What is a conformal coating on a Circuit Board?
What is a conformal coating on a Circuit Board? Conformal coatings are clear topcoats applied over PCBs to protect the boards from common contaminants like moisture, dust, fungus and dirt. These coatings are highly flexible and thus conform to the irregular geometries of circuits, and also have excellent dielectric properties which prevent arcing and corona discharge.
How do I conformal coat a circuit board?
Choosing the right technique depends on your capabilities and production volume. For small batches, either the brush or dip technique is appropriate as they are quick, easy, and require almost no capital expenditure. For higher volumes, spray coating is the preferred method, as it provides not only a superior finish and better coverage, but also facilitates higher throughput. Spray coating can be accomplished either with manual spray equipment such as HVLP spray guns, or more sophisticated automated spray equipment manufactured by companies like PVA, Nordson Asymtek and Specialty Coating Systems.
How long do conformal coatings take to dry?
A few minutes to several hours, depending on the chemistry involved. The table below gives typical ranges of dry times for different coating chemistries.
|Material||Typical Dry Time Range|
|Acrylics||3 – 30 minutes|
|Polyurethanes||15 – 60 minutes|
|Silicones||2 -10 minutes|
|Epoxy||4 -12 hours|
|UV Cure||Seconds after UV exposure|
|Ultra-Thin||Under 2 minutes|
|Styrenated Block co-polymer||5 – 10 minutes|
How do I measure conformal coating thickness?
Typical film thicknesses of conformal coatings range from 1 – 5 mils (25.4 – 127μm) and can be verified using the following techniques:
- Wet film gauge – The easiest non-destructive tool to use when measuring coating thickness is a wet film gauge with a black lamp. However, this method can only be applied with high solids systems, such as UV coatings.
- Micrometer – A simple non-destructive method is to measure points along the uncoated substrate with calibrated micrometers then re-measures these points afterwards, with the difference being the coating thickness.
- Positector – A less cumbersome, non-destructive technique employs a digital coating thickness gauge which measures changes in either magnetic or electric fields to determine film thickness. While this technique is much quicker and easier, it is limited to use on conductive substrates, and therefore cannot measure coating thicknesses over solder mask or other insulated components.
- Micro-Section Analysis – For highly accurate coating thickness measurements, cross-section analysis can be applied in which a section of the coated substrate is cut out and mounted in a plastic mold. The cross section is then polished and analyzed under a powerful microscope to measure the coating thickness at different points. While this technique provides the most accurate measurements, it requires much time and costly equipment, and destroys the sample.
How do I remove conformal coatings from PCBs?
It depends on the coating’s chemistry. For thermoplastic materials such as acrylics or silicone coatings, the cured film can easily be removed with common solvents like acetone or n-butyl acetate. For more durable thermoset coatings such as polyurethane, epoxy and UV cure, use a more aggressive stripper like MG’s 8309 liquid or 8310A gel. The table below summarizes most effective methods for removing coatings of different chemistries.
|Acrylics||Easily removed using common solvents like or esters.|
|Polyurethanes||Can be removed using 8309 or 8310A stripper. Takes 5-10 minutes.|
|Silicones||Easily removed using common solvents like acetone or esters.|
|Epoxy||Remove using 8309 or 8310A stripper. May take up to 2 hours.|
|UV Cure||Remove using 8309 or 8310A stripper. May take several hours.|
|Parylene||Must be removed with micro-abrasion. Very difficult.|
|Ultra-Thin||Easily removed using common solvents like Ketones|
|Styrenated Block co-polymer||Easily removed using common solvents like Ketones|
Are conformal coatings waterproof?
No. These materials act as a membrane that water can penetrate through osmosis. To completely waterproof PCBs, encapsulate them in epoxy, polyurethane or silicone.
How do I waterproof electronic circuits?
Waterproofing electronic circuits requires the application of an unbroken conformal coating that is free from any voids and has a watertight seal around the edges. Some believe this is the actual function of a conformal coating, but this is simply incorrect. Rather, a conformal coating acts as a semi-permeable membrane through which water can pass by osmosis. It protects the circuit from the harshest effects of long-term moisture exposure, such as the corrosion of copper traces, but in order to truly waterproof a PCB, the circuit must be fully encapsulated in epoxy, polyurethane or a silicone resin.
What is conformal coating dielectric strength?
Conformal coatings are highly flexible materials that conform to the geometries and irregularities of circuit boards. Typically, resins in conformal coatings are highly insulating, with typical dielectric strength values of 1000V/mil or higher. The dielectric strength inherent in conformal coatings enables circuit miniaturization and allows traces to be placed closer together. However, to fully insulate conductors and prevent shorts in higher voltage components like motors and coil windings, they are not ideal materials. Instead, insulation varnishes such as 4226 and 4228 are recommended for their superior dielectric strength and corrosion protection.
Do conformal coatings protect components from Electrostatic Discharge (ESD)?
Yes, coated components will be protected from discharge. However, a charge can build up on the surface, and any uncoated connectors exposed to a conductive part can subsequently suffer ESD. For protection against ESD, visit our ESD Safe Coatings line. We also offer an anti-static foam spray that effectively polishes and eliminates static charge with a single application.