Conformal Coating Education Center

Parylene for LEDs

Parylene coatings ensure an even, conformal, lightweight coating offering enhanced protection for LEDs.

In this Section:


Parylene for LEDs

How it's Used

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.

LED Coating

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.

Conclusion
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.


Parylene Conformal Coatings and UV Light

Effectiveness Explained

Parylene has numerous outdoor applications. However, a major drawback of most parylene types is limited resistance to direct contact with UV radiation. Daylight is the most common source of UV light. Prolonged exposure to its high energy radiation can cause objects extensive surface damage and lead to eventual malfunction of electrical light-generating assemblies within.

This is a significant drawback, since the objective of conformal coatings is the sustain assembly function in the long term. Generally fine when not directly exposed to UV -- as in the many cases where components are internally situated within a product – most generally transparent parylene films yellow and degrade from prolonged contact with sunlight.

Limited UV Protection Provided by Most Parylenes
This is not the case for all types of radiation, where parylene’s coating properties are maintained. For instance, in a vacuum, parylene’s radiation resistance to gamma ray degradation is consistently impressive. Conformal films composed from parylene for types C, D, N. and AF-4 retain electrical and tensile properties at dosages of 1,000 kilograys (kGy), through a dose rate of 16 kGy/hr. However, outside a vacuum and exposed to air, rapid embrittlement develops. Nevertheless, the mentioned parylene types do generate rather reliable gamma ray protection in a vacuum.

The same cannot be said for exposure to UV light (direct sunlight). Much has to do with the substances’ chemical composition:

  • Parylene N is an unsubstituted hydrocarbon molecule.
  • Parylene C has one added chlorine group per repeat unit.
  • Parylene D has two added chlorine groups per repeat unit.
  • In contrast, parylene AF-4 replaces the hydrogen atoms on the chemical's benzene ring with fluorine atoms, significantly enhancing its UV-stability.

Diverse UV Protection, According to Parylene Properties
Basically hydrocarbon dimers, parylene types have different properties. Although it has the best surface-permeating capacities of the parylenes, type N is especially susceptible to UV-generated damage. It has a significantly higher oxygen permeability than parylene C, whose composition has an added chlorine atom; Parylene D adds a second chlorine atom. Nevertheless, the superior impermeability of types C and D declines markedly with UV exposure. Oxygen in UV light causes their coating to decompose into aldehydes and carboxylic acids near the conformal film’s surface, yellowing them and reducing their barrier protection, endangering component function.

Parylene AF-4
Parylene AF-4 is the only parylene type that displays a consistent degree of UV-resistance, one wherein performance is maintained relatively long-term. Like the other parylene types, AF-4 also forms an effective, structurally continuous protective film as thin as several hundred angstroms, due to uniqueness of the chemical vapor deposition (CVD) process used to coat substrate surfaces.

Also called aliphatic fluorinate, AF-4 replaces the alpha hydrogen atom of the N dimer with fluorine, and totals 4 atoms. This compositional variation allows AF-4 to generate the lowest parylene coefficient of friction and dielectric constant throughout UV-exposure, and the highest penetrating ability. AF-4 is also useful in high temperature, short-term applications up to 450°C.

Thus, while parylenes C, D, and N provide dependably stable conformal coatings indoors, degradation commences shortly after exposure to UV light. Only parylene AF-4 displays longer-term and consistent resistance to UV light, for wavelengths between 272 – 400 nm; component degradation is eliminated even after 2,000 hours in air. However, AF-4 is the most expensive of the parylenes, with costs between $8,000 and $10,000 per kilogram. Processing for AF-4 adds a necessary third-step to the synthesis of its precursor, generating only low yield and a reduced deposition efficiency during production, driving total costs higher. Less expensive at this stage may be using another substance – acrylic or silicone, for example -- to provide UV-resistance to parylene-coated assembles exposed to UV light.

Table 1 compares the spectra of parylene types C, D, N, and AF-4 to UV light.

