“PCB coating” on an RFQ is three different factory jobs wearing one English word. Buyers mean solder mask on the bare board, a thin conformal film on the assembled board, or a thick potting shell around a module. Mixing them produces the wrong traveler, the wrong keep-outs, and the wrong waterproof claim.
This post is the word split for overseas buyers and engineers. It keeps the useful industry facts—moisture-resistant is not waterproof, keep-outs matter, chemistry families exist—and leaves spray-can tutorials and type bake-offs to posts that already own them.
Job 1: Solder mask (fab)
Solder mask is the polymer imaged on the bare PCB before parts exist. It defines where solder is allowed, protects copper, and is part of the fabrication stack and color callout. It is not applied over connectors after SMT. If your RFQ says “coating” and you mean green mask and open pads, say solder mask on the fab drawing. Mask color and opening rules are fab topics; do not send them as an assembly conformal note.
Job 2: Conformal coating (assembly)
Conformal coating is a thin film applied after assembly so it follows the shape of parts and traces. It slows moisture, dust and some contaminants. It is moisture-resistant, not a submarine seal. Acrylic, urethane, silicone, epoxy and parylene are different chemistries with different rework and process seats—our conformal types and assembly-process posts cover that selection. Here the point is only: this job needs keep-outs for connectors, test points, adjustable parts and mating surfaces, plus a cleanliness state before coat.
Our PCB Assembly scope can include conformal when you quote it. Silence means no coat.
Job 3: Potting or encapsulation (module)
Potting fills a housing or dams a region with a thick resin so liquid immersion and mechanical abuse are in a different league than a 25–100 µm film. It changes weight, rework and thermal paths. Epoxy versus silicone potting is its own materials decision. If the product will be submerged or hose-washed as a sealed unit, potting or a gasketed enclosure is the conversation—not a thicker conformal spray.
Moisture-resistant is not waterproof
Conformal film slows vapour and splash. It remains somewhat permeable. Marketing that says “waterproof coating” without an IP rating of the finished enclosure is a claim the film cannot carry alone. Write the environment: humidity, salt fog, splash, immersion. Then pick mask-only, conformal, potting, or enclosure sealing—possibly more than one.
What the RFQ must name so the factory does not guess
- Which of the three jobs (mask / conformal / potting).
- For conformal: chemistry family or equivalent rule, keep-out drawing, thickness window if you care, cure and inspection (UV tracer is common).
- For potting: resin family, fill height, keep-outs, and whether the housing is part of the PO.
- Cleanliness requirement before coat (especially after no-clean versus washed flux).
Our Manufacturing Files package should include the keep-out layer or assembly note. A photo of a spray can is not a keep-out.
Defects that are process, not “wrong brand”
Bubbles, fish-eyes, orange peel, edge pooling and soft cure are usually contamination, thickness, flash time or cure—not a reason to rename solder mask as conformal. Fix the process seat. Rework rules differ by chemistry; acrylics are generally more repairable than epoxies. Do not invent a universal stripper on the PO.
Harsh environments without copying a pitch list
Outdoor, automotive, industrial and high-humidity products often need more than indoor consumer handling. That still does not mean every board in those markets gets parylene by default. Rate the stresses (humidity, salt, chemicals, temperature swing, vibration), then choose mask-only, conformal chemistry, potting, or enclosure sealing. Creepage and clearance for mains voltages remain layout rules; coating can help dielectric strength but does not erase a spacing mistake on the drawing.
What we are not claiming
No “best coating” ranking, no spray distance recipes, no invented micrometre mandates as XFPCB defaults, and no claim that conformal replaces an IP-rated enclosure. Say mask, conformal or potting. Then the factory can open the right traveler.