UK The Short Answer
A circuit board inside a sealed enclosure can still see moisture: air trapped inside condenses when the temperature drops, and salt or sulphur in the atmosphere slowly corrodes exposed copper and solder. Conformal coating puts a thin insulating barrier over the assembled board, with connectors and contacts masked so they still mate.
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Acrylic (AR) Easy to apply and to remove for rework, with good moisture protection. The most common general-purpose choice.
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Silicone (SR) Stays flexible across a wide temperature range, so it suits boards that cycle between hot and cold.
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Urethane (UR) Tough and resistant to solvents and fuels, at the cost of harder rework.
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Parylene (XY) Vapour-deposited in a very thin, pinhole-free layer that covers every surface, used where reliability is critical.
Why It Matters
For a rugged computer, coating closes the gap that sealing leaves: an IP-rated enclosure keeps water out, but it cannot stop condensation forming inside during a cold start. Coated boards are common in rail, marine, agricultural and outdoor deployments, and coating is available as an option on many rugged boards and systems, so ask for it when you specify. Our guide to what makes a computer rugged covers the other protections, and our rugged single board computers show board-level options.