How to Choose the Right Conformal Coating

Conformal coating protects circuitry from environmental contaminants

Electronics are everywhere in today’s world. From submarines to NASA spacecraft, traffic lights to iPhones, circuitry keeps day-to-day life ticking over smoothly. Keeping circuitry ticking over smoothly is the job of conformal coating. Conformal coating is a thin film coating which protects printed circuit boards (PCBs), components, and electrical devices from harsh or damaging environments and conditions. Moisture, salt, chemicals, ranging temperatures, dust, and other contaminants could cause the circuitry to fail if left unprotected and uncoated. Conformal coatings prevent this by providing a dielectric barrier between the circuitry and the atmosphere.

In this article we look at conformal coating, how it works, and the different conformal coating types and their applications. We also list conformal coating applicators and companies in the US.

What makes conformal coatings necessary?

There are a range of polymers used for conformal coatings – polyurethane, acrylic, fluoropolymers, silicone, parylene – and though each has it advantages or disadvantages, on the whole they do the same job: They protect the circuit board. The transparent polymeric coating is a barrier between the harmful elements of an environment, such as moisture or temperature extremes, while also being breathable enough to let trapped moisture escape. Typical application sectors include automotive, aerospace, military, marine, and even domestic and consumer items like washing machines and phones.

In short, conformal coatings:

• Protect products from heat, moisture, chemicals, and particulates and thus from corrosion and possible short circuits
• Prevent damage from handling, installation, mechanical stresses
• Prolong the lifespan of the PCB, component, or device by eliminating environment-caused degradation
• Only minimally add to the weight of the component due to an application thickness of 25-200µm dependent on the conformal coating types
• ‘Conform’ to the irregularities of the circuit board
• Enable circuitry to get increasingly smaller due to their dielectric properties and insulation

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An overview of the conformal coating types – acrylic to PTFE

Conformal coating types vary in their properties and strengths, and which one you choose for your application needs to be carefully matched to the requirements of the use, environment, and expected lifespan of the substrate. Below we look at the different conformal coating types, their strengths, and their weaknesses, and sum it all up in a handy table.

1. ACRYLIC
   Acrylic conformal coating is the easiest applied, easiest repaired, and easiest removed of the conformal coatings, as well as being the most affordable option. Because of this, they are also the most popular of the options. They are typically one part coatings which dry rapidly and display good temperature range tolerances, good insulation, good durability, and good humidity resistance. They do not shrink as they cure and can be applied to heat-sensitive substrates as they do not give off heat as they cure.
   Disadvantages: Acrylics are not suited for chemical environments and do not have great mechanical strength.
2. EPOXY
   Epoxy conformal coating has good humidity resistance and excellent mechanical strength, chemical resistance, abrasion resistance, moisture resistance, and, like epoxy coatings as a group, generally performs very well in harsh environments.
   Disadvantages: Epoxy conformal coating is extremely difficult to remove. This is because the strippers used to remove it would also attack the circuit board itself, so it is nigh on impossible to achieve cleanly and without problems. Epoxies also have a short pot life and specific mixing requirements as they are usually two part, and are more prone to surface defects.
3. FLUOROPOLYMER (PTFE)
   PTFE conformal coating (also known as Teflon) is low-friction as well as being corrosion, heat, and chemical resistant. PTFE also displays dielectric stability.
   Disadvantages: PTFE is soft and easily damaged. It requires specialized equipment for application through vapor deposition and is very difficult to repair.
4. PARYLENE
   Parylene conformal coating is a thermoplastic coating with a low coefficient of friction and the best solvent and extreme temperature resistance of the conformal coatings. It also has a high dielectric strength, can form very thin films which completely cover the substrate, and has no curing time.
   Disadvantages: Parlylene is applied through chemical vapor deposition in a vacuum chamber, so application requires specialized equipment which is reflected in the higher cost. It is also very difficult to remove, easy to damage, and does not bond as well as the other coatings.
5. POLYURETHANE
   Polyurethane or urethane conformal coatings are best suited for extreme chemical conditions. It has excellent chemical resistance, humidity resistance, abrasion resistance, and excellent dielectric properties as well as being suitable across a range of temperatures. It is low in cost and has flexibility in application method. Polyurethanes are often used for aerospace applications or applications where fuel vapor exposure is an issue.
   Disadvantages: It is very difficult to repair and remove, and has a short pot life.
6. SILICONE
   Silicone conformal coating is a flexible option which is also good against high temperatures, moisture, corrosion, chemicals, and thermal shock – they are one of the most popular conformal coating types. They are highly adhesive and are used for substrates that will be subject to high temperatures or include components like power resistors.
   Disadvantages: Susceptible to abrasion, very difficult to remove, high cost, and, if atomized, it can migrate.

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Conformal coating application methods

The main application processes for conformal coatings can be divided broadly into manual and automated application. Which you choose will depend on variables such as how much preparation (such as masking or taping) is required, how fast the coating process is, how fast the curing process is, the requirements of the electronics themselves such as solvent sensitivity, the equipment requirements of the coating, and the quality requirements of the final coating.

• Dip coating is the fastest application method, though it requires a lot of preparation in masking and sealing. In dip coating, the entire circuit board is dipped into a vat of coating material and then allowed to dry. It can be done manually or through an automated process but can only be done with coatings of a lower viscosity such as acrylic.
• Brushing applies the conformal coating with a brush to specific areas. It is labor intensive and mostly used for repair work or reworking areas.
• Aerosol/spray application atomizes the coating and sprays it at the circuit board with air or gas. Can be done manually or by equipment through the selective coating process. All non-deposition coatings can be applied this way.
• Automated selective coating is an aerosol process where certain areas of a circuit board are selected and a programmed machine performs the coating, such as Asymtek. It cuts down on preparation such as masking and guarantees an even, uniform, and repeatable coating. Selective coating can be used with all non-deposition coating types.
• Vapor deposition is the more expensive of the coating processes, as it has more specialized machinery requirements. It vaporizes the coating into a mist which then deposits on the circuit board. This is a highly specialized application method used for parylene and PTFE.

Once the coating is applied, the three curing methods are air dry, oven dry, or UV cure.

Where to find conformal coating specialists in the US

There are many companies operating in the US who provide conformal coating services. These include MIS Electronics Inc, ProCOAT, Summit Technologies Inc., Plasma Ruggedized Solutions, Electronic Coating Technologies, and Dymax. Below is a a list of prominent companies and the conformal coatings services they provide.

If you would like any more information about conformal coatings, get in touch! We will collaborate with our coating partners to find the right coating for your project. Simply use the “Request a Quote” button beneath this article and tell us a little about your project to get started. Our quotation service is 100% free, and our experts are here to help.