Conformal Coating Defects & How To Prevent Them

Defects of Conformal Coatings

A single defect in conformal coating can compromise the long-term reliability of PCBs, necessitating time-consuming and costly repairs or rework. This issue can be avoided if defects are prevented in the first place.

The 6 most common defects and their solutions:

It occurs when conformal coatings fail to coat PCB surfaces evenly due to contamination from flux residues, process oils, mold release and fingerprint. This can be avoided if the substrate is thoroughly cleaned before applying the coating.
Due to a contaminated surface or an insufficient flash-off time between coats, a section of the conformal coating loses adhesion to the substrate and lifts away from the surface. This usually goes unnoticed until after the PCB are in use. If this is a common issue, ensure the surface is wiped before applying the coating, and increase the flash-off time to allow for proper adhesion.
Air Bubbles
Bubbles may form when pockets of air trapped in an uneven layer of conformal coating rises or when too much pressure is applied during brushing. To prevent this, apply several thin coats to allow bubbles to dissipate between layers and gently blend the coating so it flows evenly on the substrate.
Orange Peel
When a conformal coating that has been applied too thickly undergoes accelerated curing, excess solvent trapped within the coating evaporates. This may result in an uneven and textured surface, similar to the skin of an orange. Applying thinner coats with flash-off time between each one will allow any excess coating to evaporate.
Fish Eyes
Oily spots or silicone particles deposited on a surface while spraying conformal coating creates small, crater-like openings known as fish eyes. A well-maintained filtration system will prevent any contaminants from entering the sprayer.
The high temperature in accelerated curing will cause the surface to dry too quickly before the remainder of conformal coating has dried, resulting in cracks and potential contamination. Allowing for an initial drying time at room temperature before exposing to elevated temperature can minimize cracking.

Conformal Coating Best Practices

  1. Ensure that the surface is thoroughly cleaned with a surface cleaner or a duster
  2. Work in a clean environment to prevent contamination, such as in a clean-room or in a desktop ventilated enclosure such as the Vessel Ion Parts Cleaner.
  3. Apply the coating in thin coats to ensure a smooth and level coating. If a thicker coating is required, simply apply more thin coats until the desired amount has been built up. 
  4. Allow for sufficient flash-off time (the necessary waiting time taken between coats of conformal coating). A short flash-off time may mean the layer has not fully dried and may become bumpy when the next coat is applied. 
  5. Leave the conformal coating to dry naturally to ensure the entire piece dries at the same time. This will prevent cracking of the coating.