Defects to either the PCB assembly or its conformal coating can be sufficient to cause coating removal. Whether repair technologies address the circuit board’s components or the conformal film, subsequent post-repair coating (recoating) processes need to address:
- the correct match between conformal material and PCB function, and
- the most advantageous method of applying recoating materials.
Old Coating Removal
The complexity of stripping damaged or otherwise unwanted conformal coating from a circuit board is largely dependent upon the type of coating targeted for removal. Different film materials require different removal strategies, and need to be appropriately monitored to ensure further damage is not done to the assembly. The type of components comprising the assembly, and the extent to which conformal coating actually covers the board (and in what surface areas), are also important factors. Safely removing old coating supports the subsequent recoating process.
Conformal Coating Materials
Basic conformal coating materials are acrylic, epoxy, parylene, silicone and urethane. Only parylene does NOT use a liquid application procedure, wherein wet conformal coating is applied to the circuit board via brushing, dipping or spraying methods. Difficulty of their removal varies according to material type; only acrylic can be removed very easily. However, most liquid coating types can recoat PCBs, if the targeted surfaces have been appropriately cleaned of old coating residue and prepared for recoat.
In contrast, parylene uses a chemical vapor deposition (CVD) procedure that penetrates deeper into the substrate surface. Although its resultant coating is in many cases superior to wet material films, parylene has disadvantages for removal/repair/recoating. Removal for rework is very difficult, requiring abrasion techniques, and without access to vapor phase deposition equipment, recoating with parylene is impossible.
Conformal Coating Reapplication
Recoating procedures focus on pairing the new film with the PCB’s operational environment. In most cases, this will entail:
- testing assembly components to ensure they function as intended,
- updating/repairing component performance standards as necessary,
- ensuring board cleanliness, and
- recoating per client/component specifications,
- with close attention to film material selection and coating procedures.
When selecting the coating material, you’ll want to align its properties with those of the PCB, in terms of the unit’s:
- dependability requirements,
- electrical effects,
- life cycle expectations, and
- total operational environment,
- for determining the optimal mix of coating/assembly materials and methods.
This will entail accurately ascertaining the assembly’s surface energy during operation, to ensure the PCB can be coated with the chosen material. It is also important to establish (1) what the PCB needs to be protected from during use, (2) the most advantageous coating-thickness, and (3) the kind of coating materials that will support this capacity. Keep-out areas, regions of the board not to be coated, also need to be identified and appropriately masked prior to initiating recoating processes.
Conformal coating reapplication implemented according to these procedural standards has a superior chance of eliminating defects to the new film.
After removal of the original conformal coating from the PCB, recoating with the conformal coating type most pertinent to the assembly’s operational environment is essential. This priority is further abetted by use of the coating method that best supports the PCB’s operational requirements. Circuit boards need to be thoroughly inspected both prior to and after recoating to assure conformal film reapplication is complete and directed to the correct physical regions of the PCB. For instance, reapplication is of no use if coating materials cover either the underside of the board or interior regions, for parts like connectors, that should never be coated. Reapplication is already process downtime, and must be carefully managed in all cases, to limit further production delays. For recoating to be effective, there needs to be a reliable confluence of material, process and board. Ensuring this condition minimizes the development of future failure mechanisms within the PCB due to conformal coating problems.
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