Parylene Type % Transmittance Wavelength, nm
D, Lower range 0 290
D, Mid-range 75 350
D, Upper range 90 398
C, Lower range 0 277
C, Mid-range 68 348
C, Upper range 74 395
N, Lower range 0 279
N, Mid-range 81 349
N, Upper range 88 397
AF-4, Lower range 0 272
AF-4, Mid-range 90 351
AF-4, Upper range 91 400

Summary
This evidence suggests AF-4's superior coating and resistant qualities for UV uses, throughout the widest UV-wavelength range. The parylene types N, D and C can provide much shorter-term UV protection when treated externally with other coating substances, such as acrylic or silicone; however, even under these conditions UV-protection is limited.

In contrast, AF-4 provides 2,000 or more hours of UV-usage without suffering decomposition or surface yellowing. Moreover, AF-4 is effective with a much thinner protective coating layer. Applied as external coatings to other parylene types, substances like or acrylic or silicone need to be deposited in much thicker layers, severely restricting their applications for the microelectricalmechanical (MEMS) and nano-systems' functionality rapidly becoming more prominent for most digital assemblies and components. Under these conditions, the prospect of item malfunction or breakdown can actually make AF-4 parylene more economical to use in the long run.

Parylene is proving to be an ideal material for sealing, insulating, and protecting electronic modules, devices, and circuit boards. It’s easy to apply, goes down in uniform layers, and stands up to many environmental hazards. These same properties also make Parylene an excellent choice for manufacturing micro-electromechanical systems (MEMS) – sensors, actuators, and structures forged from silicon using industry standard semiconductor-processing techniques. In fact, Parylene is helping MEMS designers overcome some of their toughest challenges, clearing the way for new features and functions and a fresh growth spurt for MEMS applications.

From the time silicon was first used to form mechanical structures and devices, designers have struggled with space constraints on both ends of the dimensional spectrum. MEMS devices – accelerometers, gyroscopes, and flow sensors, for example – typically contain many complex moving parts as well as signal processing and interface circuits, and all these elements must fit comfortably in packages scaled to size of tiny silicon chips. At the same time, every nook and cranny in the labyrinth of silicon micro-machinery must be accessible to fluids and light fields employed in the many processing steps. Parylene excels under such circumstances because it’s applied in a gaseous or vapor state under vacuum. The vapor deposition process uniformly coats even the most inaccessible surfaces, penetrating spaces as narrow as 0.01 mm. It also covers sharp edges, points, and exposed internal surfaces, resulting in a thin conformal layer that’s free of pinholes and impermeable to anything larger than 1.4 nanometers (nm).

Another challenge MEMS designers face stems from thermal and mechanical stresses imposed by the presence of sealing materials as well as the processes by which they’re typically applied. Thick heavy coatings, for example, can reduce the sensitivity and dynamic range of motion sensors and actuators. The effects of high processing temperatures, on the other hand, are potentially worse and may even be catastrophic. Parylene circumvents both problems. For one, it’s applied at room temperature and it doesn’t require curing like many other coating materials. It also minimizes mechanical stresses and loads because it can be applied at a precisely controlled thickness. Parylene layers form on substrates literally one molecule at a time, resulting in uniform films that can be anywhere from a few angstroms to several microns thick.

Even at a thickness of just 0.5 microns (µm), Parylene achieves a near impenetrable defense for the surfaces and structures it protects. Not only does it provide a pinhole-free moisture and chemical barrier, but also a biological barrier. What’s more, components sealed with Parylene are unaffected by solvents, including gasoline and acetone, and can easily pass a 100hr salt-spray test. Besides being chemically and biologically inert, Parylene offers outstanding wear and dry-film lubricity properties with a static coefficient of friction near that of Teflon – as low as 0.25 to 0.30. It’s also stable over a wide temperature range (-200‘C to +200‘C) and is extremely rugged, having high tensile and yield strength in the range of 50 to 70 MPa.

Parylene’s electrical and optical properties are also well suited for MEMS applications. Parylene is a good electrical insulator with high dielectric strength and high bulk and surface resistance. It also has negligible capacitive effect thanks to its low dielectric constant. This mitigates parasitic losses that would otherwise occur at high frequencies. As for its optical properties, Parylene is relatively transparent and can be used to coat LEDs, light sensors, mirrors, and lenses. It also withstands UV radiation and protects optical components from UV-induced damage. Parylene can be applied to most vacuum-stable materials, including optical plastics, metals, quartz, and semiconductors.


