What is Conformal Coating on a PCB?

A PCB’s materials and components are not strong enough to withstand exposure to harsh environments. Therefore, circuit boards made for applications in the automotive, aerospace, medical, and other demanding sectors must have a protective layer. PCB manufacturers usually layer conformal coatings to provide this shield.

We’ve analyzed conformal coatings below, including their types, applications, curing methods, measurement techniques, and regulatory standards.

What is Conformal Coating on a PCB?

A conformal coating is a thin, lightweight, protective layer applied on a PCB to protect the circuit from dust, moisture, and other elements. This coat is not a sealant, so it allows trapped moisture inside the board to escape.

However, prolonged exposure to vapor will degrade the material, allowing moisture to get into the board. Other possible failures include cracking, fish eyes, blushing, delamination, and discoloration.

It is important to note that conformal coats don’t have to cover the entire board. For instance, COBs use this layer to prevent accidental contact with the dye or bonded wires.

How does Conformal Coating Protect Electronics?

The thin polymeric film covers and conforms to the PCB components, wrapping them inside. Once it cures, the coat forms a tough, impenetrable layer that safeguards the PCB surface and components from corrosion and the elements, while providing electrical insulation.

What are the Different Types of Conformal Coatings?

Silicone Conformal Coatings (SR)

Silicone is an inorganic material that can withstand a wide range of temperatures. It also has a high glass transition temperature (like epoxy), so it remains solid at these high temperatures. In chemical environments, this coating is handy because it is solvent-resistant. SR has a good dielectric strength, as well, and fluoresces in UV light, which simplifies inspection and repair.

However, it has its downsides; for instance, you can only rework small sections of the PCB.

Acrylic Conformal Coatings (AR)

Acrylic is inorganic like silicone, and has good moisture, dirt, and fungal resistance properties. But unlike silicone, acrylic is easy to rework, and the coat dries quickly. However, this conformal coat has weak abrasion, solvent, and chemical resistance, making it unsuitable for extremely harsh environments.

Urethane Conformal Coatings (UR)

Urethane is an organic liquid that is resistant to chemicals, solvents, moisture, and abrasion. This combo makes the coat extremely tough and ideal for applications like automotive (under-hood) and industrial control PCBs.

But this toughness has its disadvantages when it comes to removing or reworking. Moreover, UR has a long curing time and leaves an unaesthetic look on the soldered components.

Epoxy Conformal Coatings (ER)

Epoxy is a 2-part compound that begins curing after mixing. It delivers good abrasion, moisture, and chemical resistance and has a high glass transition temperature. These properties make this coating ideal for industrial and heavy machinery where abrasive elements and harsh chemicals are present.

However, this material might shrink during the curing process, and as it is rigid, it can crack.

Parylene Conformal Coating (XY)

Parylene has a high dielectric strength at thicknesses lower than a micrometer. The material also has low thermal expansion with good moisture, abrasion, and chemical resistance across a reasonable temperature range. The US FDA approved this coat for medical devices and implants, so you’ll mostly find it in medical and health industry circuit boards.

On the other hand, applying this coat requires the chemical vapor deposition method, a complicated process that requires expensive equipment. The coat doesn’t need curing after application though, which is a plus for high-volume production.

What are the Benefits of Conformal Coating?

• Protects the circuit board from the degradation effects caused by chemicals, abrasion, and moisture
• Provides insulation, enabling PCB conductor spacing to be reduced by up to 80%
• Eliminates the need for complex and sophisticated casings or enclosures, which are costlier than the coating and take up more space
• Reduces environmental stress and aging on the PCB assembly, resulting in a longer lifespan
• Minimizes the risk of whisker formation and current bleeding between closely positioned components

How are Conformal Coatings Applied?

Brushing Technique

Brushing is a manual process that is best for low-volume production, rework, and repair. Since it’s done by hand, the brushed coat is thick and not aesthetically pleasing. There’s also the potential for human error, so this process is not feasible for high-volume production.

Dipping Technique

With dipping, you have to submerge the PCB assembly into the coating solution and then pull it out. The process is quick and accurate, and covers the board on all exposed areas, creating a uniform coat around the assembly.

Dipping is ideal for high-volume production, but you can only submerge boards that accept coating on both sides. Additionally, when dipping, it is important to watch and control the withdrawal speeds to prevent bubbles and ensure an even coating thickness.

Spraying Technique

Spraying can be manual or automated, but both processes use aerosols to apply a superior surface finish. This process is time-consuming, so it is not the best for high-volume production. However, spraying provides the flexibility to vary the conformal coat’s thickness, although you have to control various environmental conditions around the process to avoid overspray or uneven layering.

