Parylene vs Urethane Conformal Coatings

Parylene and urethane conformal coatings share many characteristics. Both are physically robust, resistant to chemicals and mitigate tin whisker formation. However, this does not mean that the two coatings are interchangeable. While Parylenes offer a unique blend of capabilities, many projects choose urethanes because of their cost, durability and other advantages.

Parylene and Urethane Shared Characteristics

Parylene and urethane share both disadvantages and advantages. For example, both are relatively challenging to rework. Because no known chemical can break Parylene down, Parylene can be removed through microabrasion. This technique uses a small jet of air that carries abradants to gradually remove the coating. While urethane can be removed with chemicals, due to its chemical resistance, it too can require mechanical reworking techniques.

Both coatings share a common drawback — their temperature ratings. Most Parylene variants can withstand 80°C continuous operating temperatures and most urethanes in the 125°C range. This means that they can protect most items that will be used in the real world. However, neither can withstand the kind of heat inside of an operating engine or an oven. This limits their applicability in high heat applications. Specifically designed for harsh environments, Parylene HT^® is an exception and can withstand temperatures exceeding 350°C.

On the other hand, they also both share a common benefit. Tin whisker formation can be delayed to prevent them becoming a serious issue with the right coating. Both Parylene and urethane provide an adequately thick and durable coating to protect the surface of the tin and prevent additional structures from becoming a threat that could end up short circuiting electronic devices.

Both urethane and Parylene coatings share a unique advantage in their resistance to solvents. They stand impervious to a wide range of solvents, making them ideal selections for applications exposed to harsh chemicals.

Parylene and Urethane Differences

Deposition and Curing

One of the areas where the two coatings diverge significantly is in how they are applied on devices and components. Parylene is applied via vacuum deposition and has no cure, while urethane is the opposite.

Urethane is applied much like other liquid coatings. It can be applied via dip or by being brushed on. Other methods include spraying it onto the substrate using either human or robotic labor. Once applied, urethane requires an extended period of time to cure. While it can be dry to the touch within a few minutes, a full cure is usually measured in hours or, with some types of urethane, in as long as 30 days.

Parylene cannot be applied the same way as urethane and has no cure. Deposition requires specialized equipment that vaporizes the Parylene dimer and applies it in a vacuum. This process can take over 24 hours, depending on the variant of Parylene used and the thickness desired. Once it is deposited, though, it has essentially no curing time since no further chemical reactions occur after deposition.

Hardness and Vibration

One of urethane’s other strengths is also a potential weakness. It is a durable coating, creating a hard shell around the item that it covers. However, this characteristic can also make it brittle. In environments where it will be subjected to high levels of vibration, its structure can break down, eventually causing it to fail. While Parylene is hard enough to provide a degree of physical protection, it also has more flexibility. This makes it much less likely to fail under vibration. On the other hand, urethane’s hardness makes it an excellent choice for settings where it will be exposed to abrasion. While both coatings resist abrasive forces, urethanes offer an extra degree of protection.

Making the Best Choice

It is important to select the coating that will withstand the challenges your application will face and best aligns with your needs. For instance, if you can have the coating applied through traditional methods, do not require a particularly thin coating, and do not expect to have high levels of vibration, urethane could be a good choice. However, the best way to choose a coating is to work with an expert in conformal coating. That expert can help you to weigh the different features and benefits of each type of conformal coating to give you the best possible performance for your application.