PCB

A conformal coating is a thin polymeric film applied over an assembled printed circuit board so that it follows — conforms to — the contours of the components and the bare laminate underneath. Its job is to keep moisture, condensation, dust, airborne contamination, corrosive gases and stray conductive debris away from conductors that are only a fraction of a millimetre apart, and to raise the voltage that board can stand off between them.

What follows is the sequence our engineers use when a customer sends us a board and asks what to coat it with. It is deliberately ordered, because each answer narrows the next.

Step 1 — Define the environment, precisely

Almost every poor coating decision starts with an environment described as "industrial" or "outdoor". The specification that actually drives selection is narrower than that:

• Temperature range and dwell. Not just the maximum, but how long the board sits there. A part that sees 125 °C continuously is a different problem from one that touches it for ten minutes during reflow-adjacent operation.
• Humidity and condensation. Steady high humidity is survivable for most chemistries. Repeated condensation cycling — a cold board in warm humid air, over and over — is the condition that finds every pinhole and every thin edge.
• Chemical and salt exposure. Fuel, oil, solvents, cleaning agents, coastal salt fog. This is where acrylic runs out of road fastest.
• Mechanical and vibration load. Continuous vibration argues for a coating that stays flexible rather than one that goes hard and cracks at a solder fillet.
• Electrical stress. Conductor spacing and working voltage set how much dielectric withstand you actually need from the film.
• Service life and repairability. A ten-year field product that will be repaired is a fundamentally different brief from a sealed consumer assembly that is replaced on failure.

Step 2 — Match the environment to a chemistry

Five ELPEGUARD® families cover the overwhelming majority of Indian electronics manufacturing. In broad terms:

The full side-by-side, including cure behaviour and rework routes, is in acrylic vs polyurethane vs silicone. If the environment is severe enough that a thin film may not be the right answer at all, read conformal coating vs potting and encapsulation before going further.

Step 3 — Choose the application method for the board, not the preference

Chemistry decided, the board and the volume decide how it goes on:

• Brush. Prototypes, touch-up, repair and very low volumes. No tooling, no programme, complete operator dependence.
• Pneumatic spray. Mixed board types and moderate batches. Needs masking, and needs a booth.
• Dip. Full coverage including underside and shadowed areas, at the cost of extensive masking. Efficient where the same board repeats.
• Selective coating equipment. A programmed nozzle path that avoids keep-out areas entirely, so masking largely disappears. Right for repeating production; the programming effort has to be amortised. See our selective coating machines.

The masking burden is the hidden cost here. A board with fourteen connectors coated by spray can consume more labour in masking and de-masking than in coating, and that is usually the argument that carries a line onto selective equipment.

Step 4 — Set a thickness band, and agree how it is measured

For conventional solvent- and water-based systems, dry film thickness of roughly 25–75 µm (1–3 mils) is the working range, with 50 µm a frequent target. UV thick-film systems are applied deliberately heavier. Two practical points matter more than the number itself:

• Thickness is a measured property. IPC-CC-830B and IPC-A-610 both treat it that way, and a lot released on visual impression alone has not really been inspected.
• Coverage on a populated board is not uniform. Component shadows, tall parts and board edges are where a nominally correct average hides a thin patch — which is exactly where condensation cycling attacks first.

Step 5 — Decide what stays uncoated

Before the first board is coated, mark up the keep-out areas: connectors and mating surfaces, test points and programming headers, gold fingers, sockets, adjustable components, sensor apertures, heatsink and thermal-pad interfaces, and any grounding or bonding point. Mask them for spray and dip, or programme around them on selective equipment — then verify they are clear after cure, not just after masking.

Step 6 — Fix cleanliness and cure before you fix anything else

Coating adhesion is decided before the coating arrives. Flux residue, handling oils, mould release, fingerprints and moisture trapped in the laminate all cause dewetting, blistering and delamination that look like coating defects and are not. No-clean flux is not automatically compatible with every coating system; where it is not, the board is cleaned. Equally, "tack-free" is not "cured" — moving or packing a board before its cure schedule completes is a common and entirely avoidable source of surface defects.

Step 7 — Write down the inspection regime

ELPEGUARD coatings carry a UV tracer, so a film invisible in normal light fluoresces under UV. That turns an unverifiable process into a visible one, and it is why every board we coat is inspected under UV rather than sampled. Alongside that, agree the acceptance class up front — IPC-A-610 Class 2 covers most industrial and automotive electronics, Class 3 applies where the assembly is life- or safety-critical. Our standards explainer covers what each class actually requires.

The short version

1. Write the environment down properly — temperature, condensation, chemicals, vibration, voltage, service life.
2. Let it choose the chemistry: acrylic for general use and repairability, polyurethane for chemical and abrasion exposure, silicone for temperature and flex, UV thick-film for volume, aqua for lower VOC.
3. Let the board and the volume choose the application method — and count the masking labour honestly.
4. Agree a measured thickness band, not an impression.
5. Mark the keep-out areas before the first board is coated.
6. Fix cleanliness and cure discipline first; they cause more "coating failures" than coatings do.
7. Inspect under UV against a stated IPC-A-610 class, every lot.

EMS Technologies supplies the full ELPEGUARD range in India as distributor for Lackwerke Peters, and runs the same process as on-site job work with 30+ trained engineers. If you would rather have the answer than the framework, send us the board details.