Definition

Author

  • Dr Lee Hitchens, Nexus

Layering is where multiple layers of conformal coating are applied to a circuit board and do not adhere to each other or cause incorrect curing of the individual layers.

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Causes of layering in conformal coating

There are several conditions where layering problems can occur:

  • Coating Adhesion: Some conformal coatings do not stick to themselves. For example you cannot apply multiple layers of Parylene to a circuit board as as they will not adhere to each other.
  • Coating Curing: Some coatings must be fully cured before you apply a second layer. Otherwise, incorrect curing can occur due to the second layer inhibiting the first.
  • Coating Keying: Finally, some coatings must not be fully cured before the second coating layer is applied to ensure good adhesion. This can be the case with many solvent based coating processes which require multiple layers of coating to achieve the film thickness defined.

How to prevent layering problems in conformal coating

  •  Apply the Coating Correctly: It is possible to minimise the majority of layer problems by following the manufacturers guidelines.
  • Get it right first time: For Parylene, ensure the correct film thickness is achieved in one coat application. This is achieved through trials and calculations. If repair of Parylene is required, then normal practice is to use a solvent or water based film forming coating as a “patch” over the repaired area.
  • Make the coating adhere: To avoid coating inhibition, follow the manufacturers cure schedules.
  • Time the Coating Layers Correctly: To avoid lamination of coating in solvent-based coatings, ensure that the optimum time between coating layers is selected. This time is dependent on several factors including the solvent flash off time, the temperature and humidity in the area, the extraction rate and the chemical nature of the material.

The key to success is to dry the coating enough where the second layer bonds to the surface of the first layer but does not trap excessive solvent in the lower layer. This trapping can lead to excessive bubbles.

Download our PDF guide Solving conformal coating problems in the application process now.


Links

BlisteringBlushingBubbleContamination
Capillary FlowCorrosionCrackingDelamination
De-wettingDiscolourationFish EyesLayering
MealingOrange PeelShadowingUneven coating