What if my Conformal Coating is Applied too Thick?
Posted by Sean Horn
Thursday, April 21, 2016 7:30
@ 7:30 AM
Conformal Coating Thickness
Coating thickness is critical to the proper functioning of your printed wiring assembly, circuit board, or electronic device.
If a coating is too thin, proper coverage is impossible. But, if your coating is too thick, it may create excessive stress on solder joints and components (particularly glass-bodied components).
What’s more, controlling coating thickness is of special importance with rigid coating materials such as epoxy. Excessively thick conformal coatings can cause residual stress that can damage the electronic device that’s been coated.
If your coating is too thick, you’ll need to remove and reapply the coating or grind it down. The specific method you use will depend on the coating you’re working with.
Before we look at removal methods, here’s a cheat sheet showing recommended thickness for the five major types of conformal coating.
|Type||Full Name||Thickness When Applied|
|Type AR||Acrylic Resin||0.00118 to 0.00512 in.|
|Type ER||Epoxy Resin||0.00118 to 0.00512 in.|
|Type UR||Polyurethane Resin||0.00118 to 0.00512 in.|
|Type SR||Silicone Resin||0.00197 to 0.00827 in.|
|Type XY||Parylene Resin||0.000394 to 0.00192 in.|
Conformal Coating Removal Methods
If your coating has been applied too thick or there is a defect that requires removal of the coating, there are three broad categories of removal techniques:
- Thermal removal
- Chemical removal
- Mechanical 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 you decide to use the thermal removal process, 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 are hazardous.
Chemical methods are the most popular method for removing conformal coatings. As long as the solvents used do not adversely affect the printed wiring board or components, and there are no environmental issues, this technique works well.
However, there is no one perfect solvent for all applications. In some cases it may be difficult to find a suitable solvent.
The following sections discuss the chemical removal methods for various types of coating:
How to Remove Urethane
There are several solvents you can use 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
Methylene chloride based solvents are very effective in removing silicone conformal coatings. Several other hydrocarbon-based solvents are also used as alternatives.
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
In the past, acrylic coatings were removed using highly volatile and flammable solvents such as methylene chloride, trichloroethane or ketones.
A relatively safe alternative has been developed based on butyrolactone. It can be used for the removal of most acrylic coatings.
How to Remove Epoxy
The complete removal of epoxy coatings for repair is nearly impossible by chemical means. The solvent can’t discriminate between the epoxy coating, the underlying printed circuit board, and any epoxy-coated or potted components.
However, if done carefully, spot removal may be accomplished by the application of methylene chloride and an acid activator with a cotton tipped swab.
Mechanical Removal Methods
Epoxy and parylene coatings are resistant to both thermal and chemical removal techniques. In most cases to remove these types of coatings, you must use mechanical or microabrasion techniques.
Mechanical removal techniques require precise manual labor from a highly-experienced technician. He or she must grind down the coating to remove it, without damaging the circuit board or device underneath.
To learn more about conformal coating defects, download our whitepaper: