Conformal Coating: Cure Methods

While the cure mechanism isn’t a primary criterion when selecting a coating, it has a direct impact on the type of application method that will be feasible, and the production throughput that can be expected. Some mechanisms are relatively fool proof, while others are very complex and leave room for application errors when used in an uncontrolled process.

  • Evaporative Cure Mechanism
The liquid carrier evaporates, and what is left behind is the coating resin. Although very simple in theory, circuit boards usually need to be dipped at least two times to build up an adequate coating on component edges. Whether the liquid carrier is solvent or water‐based, humidity affects application parameters. Solvent systems tend to be easy to process, provide consistent coverage due to good wetting, and fast cure times. However, solvents are often flammable, so adequate ventilation and fume extraction methods are required. Using water as a carrier can eliminate the flammability concern, although they tend to take much longer to cure, and can be very sensitive to ambient humidity.

  • Moisture Cure
Primarily found in silicone and some urethane systems. These materials will react with ambient moisture to form the polymer coating. This type of curing mechanism is often coupled with an evaporative cure. As carrier solvents evaporate, moisture reacts with resin to initiate final curing.

  • Heat Cure
Heat cure mechanisms can be used with one or multicomponent systems, as a secondary cure mechanism for UV cure, moisture cure, or evaporative cure. The addition of heat will cause the system to polymerize, or speed the cure of the system. This can be advantageous when one cure mechanism is insufficient to gain the cure properties required or expected. However, thermal sensitivity of circuit boards and components must be taken into consideration when curing in elevated temperatures.

  • UV Cure 
Coatings that are cured by ultraviolet light offer very fast production throughputs. They are 100% solid systems with no carrier solvents. UV curing is line‐of‐site, so a secondary curing mechanism is needed under components and in shadow areas. UV cured coatings are more difficult to repair and rework and require UV curing equipment and UV radiation protection for workers.

Conformal Coating Removal

On occasion, it is necessary to remove a conformal coating from the circuit board to replace damaged components or perform other reworking procedures. The methods and materials used to remove coatings are determined by the coating resins as well as the size of the area and can impact the time required.

The basic methods as cited by IPC are:

  • Solvent Removal: Most conformal coatings are susceptible to solvent removal, however, it must be determined if the solvent will damage parts or components on the circuit board. Acrylics are the most sensitive to solvents hence their easy removal; epoxies, urethanes, and silicones are the least sensitive. Parylene cannot be removed with a solvent.
  • Peeling: Some conformal coatings can be peeled from the circuit board. This is mainly a characteristic of some silicone conformal coatings and some flexible conformal coatings.
  • Thermal/Burn‐through: A common technique of coating removal is to simply burn through the coating with a soldering iron as the board is reworked. This method works well with most forms of conformal coatings.
  • Microblasting: Micro blasting removes the conformal coating by using a concentrated mix of soft abrasive and compressed air to abrade the coating. The process can be used to remove small areas of the conformal coating. It is most commonly used when removing Parylene and epoxy coatings.
  • Grinding/Scraping: In this method, the conformal coating is removed by abrading the circuit board. This method is more effective with harder conformal coatings, such as parylene, epoxy and polyurethane. This method is only used as a method of last resort, as serious damage can be incurred.

If all you are doing is replacing a component or working on an isolated area, it is common to simply burn through the coating with a soldering iron. In cases when this is aesthetically unacceptable, contamination is a concern, or components are densely spaced, there are coating removers available in pen packaging.


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