Conformal Coating Masking: PCB Keep-Out Drawing & Checklist

This guide answers a practical engineering and purchasing question: what must you put in a coating masking drawing so your PCBA supplier can quote and build the intended result? It focuses on the release package and acceptance decisions, rather than choosing a universal coating chemistry or inventing one masking clearance for every board.

For a complete build, send your PCB files, BOM and coating requirements for a turnkey PCBA review. Include your quantities for each model and variant.

1. What masking must preserve

A conformal coating can protect designated circuit areas, but the same material can interfere with a connection, opening or service operation if it reaches the wrong surface. Specify the interface that must remain usable, not merely the name of the nearby component.

For example, a connector’s mating contacts and a connector’s soldered terminals have different functions. Whether the body, terminals or surrounding board can be coated depends on the exact part, coating and approved process. A sealed-looking housing is not evidence that liquid or vapor cannot enter it.

Masking also has a removal stage. A temporary protection that works during application must come off at the required point without leaving obstructive residue or damaging the assembly. Application, curing and removal conditions therefore belong in the process review.

2. Which areas need a coating keep-out review?

The table is a review checklist, not a blanket prohibition on coating every component in a category. Follow the exact component manufacturer’s instructions and your product requirements.

Sensor example: an exposed humidity-sensing opening must remain functional and free of the prohibited coating. A manufacturer-approved temporary cover can have its own removal instruction. Do not apply generic tape directly to every sensor: the permitted method and sensitivity to processing chemicals depend on the actual device.

3. Build a masking drawing that can be inspected

Start with the released assembly identity: board number, PCB revision, assembly revision and variant. Show the top and bottom sides with enough reference designators to identify each restriction. Use a clear legend for “coat,” “do not coat” and “engineering review required.” Color helps, but hatching and text should also make the meaning readable in a monochrome print.

For each restricted area, define the surface or boundary in a view that the operator and inspector can actually use. A note pointing at a connector may not show whether the restriction covers only its opening, the whole body or adjacent PCB copper. Add a detail view when the main view is ambiguous.

1. Identify the build. State the board, assembly revision, variant and coating drawing revision.
2. Define each surface. Link the region to a reference designator, interface or dimensioned feature; identify the side.
3. Specify the boundary. Provide the required limit and permitted boundary variation, with units and a datum where needed. Have the assembler confirm that the process can achieve it.
4. Define the coating system. Identify the approved product or controlled specification, thickness requirement and cure/handling criteria appropriate to the selected material.
5. Record process-sensitive parts. Include component restrictions and the approved protection/removal method where needed.
6. Define acceptance. Specify coverage, restricted surfaces, required inspection evidence and any functional checks.

There is no single masking clearance that is suitable for every spray valve, coating viscosity, connector geometry or assembly. A tight boundary beside a tall component deserves a manufacturability discussion before the PCB layout and quote are frozen.

Keep this drawing linked to the PCB assembly drawing. If a BOM alternative changes a connector body or sensor package, revisit its masking instructions before approving that substitution.

4. Compare the masking method with the build

Temporary tapes, caps, plugs, peelable masks and fixtures can serve different geometries. Selective application controls where coating is delivered; it does not remove the need to define the finished keep-outs. The supplier should review the actual part geometry, material compatibility, exposure conditions and removal access.

Do not transfer the removal timing of one peelable product to another. Coating and masking suppliers provide material-specific instructions. The same caution applies to cure temperatures and cleaning: a generally suitable board-cleaning step may be unsuitable for an exposed sensor.

5. Agree on testing, removal and first-build acceptance

A practical sequence starts with an approved release package and includes inspection of the assembled board before coating hides access. Programming or functional testing may occur at a different point for different products. State the intended sequence and required contact access explicitly.

1. Confirm assembly identity and complete the agreed pre-coating checks.
2. Prepare the assembly using compatible, approved cleaning and drying procedures.
3. Apply the released masking and verify the protected interfaces.
4. Apply and cure the specified coating under its approved process.
5. Remove temporary protection at the stage required for that material and assembly.
6. Inspect coverage, boundaries, removal and interface access; perform the agreed final checks.

A visual or UV inspection, where supported by the coating system, helps assess coverage. It does not by itself prove film thickness, electrical function or environmental qualification. Define each required measurement and test separately.

For a first build, agree which views or records must be reviewed and whether a customer approval hold is required before the remainder is processed. A sample photograph is useful evidence of an identified feature; it is not a substitute for all acceptance requirements.

6. Example: resolve one masking conflict before production

Consider a fictional controller assembly whose drawing says “mask J1,” while the BOM alternative list permits two connector bodies with different shapes. The coating program was prepared around only one body. The proper next step is a controlled clarification, not an operator guessing which file wins.

For high-mix production, use a variant matrix rather than assuming one mask fits every product. The matrix can identify assembly revision, fitted MPN, masking drawing, fixture/program revision, removal instruction and test procedure. A new component source alone may not change geometry; a different approved MPN can. Review the actual change.

7. Checklist for a turnkey PCBA quote with coating

Help the assembler quote the finished requirement, including masking work, tooling, curing and inspection. Useful inputs are:

• Gerber/manufacturing files, released BOM, placement data and assembly drawing.
• Model/variant quantities and whether shipment is coated PCBA or a further assembled product.
• Top/bottom coating views, keep-out details, boundary requirements and exact coating specification.
• Restrictions from the component documentation, including exposed sensors, connectors and temporary covers.
• Programming/test sequence, post-coating test access, fixtures and pass/fail criteria.
• Required inspection records, first-build approval and any qualification or traceability requirements.
• Target delivery date and whether proposed material changes need written approval.

Ask that one-time tooling or programming and recurring masking/inspection work be identified clearly in the quote. Price and schedule depend on the released design, selected materials, quantities and process scope. A standard coating material alone does not establish the suitability of the complete product for its environment.

8. Conformal coating masking FAQ

Does selective coating mean that physical masking is unnecessary?

Not automatically. Selective coating controls the application path. The actual design may still need protection against flow, access limitations or other process effects. Release the required finished boundary and have the supplier confirm the method.

Should all connectors be left completely uncoated?

Define the exact surfaces that must remain free of coating, particularly mating contacts and required openings. Treatment of the body, terminals and adjacent board depends on the specific connector, coating and approved process.

What masking clearance should I put on my drawing?

Use the boundary and tolerance your design requires, then review manufacturability with the assembler. There is no universal clearance suitable for every coating, application method and component geometry.

Can test points be coated after the board passes its first test?

Only if the released requirements allow it. Consider tests after coating, programming, rework and service access. Specify which contacts must remain accessible in the shipped product.

Does a temporary sensor cover make every coating process safe?

No. Check the exact sensor and cover instructions, coating compatibility and removal sequence. A cover that blocks liquid does not necessarily protect against every processing vapor or chemical exposure.

Can one masking drawing cover several PCBA variants?

Yes, if it unambiguously identifies all applicable variants and their differences. Use a controlled matrix or separate details when fitted components, keep-outs, tooling or test access change.

What if my coating drawing is not ready?

Send the assembly files and describe the interfaces that must remain usable, the environment and any specified material. Ask for a preliminary review. Approve the coating and masking requirements before production proceeds.