Conformal Coating Repair After Rework

Rework after conformal coating is a normal part of production and a common source of field failures. The coating must be removed to reach the joint, and the repair must restore the protection the design assumed.

Why Coating Repair Matters

The coating is usually the protection strategy for a humid or contaminated environment. A repaired area without the coating becomes the entry point for moisture, and the failure appears in the field rather than at the repair.

The repair is also the point where the assembly’s qualification is least well defined, because the cured material is being replaced by a different application with a different cure schedule.

The repair should therefore be treated as a process with a documented procedure rather than as an improvisation. Our conformal coating notes describe the original process that the repair must match.

Removing the Coating Locally

The material is removed by chemical softening, by abrasion or by a combination. The method must not damage the soldermask, the components or the adjacent coating.

Chemical removal uses a solvent matched to the coating chemistry. The solvent is applied to the area, allowed to soften the coating, and the material is then lifted. A solvent that attacks the soldermask or the component marking is the wrong solvent.

Abrasion with a fine tool removes the material mechanically and can also remove the soldermask. Where the surface beneath matters, the chemical method is preferred. Our coating inspection notes describe how the result is verified.

Cleaning Before Recoating

After the coat is removed and the joint has been reworked, the area must be clean. Flux residue from the rework and particles from the removal are both sources of poor adhesion.

The cleaning must be compatible with the components and must not spread contamination to the surrounding area. A local cleaning with a compatible solvent and a lint free material is the usual method.

The cleaned area should be dried, since a solvent that remains under the coating evaporates later and produces a blister.

Recoating the Area

The replacement material should be the same as the original, or one that has been qualified as equivalent. A different material may not adhere to the surrounding coating and may not have the same properties.

The application method depends on the material. A brush is common for a local repair, and a dispenser is used where the area and the volume must be controlled.

The new coating should overlap the existing coating around the perimeter so that there is no exposed boundary. A patch that stops exactly at the edge of the repaired area leaves a line where moisture can enter.

Curing a Local Repair

The cure is the difficult part, because the repair material cures differently in a small area than the original coating cured across the board.

A material that cures by ultraviolet light cannot cure where the light does not reach, and a local repair may be entirely in shadow. A moisture curing or a thermal curing material is used for the repair, or the repair is cured at a temperature the assembly tolerates.

The cure schedule must be recorded, and the result should be verified by an adhesion test rather than by appearance.

Verification of the Repair

The inspection should confirm that the coating covers the repaired area, overlaps the surrounding coating and has no pin holes or thin spots.

An ultraviolet inspection reveals the coating and its coverage, and a thickness measurement confirms the amount applied.

An adhesion test on the repaired area, performed on a sample or on a sacrificial board, confirms that the material has cured and bonded.

Records and Traceability

The repair should be recorded against the unit, including the area repaired, the material used and the cure applied. A field failure in that area can then be assessed against the repair.

Where several units have been repaired in the same area, the record shows whether the repairs are associated with a process problem.

The repair procedure itself should be a controlled document, since it is performed by different people over the life of the product. Our quality notes describe how such procedures are written and revised.

Process Control and Verification

On a design of this kind, adhesion is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, adhesion is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, adhesion is the item that decides how the rest of the board is arranged. 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.

FAQ

Can the coating be left off after a repair? Only where the design does not depend on it. Where it does, the repaired area is the most likely place for a failure.

Can a different coating be used for the repair? It can where the two adhere to each other and the properties are equivalent. The compatibility should be verified rather than assumed.

What does gopcb provide for coating repair? We provide a documented removal, cleaning, recoating and curing procedure using the qualified material, ultraviolet and thickness inspection of the result, adhesion testing, and records that tie each repair to the unit and the area.