Creating a Conformal-Coating Keepout Map

A drawing that says “coat the PCB except connectors” leaves too much interpretation to production. A connector may have solder joints that should be coated and contact surfaces that must remain clear. Define keepouts by their function and physical boundary so the applicator and inspector make the same decision.

Inventory sensitive surfaces

HumiSeal distinguishes overspray during application from material flowing into a keepout after deposition. Its guidance discusses the roles of chemistry, valve settings, fixturing, and inspection timing. A masking plan therefore needs to consider what happens after the applicator moves away, not only its programmed path. HumiSeal: selective-coating boundaries.

List electrical contacts, test points, adjustable parts, acoustic or pressure openings, optical surfaces, thermal interfaces, and mechanical mating features. Check component documentation where coating compatibility is uncertain. Assign each exclusion a reference designator or drawing coordinate and explain the function being protected.

Specify boundaries that can be inspected

• Mark required coverage, prohibited coverage, and any permitted transition region.
• Show both board sides and relevant component surfaces.
• Identify whether the keepout applies before cure, after cure, or both.
• Describe how temporary masks will be installed and removed.
• Include features that become hidden after final assembly.

For example, an edge connector requires a clear contact area, but the nearby board surface may need environmental protection. A dimensioned boundary and an agreed transition allowance are more reproducible than a hand-drawn circle around the whole connector. Confirm that the selected coating process can maintain that boundary on the populated assembly.

Qualify the masking material and process

Use masking products intended for the coating chemistry and application conditions. Electrolube, for example, offers a peelable mask specifically for protecting components and connectors during coating; that product description does not establish compatibility with every process. Validate adhesion, removal, residue, and interaction with the actual parts. Electrolube: peelable coating mask.

Check coverage and exclusions separately

Inspect the coated areas for the agreed condition and separately verify that the protected features remain usable. Fluorescence can assist inspection when the material supports it, but it should not be treated as a universal measurement of cure or thickness. Retain photographs of the approved first article and any difficult boundaries. Revisit the map when components, labels, fixtures, or coating chemistry change. The aim is a functional protected assembly, with clear evidence that the surfaces required for connection, sensing, cooling, and service remain available.

Make demasking a recorded operation, with a final check that no temporary material remains in contacts, apertures, or service interfaces.