All LEDs, OLEDs, and AMOLEDs (light-emitting diodes, organic LEDs, and active-matrix OLED, respectively) are miniaturized solid-state technologies. Some are moving into the realm of quantum dot nanotechnology. Because they are all highly miniaturized electronic assemblies, they face the same basic challenges faced by all modern electronic nano-assemblies — dense, powerful capabilities designed into a micro-packaged device.

Over recent years LED lighting has grown from a few specific uses to replacing many types of standard lighting due to its reliability, energy efficiency, and cost-effective operation. (©iStockphoto)

Today’s LED technology can be found in a wide array of applications, from large video display signs and traffic signals to the latest technology in smart cell phone displays. The never-ending goal of LED technology is to make all types of displays brighter, longer lasting, and more dimensional. For example, LEDs have become well-established in automotive and aerospace industries for both interior and exterior lighting applications. The benefits of small size, low power consumption, and long life make them very attractive. Some operating environments, however, pose a challenge to manufacturers as LEDs must survive extreme temperatures, moisture, and ultraviolet (UV) exposure. If the assemblies are not sufficiently protected, prolonged exposure to these elements will shorten the 100,000 hour LED life expectancy.

Because Parylene is formed from a gas, it penetrates into every crevice, ensuring complete encapsulation of the substrate without bridging small openings.

While a flat screen TV or computer display won’t see the same environmental challenge, long life is still in demand by customers. Even though hand held devices don’t demand the same long life, most will need to survive their short lives in a pretty hostile environment of careless handling, exposure to moisture, humidity, and temperature — until they are replaced with a newer version. Another is - sue is weight and bulk. These very small electronic LED packages have very limited space for protective coating.

Uncoated board showing corrosion from element exposure (left). Same board configuration protected with a Parylene coating after equal element exposure (right).

For large displays, the total weight of a massive outdoor video signage can reach up to 250,000 lbs and beyond. Adding weight through protective coatings for circuit boards and LEDs is not advantageous.

Issues with Conformal Coatings

Conformal coatings such as silicones, acrylics, urethanes, and potting compounds were sufficient for older types of electronic assemblies. However, liquid coatings are often uneven and thus, may have voids. These voids are pathways for moisture to get through to circuit boards and cause performance failure of the LEDs. These failures can be expensive when considering the large number of LED boards used in many digital display and video signage.

Additionally, the heaviness of many standard coatings is a drawback as they can induce stresses to delicate components. This added weight on large signage displays can pose an engineering challenge due to the additional structural framework necessary to mount the large display to buildings or other locations. Finally, it is not uncommon for thick or uneven coatings to distort the light coming from the LEDs.

Most liquid coatings or compounds can hold up to some exposure to UV light. However, for LED applications that are exposed to UV light on a continual basis, the conformal coating material must provide the board with long-term UV stability for its expected long life.

The Parylene Alternative

altAn alternative to common industry coating materials is Parylene conformal coatings. With several variants, including a new high-temperature and UV-stable variant, Parylene offers a coating alternative that is extremely thin and perfectly conformal, and provides long-term protection for the full product life — regardless of the size or complexity of the product.

The Parylene process is carried out in a closed system under a controlled vacuum, with the deposition chamber remaining at room temperature throughout the process.

Parylene is the generic name for a unique series of polymeric organic coating materials. They are polycrystalline and linear in nature, possess useful dielectric and barrier properties per unit thickness, and are chemically inert. Parylene coatings are ultra-thin, pinhole-free, and truly conform to components due to its molecular level polymerization — basically “growing” on the deposition surface one molecule at a time.

When looking at these massive outdoor sign displays, protective coatings should be ultra-thin and lightweight so as not to add significant weight to the boards. (Photo courtesy of Bruce A. Bennett, Editor, Photonics Tech Briefs and Embedded Technology)

Parylene coatings are applied via a deposition process in which the parts to be coated are placed in the deposition chamber. The powdered raw material — known as dimer — is placed in the vaporizer at the opposite end of the deposition system. The dimer is heated, causing it to sublimate to a vapor, then heated again to break it into a monomeric vapor. This vapor is then transferred into an ambient temperature chamber where it spontaneously polymerizes onto the parts, forming the thin Parylene film. The Parylene process is carried out in a closed system under a controlled vacuum, with the deposition chamber remaining at room temperature throughout the process. No solvents, catalysts, or plasticizers are used in the coating process.

Because there is no liquid phase in this deposition process, there are no subsequent meniscus, pooling, or bridging effects as seen in the application of liquid coatings, thus dielectric properties are never compromised. The molecular “growth” of Parylene coatings also ensures not only an even, conformal coating at the thickness specified by the manufacturer, but because Parylene is formed from a gas, it also penetrates into every crevice, regardless of how seemingly inaccessible. This ensures complete encapsulation of the substrate without bridging small openings.