Conformal Coatings in Electronic Devices

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Conformal coatings are a special type of coating that covers the entire surface of a material and can be used to protect various electronic components from thermal, mechanical, and environmental stresses, as well as for keeping the components sterile (for example, in the medical industry). While there is a multitude of benefits for conformal coatings, the ability to completely conform to surfaces with complex geometries has been the main go-to reason for their use. Moreover, their resistance to high temperatures and ability to dissipate heat evenly—which are known as thermally conductive conformal coatings—has become valuable in an environment where a lot of residual heat is generated and needs to be dispersed evenly to prevent damage from occurring to the components. While thermally conductive conformal coatings often get the most coverage, conformal coatings can be used for a number of reasons in electronic devices.

Conformal coatings can be made of a number of materials, and each material has its own distinct advantages depending on the application. The coating application methods can be simple, such as spray or dip coating, but the key is in the specific materials which are fluidic enough to able to cover the surface completely and are curable to turn the coating into an effective—and solid—barrier. The most common materials which are used to make conformal coatings are epoxy resins, acrylics, polyurethanes, silicone, and parylene. All have specialist properties—and different costs—which means that they have found specific uses within electronic devices.

Epoxy resins are insulating polymers that are a great medium for dissipating heat. While they are strong enough to provide a barrier to most harmful stimuli, they are inherently hard to rework and remove once they have been cured on the surface of the substrate. The use of chemicals destroys the coating, so often, burning the coating is the only option to rework it. However, for electronic devices, this is not a favorable approach.

Acrylics are a different type of polymer resin. Acrylics are applied at room temperatures, negating the need for heat to be used in the application process, and are known to have some of the highest barrier properties for liquid applied coatings. Another reason why acrylics are used is due to their long usable lifetime and that they don’t give out any residual heat to the surrounding components during the curing stages. However, the one disadvantage is that many solvents can dissolve them, so while their thermal and mechanical stability is high, their chemical stability is not.

Polyurethanes are the most common choice, partly because they are the option that costs the least. Urethanes are basic organic molecules with an ester-containing group, while polyurethanes are polymeric materials composed of urethane linkages between monomer units. Polyurethanes are also widely used because their composition is highly tuneable and can be used to make a wide range of conformal coatings, including single component, two component, UV curable, and water-borne conformal coatings. Aside from a beneficial barrier to thermal, chemical, and electrical stimuli, they are best known for their resistance to moisture. However, the one downside is that they can be easily removed using some alkaline solvents.

Silicones are a polymeric material composed of repeating siloxane units (alternating oxygen and silicon atoms with carbon and hydrogen atoms attached). The presence of silicon makes them highly resistant to temperature, as well as other stimuli, which has seen them used as heat dissipation coatings. However, while they are stable to chemical erosion, they are not the strongest coating when it comes to mechanical forces and can be easily worn away.

Parylene is the only coating here that is applied by gaseous means, but this application process means that they can penetrate very small gaps that other coating methods cannot reach. It is also known to be a very uniform coating method, and no curing stage is needed. The parylene coating is also resistant to biological erosion and can make components/devices sterile. However, the cost of applying parylene coatings is much higher than other methods, which can limit its use if the extra pin-point coverage is not needed.

Not all components within an electrical device will benefit from a conformal coating. In these cases, other coating technologies or no coating at all are perfectly fine. But there are some components that undergo a lot of thermal, mechanical, or environmental stress that do benefit from a coating that covers the whole surface regardless of the geometry. The ability to conform to many different surfaces and provide a complete barrier has been better utilized with complex components and devices where conventional coating methods will not be able to cover the whole surface, and the increase in cost over other coating methods is justified. Moreover, components also benefit from conformal coatings when there is a chance they could come under mechanical and thermal stress, could experience rough handling during installation, are likely to experience physical damage, or when the components need to be sterile.

There are far too many examples to document them all, but what we can do, is look at a selection of different components to show the range of components that can be conformally coated, and the reasons behind them. This should showcase the scope possible with conformal coatings.

The first example is electric motors. Motors contain many small and moving mechanical parts that are often hard to coat and protect. Yet, they are a vital component of many electronic devices. Epoxies, polyurethanes, and parylene are common choices for motors for different reasons. Epoxies are used because they can regulate the internal temperature of the motor, leading to a longer usable lifetime. Epoxies also provide high resistance to abrasion. Polyurethanes are used in motors to provide resistance to both moisture and abrasion, while parylene is used because, in its gaseous form, it can penetrate small gaps and can coat the complex geometries of the motor. Parylene can also partially act as a lubricant, as it has an inherent dry-film lubricity so that the movement is not hindered when the coating is on the surface of the moving parts.

Another example is circuit boards. Circuit boards are one of the most common substrates to be coated with a conformal coating. Both epoxies and parylene are widely used materials for circuit boards. Epoxies are used on circuit boards because they can regulate the temperature and avoid critical overheating, which in turn, increases the lifetime of the circuit board (as circuit boards contain many components emitting heat at the same time). Parylene is used because it can cover all the small gaps that are present on the surface of a circuit board. Moreover, its inherent lubricity is borne out of hydrophobic interactions, which makes circuit boards coated with parylene less susceptible to water damage and dust.

A third example is magnets that are used in electronic devices. It’s not the most obvious component, hence its inclusion here, but they are often susceptible to a wide range of chemicals. Parylene is the only one used because of its method of deposition. The ability to deposit the coating as a gas prevents the transition metal ions in the magnet from reacting with the deposition molecules. Liquid coatings tend to react with the surface of the magnet, causing damage to it and subsequently reducing its functionality.

The fourth, and final example is power resistors. This is one area where silicone dominates due to the high temperatures and lack of physical forces in play. The heat from power resistors is dissipated by the silicone coating, and because they can withstand high temperatures and dissipate it evenly, the silicone coatings don’t exert any localized release of heat to any one part of the electronic device.

Conformal coatings offer a way to coat electronic components, regardless of their geometry, to make them resistant to high temperatures, mechanical abrasion, and make them sterile (for use in medical applications). Moreover, the ability of many conformal coatings to dissipate heat away from electronic components offers a way of stopping electronic components from overheating due to localized heat spots that arise from the residual heat given out by the different components. It is a wide-scope area with many different application areas that can use a number of materials as the coating surface.