Vapor Deposition Technique

This technique involves vaporizing and pyrolyzing the raw material that is needed to coat the PCB at high temperatures. In this case, the material is usually parylene. Once in vapor form, the monomer gas is deposited on the PCB surface as a thin, transparent, pinhole-free polymer film.

Depositing the coating material as a gas enables it to cover all the tight and hidden areas. Therefore, vapor deposition is ideal for PCBs with complex geometries and high-reliability requirements. However, the equipment needed for this technique is expensive, which has cost implications for the end user.

What Curing Methods Work for Different Coating Types?

Most conformal coatings require curing after application to solidify and harden the materials. The specific curing process varies depending on the coat, so we’ll look at them separately.

• Acrylics: These coatings can cure either by evaporation (at room temperature) or heating. Normal drying takes 10 to 15 minutes, but the material will still be soft to handle. It needs to undergo full curing to fully harden, which will take time because all the carrier solvents must evaporate. Heating shortens this time, but the process requires an oven and more energy, adding to the cost of production.
• Silicone and Urethanes: Most silicone and urethane coatings react with moisture in the air to cure and can take days to fully cure.
• Epoxies: These conformal coats need heat curing to expedite the hardening process. It can take 2–3 hours at 100–150°C.
• Parylene: Parylene doesn’t require curing after application.

What About UV Curing?

UV curing is the fastest hardening option because it can take as little as a few seconds to cure clear resins. However, it only works on UV-curable coatings which do not have any carrier solvents.

What are the Thickness Ranges for Conformal Coatings?

Standard Thickness Range

The typical thickness of conformal coatings ranges from 1 to 5 mils (25 to 127 microns), with those applied via brushing being the thickest. This protection barrier is sufficient for protecting PCBs from the elements and abrasion while leaving them with some room to flex. Also, it keeps the costs low.

Encapsulate or Potting

If the layer’s thickness exceeds 5 mils (127 microns), it ceases to be a conformal coating and becomes an encapsulate or potting. Not only will its cost be higher due to the excess material required, but the weight and size of the PCB will increase while its flexibility will reduce.

Even so, the environmental protection and mechanical support factors will be better, and the potting will last longer.

Different Measurement Techniques to Determine Conformal Coating Thickness

Wet Film Gauge

A wet film gauge is a comb-shaped tool with graduated teeth that are pressed into the coating while still wet. The gauge has varying distances between the teeth. It determines the coat thickness based on the last wet tooth. This technique is not highly accurate but is suitable for quick, in-process checks.

Micrometer

This device measures the coat’s thickness by comparing the thickness of a coated and non-coated area or the same location before and after coating. The process is accurate and ideal for quality control.

Coulometric Method

Coulometry is an electrochemical measuring method that involves running an electrolysis reaction to dissolve the coat in a specified area and measuring the electricity consumed to dissolve that material. The process is highly precise and is suitable for applications requiring extreme accuracy.

What Standards Regulate Conformal Coating?

Regulatory and Standard Compliance

Compliance with regulatory and industry standards ensures the coat application process follows the best practices and the coat is reliable and safe on PCBs in electronic devices.

The IPC-A-610, for instance, is an important acceptability standard for these assemblies, while IPC-CC-830 establishes the qualification and conformance requirements for electrical insulating compounds (conformal coatings).

Others like the UL746E list a series of tests to evaluate whether a conformal coat can withstand sudden electrical surges and maintain its insulation properties. For military applications, the MIL-I-46058C standard gives specifications for insulating compounds.

These standards ensure consistent quality across all electronic products as well as a longer lifespan.

Coatings for PLC

Programmable Logic Controllers (PLCs) are used in harsh industrial applications, including in offshore rigs, so they must withstand the most extreme of conditions. At the least, a PLC’s conformal coat should protect the board from high temperatures, solvents, and abrasive materials. Urethane fits this bill.

Conformal Coating in PCB FAQs

Can Conformal Coatings be Removed or Reworked?

Materials like acrylic are easy to remove or rework, but silicone and urethane are tough to remove. But it is possible, and you can rework or remove these layers using mechanical, thermal (hot air desoldering tools or solder guns), or chemical methods.

What are the common Challenges With Conformal Coating?

The common challenges include cracking, bubbling, capillary flow, de-wetting, delamination, and orange peeling. You can avoid these issues by:

• Ensuring the PCB is clean
• Lowering the curing temperature
• Choosing a more flexible coating
• Allowing some additional drying time at room temperature
• Reducing the coating thickness
• Applying a primer material before the coat
• Using a lower-viscosity version of the conformal coat

How do you Choose the Right Conformal Coating?

The best way to choose the most suitable conformal coating for your PCB is to analyze the pros and cons of each material. Compare these pros and cons against the application environment conditions and electronic requirements.

Back to top: What is Conformal Coating on a PCB?