Conformal coating removal and repair

Authors and contributors

• Dr Lee Hitchens, Nexus
• Karl Hardcastle, SCH Technologies

Stripping of unwanted conformal coating or Parylene from a PCB can be a simple process but depends on what conformal coating is to be removed or stripped, the area of coating to be removed, where the conformal coating is and the type of components on the board.

There are several methods for removal of conformal coatings from circuit boards.

However, they fall into two categories. These are localised repair and complete removal.

Localised repair and rework of conformal coating

Localised repair and removal of conformal coating is where a small area of the circuit is cleaned of the coating applied.

This could be to change or repair a component or to remove a conformal coating defect.

There are several methods that can be used on a practical basis.

These include:

• Soldering through the conformal coating
• Chemical removal in a local area
• Mechanical abrasion

Soldering through the conformal coating

The conformal coating generally will volatise or melt away when a hot soldering iron tip is applied to it.

The coating may just melt. It may blacken and char.

Generally, this is a cosmetic issue but the relevant material company should be consulted.

Chemical removal of a localised area

Applying a small amount of solvent or stripping fluid to a local area can remove the conformal coating.

The effectiveness of this method will depend on the conformal coating being removed.

For example, an acrylic based conformal coating can easily be removed with most solvents.

Applying a small amount of solvent with a cotton bud and applying some mechanical pressure tends to remove the conformal coating from the circuit board by re-dissolving the material back into solution.

However, a conformal coating with chemical resistance and cured will resist general solvents.

They will require more aggressive solvents and stripping agents to rework them.

But, it may be an issue that the chemicals used to remove the conformal coating may be corrosive to the circuit itself and also highly hazardous to the operator.

In this case, care must be taken in removing the stripping fluid residues after removal to avoid long-term reliability issues.

The chemical removal process may also not work with Parylene conformal coatings.

Mechanical abrasion

Conformal coating abrasion systems work on the principle of abrading the conformal coating from the surface of the PCB.

The particles are fired at the conformal coating via a compressed air supply. The particles wear off the coating in a gradual process.

This can be a highly effective method for local removal.

Consideration should be given to the wear on the surrounding area by the particles.

The waste particles of conformal coating will also need to be cleaned off after removal.

Finally, the particles can generate severe ESD static and the system must counter this with measures built in.

So, this is a specialist micro abrasion system rather than a standard shot blasting system.

Complete removal of conformal coating

Complete removal (stripping) of the conformal coating is a very different process. This can take time and care.

Some of the same processes used in local repair can be adapted to complete stripping of a circuit board. These include:

• Chemical stripping
• Mechanical abrasion

Chemical stripping of conformal coating

The principle of operation is that the circuit board to be stripped is manually placed by the operator in the succession of tanks that gradually strip / remove the conformal coating from the PCB using proprietary blends of recommended stripping fluids.

This effectively removes the coating.

However, the compatibility of the stripping fluids used in the process with the coating and the circuit board should be a prime consideration since damage to the PCB could be done by chemical attack.

Sophisticated systems can have the option of a rinse pipe and solvent pump fitted for gentle circulation and rinsing of the fluid and the operator working behind the safety screen is able to gradually remove the majority of coating, leaving a small amount of residue remaining.

Abrasion removal

The abrasion systems can be used for removal of the coating and can be highly effective on high value circuit boards.

However, it may take considerable time to remove all of the coating.