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.