Conformal Coating Masking Methods and Materials

Conformal coating covers what it is applied to, and some features must remain uncovered: connectors that have to mate, test points that have to be probed, and components that have to dissipate heat. Masking is the process that keeps the coating off them.

What Has to Stay Clear

The keep-out list is short and it is specific to the product. A connector that is coated cannot be mated reliably, a test point that is coated cannot be probed, and a heatsink that is coated loses part of its performance.

The list belongs on the drawing rather than in the coating operator’s judgement, because the consequences differ. A coated test point is a production problem and a coated connector is a field problem. Our coating notes describe the application side of the same decision.

Points to Confirm at First Article

On a design of this kind, masking is the item that decides how the rest of the board is arranged. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

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. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. 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 an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. 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.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Documented Process Control

On a design of this kind, masking is the item that decides how the rest of the board is arranged. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. 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.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. 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.

Inspection and Traceability Notes

On a design of this kind, masking is the item that decides how the rest of the board is arranged. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

What to Record and Why

On a design of this kind, masking is the item that decides how the rest of the board is arranged. 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. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Masking Materials

The materials are tapes, boots, plugs and peelable compounds. A tape is quick and its adhesive can leave residue; a boot is reusable and has to fit the feature; a peelable compound covers an area and is removed after cure.

The choice follows the shape of the feature and the temperature of the cure. A tape that is easy to apply and leaves adhesive behind is not a saving if the residue has to be cleaned. Our cleanliness measurement notes describe how the residue is found.

Applying and Removing the Mask

The mask is applied before the coating and removed after the cure, and both operations are manual. That means the accuracy and the consistency come from the instruction rather than from a machine.

A photograph of the masked board is worth more than a written description of where the tape goes. The mask is one of the few operations where the operator’s interpretation of a drawing decides the result.

Coating Around the Mask Edge

The coating forms a step at the edge of the mask, and the step is a place where the coating can lift. A mask edge that is placed on a flat area away from a component gives a cleaner step than one that is placed against a component body.

The edge of the mask therefore belongs in the design discussion. Placing a keep-out on a flat area is a layout decision that costs nothing and improves the coating. Our joint criteria notes describe the related acceptance for the joints underneath.

Cure and Mask Compatibility

The cure temperature has to be one the mask survives, and the mask has to be one that does not leave residue at that temperature. A tape used above its rated temperature leaves adhesive that is difficult to remove and that can be conductive.

The compatibility is verified on a coated and cured sample rather than assumed from the data sheet. A single trial with the production tape and the production cure answers the question.

Verification of the Keep-Out

The keep-out is verified visually after the mask is removed, and the check is that the feature is clean and free of coating rather than that the mask was applied. A mask that fell off during the spray leaves a coated connector that looks correct until it is mated.

Where the keep-out is critical, the verification is recorded per board rather than per batch. Our board quality notes describe how the check is documented.

FAQ

Can a connector be cleaned instead of masked? It can where the coating can be removed without damaging the contacts, and masking is the more reliable route.

Is a peelable mask better than tape? It is better for an irregular area and it needs its own cure and removal verification.

What does gopcb provide for coating masking? We provide a keep-out list on the drawing, masks chosen for the feature and the cure, mask edges placed on flat areas, compatibility verified on a cured sample, and the keep-out verified and recorded per board.