Removing Conformal Coating

Conformally coated PCBs are expected to work without fail, largely because of the protection the coatings provide them.  In addition to PCB-manufacturing issues, coating problems can trigger failure mechanisms for the assembly.  For instance,

• Conformal coating applied incorrectly can cause PCB malfunction.
• Selecting the wrong coating material from among acrylic, epoxy, parylene, silicone or urethane can be a source of board failure, if it does not support the PCB’s operating environment.

Removing the coating may be necessary if these conditions prevail.

Reliable Methods of Removing Conformal Coating

Coating removal methods are determined by their impact on the film, its thickness, and effect on the substrate.  Major removal methods include:

• Chemical solvent methods are used most frequently for conformal coating removal. Much depends on the coating material.  No single solvent is equally effective for all applications.  Used essentially for wet coatings – acrylic, epoxy, silicone, urethane – chemical methods are generally ineffective for chemically-inert parylene.  Solvents remove specific soluble-type coatings on a spot-basis by brushing or swabbing the local area with controlled solvent-application until it is free of the conformal film; larger-scale immersion may be necessary for quicker coating-removal from entire PCBs.
• Laser ablation is a cost-effective method for difficult removal conditions, controlling the process to areas as small as a single micron. Removal is generally a one-step procedure, and especially useful for parylene, converting the coating to gas or plasma.  While precise removal is the result, it can be slow; each laser pulse separates only a minute segment of the existing film’s material thickness.
• Mechanical removal techniques involve cutting, picking, sanding or scraping coating from the surface, requiring time-consuming, precision control. Poorly enacted mechanical removal processing can accelerate damage to the coating and PCB.  Less dependable than other removal techniques, thorough masking of non-removal surfaces is mandatory in all cases.
• Micro abrasive blasting (abrasion) is environmentally friendly, and inexpensive. A tiny nozzle attached to a stylus directs project-specific formulas of abrasive media and inert gas/dry air at the coated surface, by means of automated or human handheld-technologies.  Electrostatic potential is dispelled by grounding devices; filtration processes dispose coating debris removed from the substrate.  The process can be focused onto targeted areas as minute as an individual test node or those as large an entire PCB.
• Oxygen-based plasma is often used in situations requiring highly selective removal of coating from specific components – such as connectors — within an assembly. However, this fine-scale procedure can also be used to strip entire PCBs.
• Peeling is used only in special circumstances, like removing thickly-applied silicone coating, using a dull knife/blade to slit the film, so it can be peeled off the PCB by hand.
• Thermal is a less-recommended method of coating removal. Relying on very high temperatures generated by a soldering iron, thermal removal’s longer duration of exposure can cause delamination/discoloration, overheat temperature-sensitive components, negatively impact solder joints, and leave surface residue.  Difficult to manage, thermal methods should be confined to spot-removal; toxic fumes can result from careless thermal application.

One needs to match the appropriate removal method to the coating material, its age, thickness and the board’s specific function.

Removal According to Coating Material

Since the possibility of defective coating or PCB components can develop after manufacture, removal strategies are an integral element of design, based largely on the assembly’s specific coating material and operational function.

• Acrylic: The solvent butyrolactone is frequently used for chemical removal of most of the acrylic coatings. This substance replaces the far more flammable ketone, methylene chloride and trichloroethane, used in the past.  In addition to chemical solvent removal, acrylic films also respond to mechanical, micro-abrasive and thermal treatment for coating removal.
• Epoxy: Thermal treatments are most frequently used to remove super-hard epoxy coatings.  Mechanical and micro-abrasive methods are also recommended. Chemical removal methods are generally ineffective.
• Parylene: Tetrahydrofuran (THF) is the only chemical solvent that effects parylene removal on a persistent basis.  Micro-abrasion and scraping are currently most consistently used, while developments in laser technology may soon transform it into a more common removal source.  Spot-removal is achieved with mechanical and plasma methods.  Thermal methods also have some use for spot-removing parylene, but are difficult to control.
• Silicone: Chemical removal via methylene chloride or hydrocarbon-based solvents is recommended.  The hydrocarbons are safer, far less likely to damage PCBs, components, metals and plastics.  In descending order, thermal, mechanical and micro-abrasive techniques will effectively remove thin-layered silicone films.  Thicker silicone coatings respond to peeling or mechanical techniques.
• Urethane: Thermal and mechanical methods are recommended for urethane removal.  The solvents methanol-base/alkaline activators and ethylene glycol ether-base/alkaline activators efficiently remove silicone films, as does micro-blasting.

Summary

Because the process of removing conformal coating from PCBs can vary from simple to complex, it is important to correctly match the removal methodology with both the type of coating material used, and the project/purpose of the circuit board.  Stripping fluids that work for some materials (acrylic) may be useless for others (parylene).  Determining the appropriate coating erasure procedure assures optimal removal without interfering with PCB function after it has been recoated.  THF is the only chemical solvent that consistently removes parylene from assembly substrates; the limited chemical options remaining are highly specialized and seldom applied.  Abrasion and scraping techniques work best for parylene films; laser may also develop further as a major parylene removal process.  Mechanical and plasma-based techniques are useful for spot-removal assignments.

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