Protecting LEDs with Parylene
Posted by Sean Horn
Friday, May 29, 2015 8:00
@ 8:00 AM
Light emitting diodes are gradually replacing all other types of lighting. As they move out of consumer electronics and into general purpose applications ,the demands on the technology are shifting. It’s relatively easy to keep an LED safe when it is mounted in the front panel of a computer or hidden under a cover on an alarm clock. Protecting it when it is going to be exposed to the elements 24 hours a day, 365 days a year is more challenging.
One of the fundamental problems with LED technology is that LEDs are not particularly rugged in and of themselves. While many are rated for 100,000 hours of life, the inherent nature of an LED’s design means that they can’t achieve this type of longevity when they are being exposed to real world situations. LEDs are sensitive to moisture, to electrical interference and to physical damage. Their internal light generating components and the plastic shells that cover them are also extremely sensitive to ultraviolet radiation. Organic LEDs are particularly prone to degradation.
While LED prices are constantly dropping, leaving them uncovered and exposed and just planning on replacing them as they fail is not a practical option. Replacing light bulbs or florescent tubes is a manageable challenge. Climbing up 100 feet to replace one of two million LEDs in a multi-color display is much less so. As such, the LEDs have to be protected to reach their maximum life potential.
The natural solution to this challenge is to conformally coat the LED. Covering it with a protective compound gives the LED a barrier from the outside world. Most liquid conformal coatings like acrylic, urethane, silicone or epoxy provide great performance in many settings. However, they have a few fundamental drawbacks that can make them a bad choice for LEDs.
- Liquid coatings can be relatively thick and heavy when applied. Especially in settings that contain multiple LEDs, they can require changes to a product’s design and underlying support structure to accommodate heft and bulk.
- When applied in thinner and lighter coats, liquids can frequently end up forming pinholes or uneven areas that fail to truly protect the LEDs they are supposed to cover.
- Some liquid coatings can absorb or diffract the light coming out of the LEDs, changing the device’s optical performance. Given that LED light output still lags behind other technologies, losing light to a coating is rarely an acceptable option.
Parylene or type xy conformal coating is frequently the best choice for protecting LEDs and assemblies containing them. Its vapor-based deposition method eliminates the challenges inherent in using liquid-based coating compounds and its chemical properties add additional benefits.
Parylene’s first benefit comes from its unique deposition method. Since it deposits as a vapor, it creates a coat that touches everything that air can touch. It also touches everything evenly. This means that it creates a truly conformal coat with no pinholes.
At the same time, parylene is an extremely effective conformal coating compound. It has some of the best dielectric and moisture barrier qualities of any coating. This helps to protect the LEDs from interfering signals and from fog or water damage. At the same time, parylene is also an excellent chemical insulator, so it helps reduce the risk of corrosion. Unlike other coatings, it also does its job at thicknesses that are so small that they frequently do not require any redesigning for the coated items.
Parylene also provides a degree of UV protection for the LEDs. Simply coating them helps to prevent their internal components from light degradation. However, certain types of parylene are, in and of themselves, prone to gradual color change due to UV exposure. Depending on the product’s intended life, parylene may be stable enough for this to not be an issue, since it can typically last years without a visible color shift. However, certain types of parylene have even longer life and can be used in those situations where color shift is not an option.
Ultimately, parylene is the best choice for coating and protecting LEDs and LED assemblies. Between its extreme thinness and lightness and its comprehensive coverage against just about any risk that nature can throw at the coated item, no other compound can touch it.