Protecting LEDs with Parylene

Advantages of Parylene

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.

Why Parylene?
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.


Conformal Coatings for LED Applications

Coatings Comparison

From front to back, LEDs are improved by conformal coatings. Whether the coating is improving the LED's color accuracy, protecting it from damage or keeping the electronics functioning well, conformal coating of LED electronics extends the suitability of LED technology. Here are the top six ways that conformal coating and LEDs go well together

UV Resistance
Many types of LEDs are sensitive to the ultraviolet radiation that comes off of the sun. Over time, these LEDs gradually yellow and experience reduced light output. While some conformal coatings do not protect LEDs from UV, others like special types of Parylene, block the light and its yellowing effects. Conformal coating of LED electronics that are outside can help to make the displays last longer while maintaining the vivid colors that LEDs are known for.

Moisture-Resistance
Putting LEDs outside means exposing them to the extremes of the elements. Among other risks is damage from exposure to water. Silicone conformal coatings can be applied relatively thickly, providing a large margin of error to prevent water from reaching and damaging the LEDs and the circuitry that drives them. Parylene coatings have also been shown to have a high degree of success preventing moisture damage to LEDs.

Chemical Protection
Providing a conformal coating for LED's can also protect them from chemicals. Urethane conformal coatings and parylene both protect LEDs from the solvents that they can be exposed to in automotive applications. At the same time, the salt protection that coating provide aren't just valuable for outdoor facing LEDs. They're also useful for products like consumer electronics that will be used in close proximity to the human body and the sweat and other byproducts that it can produce.

Optical Clarity
Many conformal coatings are optically transparent to visible and, in some case, invisible light. This means that they can add protection to LED displays without the risk of glare or distortion that can come with adding a cover glass over them. At the same time, they're also usually relatively light and thin. A truly conformal coating over LED electronics, like the kind that parylene offers, is also completely uniform, minimizing the risk of any distortion or refraction of the light.

Electrical Insulation
Many conformal coatings are also excellent dielectrics. When conformal coatings are applied to the backside of LED electronics, they seal the components off from each other. This helps to reduce the risk of any short circuits occurring when parts shift or when conductive materials fall on the PC board.

Tin Whisker Management
Unfortunately, some connections are prone to the formation of tin whiskers. These are small conductive bridges of tin that gradually grow out of tin-coated surfaces and can cause short circuits. One of the best ways to manage them is through the use of conformal coatings. Conformal coating of LED electronics with compounds like urethane solves the problem in two ways. First, they hold back the growth of whiskers and encapsulate any growth in a dielectric barrier. Second, even if the whisker can pierce the coating, it will end up running into a conformal coating barrier on the other side, instead of contacting a bare conductor.


Ultraviolet (UV) Stability of Parylene

Parylene Chemistry

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 Chemistry
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.


What is the right conformal coating for LEDs?

How to Choose

ight emitting diodes (LEDs) are a huge and growing even bigger segment of the electronics industry. LEDs are expanding into environments that conformal coating LEDdemand a higher l evel of protection in order for the LED to function properly. One way to get this level of protection is by using conformal coating.

LED Conformal Coatings

Acrylics
Acrylic conformal coating is great for LEDs that are looking for solid moisture protection. Acrylics should not be used on LEDs that are going into a high-solvent environment. Some acrylics have been specially formulated to be optically clear so the LED can be coated over, reducing masking time and overall cost.

Silicones
Silicone conformal coating is used whenever there are high heat requirements for the LED. Some specially manufactured silicones can offer protection beyond 200ºC. Silicones, because they are applied thicker than other coatings, also can offer a higher level of moisture protection for harsher environments.

Parylene
Parylene is an excellent choice for LEDs that are looking for the ultimate in moisture and solvent protection. While normally parylene has poor ultra violet (UV) resistance, a great alternative is to use Diamond-MT’s parylene coating for LEDs that has the benefits of parylene with UV protection built in for outdoor applications, like signage.


Back to Education Center »