• Dr Lee Hitchens, Nexus

Capillary flow in conformal coating is where the coating pulls or runs away from certain areas of a PCB.

The coating move to more favourable sectors due to a combination of effects.

This leaves a patchy finish on the surface of the board.

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

Examples of conformal coating capillary flow

Examples are shown in the images where the conformal coating where capillary has occurred:

Causes of capillary flow (scavenging) in conformal coating

Factors that influence the capillary effect include:

  • Low viscosity: If the conformal coating has a low viscosity it will penetrate into devices much easier.
  • Volume of coating: An abnormally high amount of conformal coating applied during processing means the excess material can wick into components before “gelling” of the material is acheived.
  • Substrate Cleanliness: If the circuit board has a low surface energy then the coating will not bond to the substrate.
  • High surface tension: If the conformal coating has a high surface tension then it will also like to energetically penetrating under components.

 How to stop capillary effects in conformal coating

It is possible to minimise the capillary effects by using techniques such as:

  • Low viscosity: Increasing the viscosity of the conformal coating will lower the mobility of the coating and aid the adhesion to the right areas of the board
  • Volume of coating: Reducing the conformal coating applied will reduce the amount of free material to capillary.
  • Substrate Cleanliness: Cleaning the board before processing will aid the bonding of the coating during application.
  • High surface tension: Use a solvent based material rather than water based or 100% solid conformal coating.
  • Heat board: prior to coating the board raising the core temperature of the circuit can allow the coating to dry quicker and be less mobile on the circuit board.

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


Capillary FlowCorrosionCrackingDelamination
De-wettingDiscolourationFish EyesLayering
MealingOrange PeelShadowingUneven coating