Ultraviolet (UV) Stability of Parylene

For all of Parylene’s strengths, it has one key drawback — its resistance to ultraviolet (UV) radiation is limited. Most formulations of Parylene gradually yellow when exposed to the kind of UV light produced by the sun. While this isn’t an issue when Parylene is used to coat a printed circuit board 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-resistant is necessary.

Parylene Chemistry

Parylene generally comes in three main 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 showing that the N variant is particularly prone to damage due to its much higher oxygen permeability than Parylene C. When 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 HT replaces the hydrogen atoms on the chemical’s benzene ring with fluorine atoms. This change increases the Parylene’s stability in ultraviolet light. It can easily withstand 2,000 hours of accelerated exposure to ultraviolet light (per ASTM 154) without breaking down or yellowing. This correlates to ~100,000 hours of outdoor use.

Parylene HT 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 coating even provides insulation from other electrical signals thanks to its dielectric properties.

Additionally, another option is to use a different type of coating. Both acrylic and silicone 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.