Conformal coating

Summary¶

Conformal coating is a thin layer of silicone or acrylic applied over completed PCB assemblies, covering every solder joint, component, and exposed trace. It is electrically transparent at the frequencies electronics operate at, mechanically protective against moisture and condensation, and prevents conductive contamination (dust, water, salt) from bridging between exposed conductors. Once applied it is essentially permanent — it cannot be removed cleanly without damaging the board. Therefore: complete all soldering first, mask all connectors and pads that will be soldered later, then coat. Apply before the first flight, not after a moisture incident.

Concept¶

Why electronics are vulnerable to moisture¶

Electronic components and PCB traces are designed to function in dry air. Moisture creates two failure modes:

Conductive bridging: Water — particularly with dissolved ionic contamination from sweat, dew, salt, or condensation on concrete — is a conductor. A water film across two adjacent PCB pads or traces creates a resistive short. On a 3.3V logic circuit, this may cause misbehaviour. On a power circuit, it can cause sustained current flow that heats the water film, deposits ionic residue, and eventually creates a permanent conductive path — a salt bridge that remains even after the water evaporates.

Corrosion: Exposed copper oxidises in the presence of moisture and oxygen. Solder joints and exposed pads corrode gradually, increasing resistance and eventually causing open circuits. Conformal coating prevents oxygen and moisture from reaching the copper.

Why a drone is particularly vulnerable¶

A drone operating in the environment encounters: morning dew when flying at dawn; rain in conditions that start clear and deteriorate; condensation when moving from a warm vehicle or case into cold outdoor air (the electronics are warm from storage; cold humid air condenses on them); concrete spray from landing near water; humidity inside a storage case that is not properly dried after a wet flight.

Skateparks — one of libdrone's operational environments — are particularly challenging: damp concrete, water features, and the practice of wetting surfaces for certain activities. Even without direct water contact, the humidity is elevated.

What conformal coating does and does not do¶

Does: creates a hydrophobic (water-repelling) barrier over all exposed conductors; prevents conductive bridging from surface moisture; slows corrosion of exposed copper; provides minor mechanical protection against abrasion.

Does not: make the board waterproof — connectors, through-holes, and component openings are not sealed; prevent submersion damage; protect against direct high-pressure water contact; survive repeated abrasion.

Application sequence¶

Conformal coating cannot be removed cleanly from a PCB. Attempting removal with solvent risks lifting traces, dissolving component markings, and removing solder resist. Therefore:

1. Complete all soldering first. No further soldering after coating.
2. Mask all connectors, pads, and test points that must remain conductive or solderable. Use kapton tape or connector-specific masks.
3. Apply coating in thin, even layers. Brush application or spray are both acceptable; spray provides more even coverage on complex board topography.
4. Allow to cure fully before the first flight — typically 24 hours for silicone, 1–2 hours for acrylic (check manufacturer's datasheet).

Reference¶

Components requiring coating on libdrone¶

Coating material options¶

Silicone is preferred for drone use because it maintains flexibility at low temperatures (important for winter operations) and provides the best vibration resistance.

Inspection after coating¶

Hold the board at an angle under a UV lamp. Silicone and acrylic conformal coatings fluoresce under UV — coated areas glow, uncoated areas are dark. Check for: uncoated areas on exposed traces (missed coverage), coated connector pins (masking failure), bubbles or thick pools (may crack under vibration).

Procedure¶

Coating the H7A3-SLIM¶

1. Apply kapton tape over: USB port, all JST/DF connectors, motor output pads, UART header pins, voltage/current monitor pads.
2. Secure the FC to a clean non-conductive surface so both faces are accessible.
3. Apply coating with a fine brush to all exposed copper: component pads, solder joints, exposed trace runs between components. Work in sections.
4. Avoid leaving pools in component cavities — thin and even is correct.
5. Allow to cure at room temperature for the specified time.
6. Remove masking tape before cure completes (if tape was not removed before application) to avoid pulling up adjacent coating.
7. Inspect under UV. Re-apply to any missed areas. Allow second application to cure fully before removing remaining masking.

Rationale¶

Why coat before the first flight rather than after a moisture incident¶

By the time moisture has caused a failure, conductive residues may already be deposited on the board. Coating after a moisture incident seals those residues in place — the problem may appear to be fixed but the residue continues to cause intermittent failures under heat or humidity. Coating a clean, never-moisture-exposed board ensures the barrier protects against contamination from the beginning, not after the damage is done.

Why the VTX is not coated¶

The HDZero Freestyle V2 VTX has a manufacturer-applied conformal coating. Applying a second layer can trap heat between the coatings, interfering with thermal management at the power amplifier — which runs at significant dissipation (1–2W) during transmission. The manufacturer coating is adequate.

Connections¶

requires: [] related: - emc-noise-sources - power-signal-separation leads_to: - airframe-integration