Failures of PCBs and similar electronic assemblies can occur despite the protection of conformal coating. Contributing reasons include:
- Incorrect selection of coating methods/materials related to the assembly’s use.
- Poor manufacture or stresses to the assembly during operation.
- Challenges to the coating’s integrity – bubbles/voids, inadequate masking, poor adhesion/surface finish, uneven thickness.
As with selecting an appropriate coating pre-application, conformal coating removal requires matching removal methodology with coating type and the component’s function.
Major Removal Methods
Matching the removal method to the coating material, its age, thickness and the PCB’s specific function is an essential first-step to the removal process. Proper identification of the coating material, and its original application method, are also basic to efficient removal. Coating removal is determined by:
- the type of coating material used,
- the PCB’s components/their position on the board,
- the film’s material, its thickness, and
- effect on the substrate.
Once these have been identified, determination of the appropriate removal method is possible. Major removal methods include:
- Chemical solvents are most-used for removal for liquid coatings. However, because no single solvent will remove every coating material with the same degree of efficiency, selection of best-choice chemical solvent is dependent on coating-type. The solvent butyrolactone is frequently used for acrylic removal. Methylene chloride or hydrocarbon-based solvents are recommended for silicone, methanol-base/alkaline activators and ethylene glycol ether-base/alkaline activators for urethane. Solvents are generally ineffective for epoxy and chemically-inert parylene.
- Cost-effective laser-ablation offers precise removal, to a single micron, but because each laser pulse separates only a minute segment of the existing film coverage, it can be slow. Generally a one-step procedure, laser is particularly good for parylene removal, re-converting the solid film back into a gas or plasma.
- Cheap, environmentally-safe micro-abrasive blasting (abrasion) can be focused onto minute board-segments (an individual test node), or an entire PCB. Automated or operator methods direct project-specific formulas of abrasive media/inert-gas/dry air through a tiny nozzle onto targeted surfaces. Resultant coating debris are disposed by filtration; grounding devices dispel electrostatic potential. Abrasion can effectively remove all liquid coatings and parylene.
- Mechanical removal techniques include grinding, scraping, cutting, or sanding coating from the surface. Time-consuming, thorough masking of non-removal surfaces is necessary. Less dependable than other removal techniques, poor mechanical processing can damage the coating/PCB. Acrylic, epoxy and urethane coatings respond to mechanical methods, as do thicker silicone films. If needed, scraping works for parylene.
- Plasma is frequently used when highly-selective coating removal from specific components within an assembly is required. Parylene responds to plasma spot-removal. However, this fine-scale procedure is also effective for removing coatings from entire PCBs.
- Peeling is suggested only for specialized removal. For instance, thickly-applied silicone films can respond well, using a dull knife/blade to slit the film, then peeling it from the PCB by hand.
- Thermal uses a soldering iron, at very high temperatures that can generate toxic fumes. It is difficult to manage; longer-duration exposure can overheat temperature-sensitive components, negatively impacting solder joints, leaving surface residue and de-laminating/discoloring remaining surfaces. Frequently used for durable epoxy coatings. Thermal methods have some use for acrylic, silicone and urethane; spot-removal is most recommended, especially for parylene.
Prevailing industry standards provide appropriate process guidelines for conformal coating removal. IPC-7711/7721, Rework, Modification, and Repair of Electronic Assemblies, imparts best-practice procedures for removing conformal films from PCBs. IPC-7711/7721 standards detail coating removal methods determined by their impact on the film, its thickness, and effect on the substrate. IPC-CC-830 defines conformance guidelines for all conformal coatings; film removal is frequently required when IPC-CC-830 requirements for each coating type’s overall quality conformance are unmet during application.
Operator experience and skill influence film removal. Appropriate material identification is necessary prior to commencing removal. Better-equipped to correctly identify coating characteristics, skilled professionals determine best-method removal procedures with greater accuracy. They can also implement removal procedures within the constraints of conformance issues like the PCB’s environmental, functional, and serviceability requirements, improving the removal process.
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