Parylene has a well-deserved reputation as one of the leading choices for conformal coatings. For many applications, it is the best choice. However, there are some real parylene disadvantages, as well. For many applications, other conformal coatings such as acrylic, epoxy, silicone or urethane offer better performance, lower cost or both.
Coating Cost and Logistics
One of the most significant parylene disadvantages is its cost. The parylene dimer that forms the coating is available from a limited number of suppliers and carries a cost of anywhere from $200 to $10,000+ per pound. Given that a typical run requires a pound of parylene, it can make the material cost extremely high, especially if you are only looking to coat a limited number of components.
In addition, parylene can only be deposited one way -- as a vapor in a vacuum. This requires highly specialized equipment that has limited capacity, making it hard to handle very large quantities of items. At the same time, since the coating process requires extensive preparation before it can start, the labor cost of coating makes parylene doubly unsuitable for small runs.
Other coatings can be applied in small or large batches using a range of different methods. In addition, many of them are based on materials that are orders of magnitude less expensive. This makes it easier and less expensive to coat both very small and very large jobs.
Total Coverage... No Matter What
Parylene covers everything, everywhere. Since it is a vapor, it coats anything that air touches. While this can be a highly desirable feature, it also means that items that aren't intended to be fully coated need to go through a masking process that adds time and expense. Acrylic coatings, on the other hand, can be sprayed or brushed on by humans or robots. This makes it easier to only apply the coating where it needs to go.
Thin and Clear
Being thin and being clear can both be parylene disadvantages. A thicker conformal coating, like the one that silicone can provide, can add an additional layer of cushioning and shock protection to the coated item. While parylene protects the items that it coats, especially given its cost, it is more likely to be applied in a thin coating.
Parylene's optical clarity makes it suitable for coating lenses and optical elements. However, this also means that anything coated with it is visible to anyone that looks at it, including someone that is interested in reverse engineering the item. Pigmented epoxy and polyurethane coatings can be used to completely cover the item, hiding its components. They also have roughly similar toughness to parylene, making it hard for someone to remove them.
Among other parylene disadvantages that can also be advantages, parylene is hard to remove. It is hard, solvent-resistant and relatively heat-resistant. This means that the only practical way to remove it is with time-consuming and inconvenient micro-abrasion. Reactive coatings like acrylic can be removed by something as simple as a cotton swap dipped in solvent. Other stripping solutions work on silicone-coated items and some even work to remove polyurethane coatings.
Many of the less expensive parylene dimers have relatively weak resistance to ultraviolet light. This means that if they are used on a component that is mounted outdoors, they could yellow. While some parylene dimers have UV resistance, they are typically the most expensive formulations, costing upwards of $10,000 per pound. For comparison, both silicone and acrylic coatings can be applied in a UV-resistant formats that allow them to be used outdoors for extended periods of time without any degradation or yellowing. This property also makes them more effective at protecting the items that they coat from UV radiation.
While there are many parylene disadvantages, it also has significant advantages. Automatically disqualifying parylene for consideration would be like disqualifying any other coating for parylene. Ultimately, different coatings are suited to different products. Considering all of the options for all of your projects can help your company find the right coating for every project, whether it ends up being parylene, acrylic, epoxy or polyurethane.