PCB Handling After Conformal Coating

A coated assembly is more robust and also more difficult to handle, because the coating can be damaged and because the film continues to change after it is applied. The rules for handling, curing and rework therefore differ from those for an uncoated board.

The period immediately after coating is the most delicate, and it is also the period when the boards are most likely to be moved because the line wants them out of the way.

Cure and Handling Time

The coating should be handled according to its cure schedule, and there is usually a period during which the film is tack free but not fully cured. Handling during that period leaves marks that cannot be removed.

The cure schedule should be recorded, and the boards should be left in the specified environment until it is complete. Moving boards to a rack early is a common cause of cosmetic and functional damage.

Touch and Cleanliness

A cured coating resists handling, while an uncured one does not, and even a cured film can be attacked by solvents and by some cleaning agents. The handling rules should be written for the cured state.

Cleaning a coated board after assembly can damage the film, so the cleaning decision should be made before the coating rather than after. This interaction is described for coating compatibility.

Mechanical Damage

The film can be cut by a sharp edge, abraded by a rough surface and cracked by bending. The packaging should protect the coated surface, and the handling fixtures should not touch it.

A coating that is damaged in a small area can expose a conductor, so the damage should be assessed rather than ignored. The repair decision is a coated version of the rework verdict described for rework and scrap.

Rework Through the Coating

Rework requires the film to be removed locally, the joint to be repaired and the area to be recoated. The removal chemistry is specific to the coating, and it should be selected with the coating rather than after it.

The recoating of a small area is difficult to make continuous with the surrounding film, so the adhesion at the boundary is the critical property. A repair that delaminates at the edge is worse than the damage it replaced.

Testing After Coating

Electrical test after coating is possible where the test points are masked, and the test should not puncture the film. Where the film must be penetrated, the repair has to be made afterwards.

The test sequence should therefore be decided with the masking plan, so that the points that will be needed are left free.

Inspection After Coating

Inspection under ultraviolet light shows the coverage and the gaps, and a thickness measurement confirms the film is within range. Both should be done before the boards leave the coating area.

The inspection requirements are described with the defects that the process can produce, at coating application defects.

Packaging

Coated boards should be packed so that the film is not in contact with a material that could abrade or stick to it. The packing material should be compatible with the coating and clean.

The packaging requirements belong with the general ones described for board packaging.

Records

The records should include the material, the cure schedule, the thickness, the inspection result and any repair. Where a field failure involves corrosion, those records show whether the coating was present.

They belong with the process evidence described for manufacturing processes.

Process Control and Verification

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.

FAQ

How long before a coated board can be handled? Until the cure schedule is complete, since the film reaches its properties at the end rather than at the start of the cure.

Can a coated board be cleaned? It can with a chemistry that is compatible, and aggressive cleaning risks the film and the adhesion.

How is a damaged coating repaired? By removing the film locally, repairing the circuit and recoating the area with attention to the boundary adhesion.

Should test points be coated? They should be masked so that the film does not have to be penetrated for test.