What if my Conformal Coating is Applied Too Thick?

The thickness of conformal coating plays a vital role in the optimal performance of printed wiring assemblies, circuit boards, and electronic devices. An inadequate coating thickness hinders proper coverage, while an excessive thickness can lead to undue stress on solder joints and components, especially glass-bodied ones.

Additionally, controlling coating thickness is of great importance with rigid coating materials. Overly thick conformal coatings can cause residual stress that can damage the electronic device that has been coated. If the coating is too thick, it’ll need to be removed and reapplied or grinded down. The removal method used will depend on the specific coating type.

The chart below lists the recommended thickness for the five major types of conformal coating.

Type	Coating Name	Thickness when Applied
Type AR	Acrylic Resin	0.00118 in – 0.00512 in
Type ER	Epoxy Resin	0.00118 in – 0.00512 in
Type UR	Polyurethane Resin	0.00118 in – 0.00512 in
Type SR	Silicone Resin	0.00197 in – 0.00827 in
Type XY	Parylene	0.000394 in – 0.00192 in

Conformal Coating Removal Methods

Coating that has been applied too thick or contains a defect may require removal of the coating. There are three broad categories of removal techniques: thermal, chemical and mechanical. Each technique has its own advantages and limitations.

Thermal Removal

Thermal removal methods use a soldering iron to heat and remove the coating. Because most conformal coatings require a very high temperature and long exposure times, the thermal removal process can cause discoloration, leave residue and adversely affect solder joints and other materials and components used in the fabrication of assemblies.

If the thermal removal process is used, it must be monitored to ensure excessive temperatures do not cause delamination or other damage to the underlying device. Extreme caution must be taken when burning off conformal coatings. Some coatings emit toxic vapors which can be hazardous.

Chemical Removal

Chemical methods are the most popular method for removing conformal coatings. This technique works well as long as the solvents used do not adversely affect the printed wiring board or components and there are no environmental issues.

However, there is no single perfect solvent for all applications. In some cases it may be difficult to find a suitable solvent.

Mechanical Removal

Mechanical removal techniques require precise manual labor from a highly-experienced technician. The coating must be ground down to remove it without damaging the circuit board or device underneath.

Removing Specific Types of Conformal Coating

The following sections discuss the chemical removal methods for various types of coating:

How to Remove Urethane

There are several solvents that can be used to remove urethane conformal coatings. These solvents include:

• Methanol base/alkaline activator solvents, which provide a range in the dissolution power and selectivity
• Ethylene glycol ether base/alkaline activator solvents, which are relatively fast and less selective than methanol/alkaline solvents

How to Remove Silicone

While not as fast as the methylene chloride, the hydrocarbon-based solvents are more selective. When not contaminated by water, hydrocarbon solvents will not attack epoxy-glass printed circuit boards (PCBs), components, metals and plastics.

How to Remove Acrylic

Historically, acrylic coatings were removed using solvents such as methylene chloride, trichloroethane or ketones. These solvents are very flammable. A safer alternative for removing most acrylic coatings was developed based on butyrolactone.

How to Remove Epoxy

It is nearly impossible to completely remove epoxy coatings using chemicals. Solvents are unable to differentiate between the epoxy coating and underlying printed circuit board or potted components. Spot removal of the coating can be achieved through the application of methylene chloride and an acid activator with a cotton tipped swab.