For all of Parylene's strengths, it has one key drawback—Parylene's resistance to ultraviolet (UV) radiation is limited. Most formulations of Parylene gradually yellow when exposed to the kind of UV light that's produced by the sun. While this isn't a problem when Parylene gets used to conformally coat a printed circuit board that's sealed in a box, it can be a problem when a display made of Parylene-coated LEDs is installed outdoors.
UV stability is an important feature when coating light emitting diodes (LEDs). LEDs themselves, especially the organic variant in OLED and AMOLED displays, are prone to yellowing in the presence of UV. At the same time, even if the LED doesn't yellow, a yellow coating will compromise the color accuracy of the LED display. With this in mind, a coating that is both tough and UV-proof is necessary.
Parylene generally comes in three different formulations: Parylene N, Parylene C and Parylene HT. All are hydrocarbon dimers. Parylene N is the best penetrating form of the chemical. Parylene C adds a chlorine atom to Parylene N that makes it more impermeable, making it a better choice for coating electronics.
Unfortunately, Parylene N and C are sensitive to ultraviolet light, with research in Europe showing that the N variant is particularly prone to damage due to its much higher oxygen permeability than Parylene C. When they're in a setting where they come in contact with oxygen, exposure to UV light causes the coating to partially break down. Parts of the coating near its surface turn into carboxylic acids and aldehydes, yellowing them.
Solving the UV Problem
Parylene AF-4 replaces the hydrogen atoms on the chemical's benzene ring with fluorine atoms. This change increases the Parylene's stability in ultra violet light. It can easily withstand 2,000 hours of exposure to ultraviolet light without breaking down or yellowing. However, Parylene AF-4 is more expensive, with costs between $8,000 and $10,000 per kilogram. This usually puts the use of Parylene AF-4 outside of the realm of cost feasibility for most LED applications.
UV-proof Parylene may be the perfect coating for LEDs. LED display structures that are exposed to the elements are prone to multiple breakdown modes. Parylene can completely encapsulate an LED, protecting all of its surfaces from moisture damage. It can also withstand high heat, which is present in some dense LED displays. The compound even provides insulation from other electrical signals thanks to its dielectric properties.
Including UV protection in your product design can also help to mitigate the problem. For instance, putting a piece of UV-filtering glass in front of an array of LEDs could protect them and their Parylene coating from UV degradation. It also shields them from the elements and makes them easier to clean.
The other option is to use a different type of coating. Both acrylic and silicon can be applied in UV-resistant formulations. While each of these components have their own strengths and benefits, they share two characteristics relative to Parylene: Both are deposited in much thicker coatings, which may not be desirable when a small size is important and both can also be less expensive to use.
Diamond-MT has over 25 years of field experience using Parylene and other compounds to create conformal coatings that serve our clients' unique needs. If you need to have your LEDs coated, contact us to request more information on our services. We can help you determine which coating is best for your application and come up with a quote that will help your project be successful.