Parylene and acrylic conformal coatings represent two extremes of the types of compounds you can use to coat printed circuit boards, sensors, or other devices. While acrylic is popular and inexpensive, parylene offers some of the best performance of any coating compound.
Acrylic resin (AR) is the most commonly used compound in the conformal coating industry for coating printed circuit boards. It is a liquid polymer that provides a moisture-proof and dielectric protective layer and type AR conformal coating is one of the least expensive options available.
Acrylic resin can be applied in three different ways. An operator can use a brush to apply it or it can be sprayed on. In addition, larger jobs can be dipped in vats of resin. Instead of using a human technician, robotic technologies can also apply AR to items needing coating. Once applied, acrylics can cure in as little as 30 minutes, making them a good choice when fast turnaround is required. Typically, acrylic coatings are applied at thicknesses of .002 and .005 inches, with coatings at the thinner end of the scale typically being a more popular choice.
The most popular AR compounds only withstands temperatures up to 125 degrees Celsius.
Acrylic is very good at protecting against moisture. As a strong dielectric, it also provides a degree of protection against electrical interference for PCBs. It is also relatively physically strong, especially compared to soft silicone. Unfortunately, it can be easily removed with solvents. While this makes it easy to rework, it also means that it is not suitable for applications where it could end up interfacing with chemicals as common as regular isopropyl (rubbing) alcohol.
Parylene Conformal Coatings
Parylene is also a type of plastic. It is transparent, extremely moisture proof and an excellent dielectric. In addition to those characteristics, it adds an extremely high degree of chemical resistance. Unfortunately, due to how parylene is applied, it is often the most expensive conformal coating choice.
Many of parylene's benefits come from the unique way in which it gets deposited. Parylene dimer gets heated and vaporized. The items to be coated get placed in the vapor chamber where a vacuum sucks the vapor in to deposit the coating everywhere that air can touch. This leads to the best coverage of any conformal coating compound with the best uniformity. Furthermore, the unique way in which parylene coats items allows it to be deposited at thicknesses as little as .0001 inches, although it is typically deposited at a thickness of .0005 to .002 inches.
Like acrylic, parylene is not a particularly strong coating at extremely high temperatures. It withstands 80°C continuously and can handle temperature spikes to 125°C. However, while it is not particularly good in the face of blistering heat, it can withstand other types of energy, such as electron beams and gamma rays.
When it comes to chemical resistance, parylene is hard to beat. Acids, bases, salts, corrosive materials and most solvents have no effect on it. This is why it is a popular choice for implantable devices, since even the body’s own chemical-rich environment cannot compromise it. The drawback of these features is that parylene is extremely hard to rework.
Choosing a Coating
The choice between parylene and acrylic is relatively simple. If you need parylene's unique capabilities such as total conformity in coating, chemical resistance and extreme thinness, it is the best choice. However, if you are just looking for a coating that will provide some protection and an extremely strong seal against moisture, acrylic will usually provide adequate protection at a much lower price.