Parylene for LEDs
Posted by Sean Horn
Friday, March 4, 2016 8:00
@ 8:00 AM
LED Lifespan as Effected by UV Light
Although LEDS are designed to provide as many as 100,000 hours of illumination under laboratory conditions, they are not nearly as resilient when subjected to persistent real-world, real-time usage. Sensitive to electrical interference, moisture, UV light, and other persistent sources of physical damage, LEDs require protection to operate at levels anywhere near maximum efficiency. Of all the conformal coatings available to deliver reliable safeguards on an ongoing basis, none surpasses parylene.
Parylene and Application with LEDs
Parylene is the conformal coating of choice for LEDs that require dependable function 24/7, 365 days a year, for several years or more. While offering this degree of protection is a comparatively trouble-free proposition if the LED is situated indoors, beneath the panel of a kitchen clock or computer, it can falter when applied to more demanding operational circumstances, particularly those of consistent, real-time outdoor usage. Such developments are no longer unusual; LEDs are increasingly deployed into outdoor working environments subjected to harsh operational conditions, requiring constant levels of component security to assure performance according to product specifications.
In this regard, parylene’s performance far outstrips that of competing conformal coatings. Substances such as acrylic, epoxy, silicone and urethane definitely have their advantages for specialized uses. But when generating an abiding conformal coating for LED printed circuit boards (PCBs) and related electrical assemblies, parylene provides the most trustworthy level of component protection from corrosion, exposure to atmospheric conditions and UV light. It should be noted that Diamond MT has developed a parylene method that provides LEDs exceptional chemical barrier and moisture protection, in virtually any open-air operational context. This superior methodology is an outcome of parylene’s unique deposition process, in comparison to other conformal coatings.
Parylene Deposition as a Benefit for LED Protection
Acrylics, silicones and other major conformal coatings have some LED uses, applications requiring less robust protection. These coating substances, including epoxy and urethane, generally employ wet application techniques, whose final consistency can add weight to LED assemblies; the often thick, uneven covering of assembly substrates generated by using wet technologies diminishes component performance and operating life. Exposure to harsh environmental conditions and UV light also add to these problems.
Among other salient conditions limiting the use of liquid coatings for LEDs are:
- Voids within the surface coatings, acting as conduits for moisture that can negatively impact PCB’s efficient performance.
- The weightiness of wet-application coatings in comparison to parylene may damage LED components, disrupting their specialized operational features.
- Poor long-term resistance to UV light can seriously restrict PCB protection when LEDs are used outdoors for illumination devices, traffic signals, transportation signage and the like.
- Continual maintenance for these LED applications can rapidly escalate, becoming very expensive.
In comparison, ongoing development of parylene types and technologies has led to development of parylene compounds that are far more UV resistant than those of the past. Rather than being dipped, painted, sprayed or otherwise applied through a liquid procedure, parylene’s chemical vapor deposition (CVD) process causes the substance to adhere to substrates in a gaseous state, allowing deeper, more uniform penetration into the LED component’s surface. Very simply, parylene’s exceptionally minute molecular size generates a more uniform coating, conformal regardless of the contours or size of the elements being covered. In addition to the UV stability of parylene AF-4, other benefits of parylene for LED coatings include:
- resistance to temperature extremes,
- ultra-thin, pinhole-free covering,
- considerable, reliable dielectric properties,
- effective component insulation, and
- the ability to repel abrasive substances, chemicals, corrosive liquids, and moisture, while assuring
- lightweight barrier protection without the significant dimension or mass of liquid coatings.
Application of the CVD process creates a kind of molecular growth within and along substrate surfaces. These conditions provide gaseous parylene properties that effectively permeate every substrate crevice, enclosing its surface entirely, safely encapsulating often delicate components.
Protection from UV Radiation and Temperature Extremes
Parylene coatings ensure an even, conformal, lightweight coating offering enhanced protection for LEDs. With parylene AF-4, the pinhole-free coating is both UV and heat resistant, providing reliable component protection through prolonged exposure to outdoor conditions. Parylene provides significant protection at thickness levels far thinner than competing coating types (500 angstroms to 75 microns), that are generally undetected on the final product. Problems of yellowing or discoloration of the clear parylene coating due to exposure to UV light are limited by application of type AF-4. While subjected to some surface discoloration after prolonged exposure to UV radiation, parylene also filters UV light from the coated item, effectively protecting the internal LED assemblies from degradation and performance malfunction caused by UV radiation.
Parylene AF-4 coating makes LED lighting more efficient, durable, longer-lasting. and operationally versatile. Maximizing parylene’s benefits for LEDs subjected to UV exposure may require developing deployment strategies with conformal-coating specialists to generate the desired performance outcomes. Diamond MT offers clients LED-coating methods that ensure UV stability, with limited discoloration in the long-term, without sacrificing the chemical, dielectric and moisture barrier protection of parylene or the longer-term performance of LED components.
Diamond-MT has developed a process in which typical parylene C or N can offer UV stability along with the great protection of parylene.
We verified our process with some accelerated UV testing. The results of the UV test of 504 hours accelerated testing with UVB 313 nm cycling between 8 hours UV at 60 degree Celsius and 8 hours condensation at 50 degree Celsius showed that all board substrates survived the test. Normal parylene C and Dow Corning 1-2577 exhibited a fair amount of UV damage, as evidenced by the significant yellowing. Using Diamond-MT’s process prevented the UV damage and is considered the best option.
Even when not exposed to persistent UV radiation, LEDs require the protection of conformal coatings to maintain functionality long-term. Conformal coatings’ reliability varies according to the coating material and conditions of use. Outdoor LED performance is complicated by the degrading impact of UV light on efficient operation and use, a condition that worsens incrementally with the length of exposure. Most conformal coatings require specialized processing and inspection procedures if PCBs and related LED assemblies are to be used in direct exposure to UV light.
Compared to parylene, competing coating types – acrylic, epoxy, silicone and urethane – employ liquid coating techniques that can result in uneven, overly thick and viscous covering layers; they offer only moderate protection that can actually interfere with LED function, especially after prolonged use. Perhaps more significant, sustained exposure to UV light can seriously damage LED components using these coating substances, limiting their ongoing functionality.
In this regard, while most types of parylene – C, D, N – generate better overall protection to UV radiation than liquid application coatings, they are also limited in the longer-term security they provide LEDs exposed to UV light. However, Diamond MT’s parylene type AF-4 has been formulated to ensure long-lasting, superior preservation of LED assemblies, supporting reliable performance in the longer-term. Although more expensive to apply, AF-4 can actually generate cost savings over time, as it supports continued LED function after other coatings have degraded beyond use.
Because UV trace and curing are based on wet application techniques, neither should be used in conjunction with any type of CVD-process parylene coatings used for LED assemblies.