Parylene and urethane conformal coatings share many characteristics. Both are physically strong, resistant to chemicals and mitigate tin whisker formation. This doesn't mean that the two compounds are interchangeable, though. While parylene offers a unique blend of capabilities, many projects choose urethane because of its cost, strength and other advantages.
Parylene and urethane share both disadvantages and advantages. For example, both are relatively challenging to rework. Because no known chemical can break it down, parylene is best removed through microabrasion. This technique uses a small jet of air that carries abradants to gradually break down the coating. While urethane can be removed with chemicals, due to its chemical resistance it too can require mechanical reworking techniques.
Both compounds 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 and extremely cold environments.
On the other hand, they also both share a common benefit. While the mechanism by which tin whiskers form remains unclear, one thing is clear is that the right coating can retard their growth to prevent them becoming a serious issue. Both parylene and urethane provide an adequately thick and strong coating to protect the surface of the tin and prevent additional structures from becoming a threat that could end up short circuiting the item.
Solvents, Parylene and Urethane
Where they are both uniquely strong is in their resistance to solvents. Both urethane and parylene coatings are impervious to them. This makes then excellent choices for applications where they will be exposed to any solvent or solvent-like chemical. Urethane, simply because of its thickness, even offers an extra degree of protection against them.
Deposition and Curing
One of the areas where the two coatings diverge significantly is in how they are placed on the item that will ultimately be coated. Parylene is applied via vacuum deposition and has no cure, while urethane is the opposite.
Urethane gets 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 it is not a liquid.
Hardness and Vibration
One of urethane's other strengths is also a potential weakness. It is a strong 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 make it an excellent choice for settings where it will be exposed to abrasion. While both coatings resist abrasive forces, urethane offers an extra degree of protection.
Making the Best Choice
Since urethane is usually less expensive than parylene, one option is to assume that urethane will be the best choice and then look for factors that will disqualify it. 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 in general. 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.