Capillary flow (or scavenging) in conformal coating processing is where the coating pulls or runs away from certain areas of a PCB to more favourable areas.

This is due to a combination of effects which leave a patchy finish on the surface of the board around the defective area.

There are several options to resolve this issue and prevent this occurring in production.

Capillary (scavenging) action is due to a combination of effects that leave the conformal coating patchy on the circuit with varied thickness
Capillary (scavenging) action is due to a combination of effects that leave the conformal coating patchy on the circuit with varied thickness

Factors that Cause Capillary Affects

There are several reasons that capillary effects (also known as scavenging) can be found on printed circuit boards. These include

1. Low viscosity of the conformal coating

2. An abnormally high amount of conformal coating applied to the PCB

3. A low surface energy of the substrate (laminate) PCB.

4. The high surface tension of the conformal coating.

Several examples are shown in the images below where the conformal coating has drawn away from the components or surface, finding it more energetically favourable to gather around the devices, which in turn produces a poor conformal coating finish.

Another example of scavenging of the conformal coating across the circuit board
Another example of scavenging of the conformal coating across the circuit board showing uneven thickness of material

How to Reduce or Eliminate Capillary Effects

It is possible to minimise the capillary effects by using different techniques to aid the process.

These include:

Increasing the viscosity of the conformal coating

If the conformal coating has a low viscosity and is in a liquid state on the PCB then it will tend to migrate under devices.

This is due to low surface tension effects. Increasing the solids content (reducing the amount of thinners used) can help with this issue.

Reducing the conformal coating film thickness

Applying less coating per application allows the coating to dry quicker and there is less liquid to migrate away from the area in question.

Cleaning the printed circuit board

By cleaning the PCB, this allows the surface energy of the laminate to be more suitable to wet to.

If the PCB is “dirty”, then the coating may not want to stick to these areas of the board. The coating will then run away from the area.

Use a solvent based material rather than water based or 100% solid conformal coating

Many times it can be found that problems occur when changing from a good solvent based conformal coating process to a different material that is solventless.

This is because the solvents in the coating “cleaned” the board as they coated and had better bonding.

Heat the board prior to conformal coating

By heating the board you can accelerate the evaporation of the solvent from the coating and make the coating “gel up” quicker. This will mean it will stop migrating quicker. This has to be balanced against drying too quick which can cause other problems.

Article by Dr Lee Hitchens