Conformal Coating Masking and Keep Out Control

Conformal coating protects a board only where it is present, and the areas where it must not be present are as important as the areas where it must. Masking is the step that keeps the two apart.

Areas That Must Stay Clear

Connectors, test points, adjustment trimmers, relay contacts, optical surfaces and any surface that has to make electrical contact are the usual exclusions. A coating on a connector body can prevent mating or trap contamination.

The keep out belongs on the assembly drawing rather than in a note. Our coating notes describe the process that follows it.

Masking Materials

Peelable masks, tapes, boots and dispensing fixtures are all used. The choice depends on the geometry, the number of boards and whether the mask has to survive a cure or a reflow.

A mask that leaves adhesive behind has replaced one defect with another. Our cleanliness notes describe the check.

Coating Thickness and Coverage

The thickness is specified for the cured film, and the film thins over edges and peaks on tall components. Measuring on a flat area gives a number that does not describe the edges where protection is needed most.

The measurement should therefore include a component edge and a board edge. Our coating inspection notes describe where to measure.

Connector Protection

A connector that is masked should be inspected after demasking for coating creep and for adhesive residue. Both are difficult to remove once the assembly is closed.

Where the connector is not masked, the coating has to be applied around it. Our connector notes describe the geometry that results.

Cure and Its Effect on the Mask

A thermal cure subjects the mask to the same temperature as the coating, so the mask has to be rated for it. A UV cure reaches the film only where light does, which is a limitation on shadowed areas.

The cure schedule should be recorded as a profile. Our thermal measurement notes describe the recording.

Verification

The verification is a coated board examined under ultraviolet light, which makes the film visible and shows thin areas and pinholes. The keep out areas are checked in the same pass.

A visual check under white light alone will not show a thin film. Our quality notes describe the lighting that is used.

Records and Rework

The mask type, the application method, the coating material and the cure profile should be recorded per lot. Where a board is reworked, the coating has to be removed locally and reapplied, and the record should show it.

Our rework notes describe the local removal.

Process Control and Verification

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Can the keep out be defined by a note? A note helps, but the areas should be drawn, because the operator applying the mask works from the drawing rather than from the text.

Is peelable mask reusable? It is not, because it deforms on removal and loses its sealing edge.

What does gopcb provide for coating work? We provide a drawn keep out rather than a text note, masking material chosen for the cure it must survive, thickness measured on a component edge and a board edge as well as a flat area, inspection under ultraviolet light, and a recorded mask, material and cure profile per lot.