Conformal Coating¶
Why Conformal Coating¶
Marine electronics live in salt-laden, condensing-humidity environments. Bare PCBs in that environment fail by three mechanisms, in roughly this order:
- Surface ionic conduction. Hygroscopic dust films collect, absorb moisture, and form a low-resistance leakage path between adjacent pins. The first symptom is usually a high-impedance node (ADC, FB pin, op-amp input) reading drifted or biased, not a hard short.
- Galvanic corrosion at via barrels and solder joints. Salt-laden water deposits on dissimilar metals (tin-lead solder, ENIG gold, copper) drive slow electrochemical attack. Visible as green or white efflorescence around joints.
- Dendrite growth between adjacent traces under DC bias. Long-term failure mode on high-voltage isolation gaps. Common on power-entry circuits where 12–48 V sits across millimetres of trace gap for years.
Conformal coating addresses all three by sealing the board surface from atmospheric moisture and contaminants. It is not a structural element — its job is purely environmental.
Coating Material — Acrylic¶
For this board the recommended coating is MG Chemicals 419D Acrylic Conformal Coating (or any UL-recognized acrylic conformal coating that meets IPC-CC-830).
Acrylic is selected over silicone, urethane, or parylene because:
- Reworkable. Field service may require removing a component (e.g., replacing a fuse or a sensor connector). Acrylic dissolves in MG 4140 stripper or even isopropanol with enough soaking — silicone and urethane do not.
- Single-part, room-temperature cure. No mix ratio, no oven required.
- Visible UV-fluorescence under blacklight confirms coverage during inspection.
- Acceptable moisture and salt-spray resistance for this duty cycle. The board is not submerged; it lives in a cabin or engine room.
Acrylic's main weakness is poor abrasion resistance and lower upper-temperature limit (~125 °C) than urethane. Neither matters for an enclosed PCB in a wooden housing.
Areas to Mask Before Coating¶
Coating must NOT cover the following surfaces:
| Area | Reason |
|---|---|
| RJ45 jack contacts (RJ3, RJ5) | Contact resistance / mating |
| USB-C connector (J19) contacts and shield | Same |
| WJ2EDGRC terminal block (J2) screw-clamp contacts | Same |
| NMEA2000 Micro-C connector (U18) | Same |
| All jumper headers (J15, J17, INP jumper) | Jumper insertion |
| ESP32-S3 module antenna keep-out | RF detuning |
| BMP388 (U14) sensor port | Pressure port must remain open to air |
| DS18B20 connection | Field service replaceability |
| Mounting screw holes | Avoid coating slipping during torque-down |
Mask with Kapton tape for connectors and headers. For the BMP388 sensor port (a small opening in the LGA package), apply a single drop of coating-resistant peelable mask or simply lift the part with thin Kapton across its top during spray.
The ESP32-S3 module is internally shielded; the bottom edge of the module is keep-out to avoid loading the printed antenna trace. A 5 mm clearance band around the antenna pattern is sufficient.
Application Method¶
Brush application is recommended for V9 build quantities (qty < 50). Two thin coats, applied at 90° rotation to each other, cured 30–60 minutes between coats and 24 hours before service.
| Step | Action |
|---|---|
| 1 | Clean the assembled board with isopropanol and let dry 30 minutes. No-clean flux residues are acceptable under acrylic but rosin is not — wash visible rosin first. |
| 2 | Mask all keep-out areas with Kapton tape. |
| 3 | Brush coat 1 in long parallel strokes. Cover all SMD pads, BGA edges, and via holes. |
| 4 | Cure 30 minutes at room temperature in a dust-free space. |
| 5 | Brush coat 2 perpendicular to coat 1, same coverage. |
| 6 | Cure 24 hours before installing in enclosure or handling. |
| 7 | UV-inspect under blacklight to confirm uniform coverage. Touch up any visible gaps. |
| 8 | Remove masking tape carefully — lift slowly to avoid pulling coating off adjacent areas. |
Spray application is faster for higher volumes but requires a properly ventilated spray booth and more careful masking. Defer until production volume justifies the setup.
Inspection and QC¶
After cure:
- UV blacklight check. Acrylic coating fluoresces blue-white. Look for dark spots (missed areas) under all ICs, around the perimeter of the board, and along high-voltage trace gaps near the TPS48000 input stage.
- Coverage at component edges. Common failure: coating bridges to the side of a component body but doesn't fully cover the solder fillet. Look at every QFN/SOIC edge.
- No coating in connector cavities. Wipe out with cotton swab while still soft if found.
Field Service After Coating¶
To service a coated board:
- For component-level replacement: apply MG 4140 stripper to a cotton swab and dwell on the area for 60–120 seconds, then wipe clean. Reapply if needed. Avoid getting stripper on the BMP388 sensor port or the ESP32 module.
- For a full recoat after service: clean with IPA, repeat application sequence above.
- Salt water exposure event: wash with fresh water, dry thoroughly (24 h in a warm dry place), inspect under UV, touch up any lifted coating with a small brush.
Operational Notes¶
- Do not coat until all factory test and calibration is complete — once coated, the test points are inaccessible.
- The board's TPS48000 input stage has 65 V across small gaps. The acrylic coating substantially raises the surface resistance there and is one of the load-bearing elements of long-term reliability on this board, not just a nice-to-have.
- Coating is one of the items called out as a Future Plan in input_protection.md — it is recommended for any board destined for installation in a real boat, not just bench use.