From advanced manufacturing operations to intricate consumer wearable devices, electronics are the heartbeat of many industries today. To meet growing demands for more advanced features and overall smarter devices, sensors, circuit boards, flex circuits, and other electronic components must continue to expand rapidly in their capabilities even as their packaging and the systems they are part of continue to shrink in size.

Across industries, design engineers are developing innovative consumer technologies that include smart wearables, drug delivery devices, personal security cameras, cellphones, and more. As in industrial markets, these devices are required to perform reliably in their intended environments, which could mean exposure to humidity, high temperatures, ultraviolet light, fluids, and chemicals. While protection from these environments is critical, it cannot generally add thickness or weight to components.

Consumer electronics also pose an environmental challenge due to their relatively short lifespan. With the introduction of the next-generation device being only months away, there are environmental concerns regarding how products are recycled after their end of life. Thus, design engineers must not only meet a device’s protective needs, devices must also comply with increasing environmental standards.

The latest innovations in electronic components and smart sensors are also key to the success of IoT (Internet of Things) devices and technologies. Today’s IoT systems are not conventional devices that simply convert physical variables into electrical signals; rather, they are highly sophisticated systems that perform various roles across industries. These systems generally include a multitude of flow, level, proximity, and/or temperature sensors that enable their functionality.

RF (radio frequency) microelectronics are used in many modern technologies, from basic communication systems to military radar systems. Advanced RF systems need to meet size, weight, power, and cost requirements and their components must be able to survive environmental exposure in order to ensure long-term product reliability. The dielectric constant and thickness of protective coatings must be considered as the operating frequency of underlying RF devices increases.

Parylene Conformal Coatings

Parylene is the generic name for members of a unique polymer series. Parylene coatings are ultra-thin (applied as a gas), conformal, and uniform, ensuring complete coverage of sensors, circuit boards, LEDs, wafers, ferrite cores, and other electronics packages including MEMS, labs-on-chips, and electro-wetting technologies. Parylene’s outstanding penetration ability ensures total and uniform encapsulation of all components and crevices, with no meniscus, flowing, or edge effects.

Rather than being applied by dispensing, spraying, dipping, or brushing onto the device, Parylene coatings are applied via a vapor deposition process that enables the coatings to “grow” on the surface. The polymers are applied at ambient temperatures with specialized vacuum deposition equipment. Parylene deposition takes place at the molecular level, where films essentially grow a molecule at a time.

Parylene Deposition Process (Fig. 1)

  • Raw material, called dimer, is heated under vacuum and vaporized into a dimeric gas.

  • The gas is then pyrolized to cleave the dimer to its monomeric form.

  • Finally, the monomer gas deposits on all surfaces as a thin, transparent polymer film within the room-temperature deposition chamber.

Because Parylene coatings are applied as a gas, the coatings effortlessly penetrate crevices and tight areas on multilayer components, providing complete and uniform encapsulation. While Parylene coatings can range in thickness from hundreds of angstroms to several mils, a typical thickness is in the micron range.

The family of commercially available Parylenes offers engineers a variety of beneficial properties including:

  • Ultra-thin nature that allows for the conformal coating of all exposed surfaces

  • Excellent dielectric properties

  • Excellent chemical and moisture barrier properties

  • Optically clear; does not interfere with electrical, optical, or RF signals

  • Biocompatible and biostable

  • Sterilization compatible

  • Low coefficients of friction

The basic member of the series, Parylene N, is poly (para-xylylene), a completely linear, highly crystalline material. Parylene N is a primary dielectric, exhibiting a very low dissipation factor, high dielectric strength, and a low dielectric constant invariant with frequency. The crevice-penetrating ability of Parylene N is second only to that of Parylene HT.

Parylene C is produced from the same raw material (dimer) as Parylene N, modified only by the substitution of a chlorine atom for one of the aromatic hydrogens. Parylene C has a useful combination of electrical and physical properties, plus very low permeability to moisture and corrosive gases.

Parylene D is also produced from the same raw material as Parylene N, modified by the substitution of chlorine atoms for two of the aromatic hydrogens. Parylene D is similar in properties to Parylene C, with the added ability to withstand slightly higher use temperatures.

Parylene HT, which replaces the alpha hydrogen atom of the N dimer with fluorine, provides protection for electronics that operate in harsh environments. Parylene HT is useful in high-temperature applications (short term up to 450 °C) and those in which long-term UV stability is required. Parylene HT also has the lowest coefficient of friction and dielectric constant and the highest penetrating ability of the four variants.

Figure 2. Key properties of commercially available Parylenes

ParyFree is a new, unique Parylene variant that provides advanced barrier protection in a halogen-free coating. It replaces one or more hydrogen atoms of the Parylene N dimer with non-halogenated substituents. This halogen-free variant offers the advanced barrier properties of Parylene C and adds improved mechanical and electrical properties compared to other commercially available Parylenes. ParyFree optimizes the critical combination of barrier, electrical, and mechanical properties to provide robust protection against moisture, water, corrosive solvents, and gases while complying with halogen-free requirements of select industries worldwide.

A summary of the key properties of commercially available Parylenes is shown in Figure 2.

Meeting the Growing Demands of Advanced Electronics

As technologies expand and devices become smaller with greater capabilities, Parylene coatings offer exceptional properties to solve numerous protection challenges. Parylene coatings are ultra-thin, lightweight, and pinhole-free. As such, they provide outstanding barrier protection without adding significant mass or dimension to delicate components.

Figure 3. Circuit boards after 144 hours of salt-fog exposure. (Top) The ParyFree-coated board shows no sign of corrosion; Parylene C and Parylene HT-coated boards have also been tested and exhibit similar results. (Bottom) The uncoated circuit board.

Circuit boards coated with Parylene C, Parylene HT, and ParyFree have been salt-fog tested by an independent facility. The coated boards exhibited no corrosion, salt, or heavy iron oxide deposits after 144 hours of exposure in accordance with ASTM B117-(03). (Figure 3)

Parylene N, ParyFree, C, and Parylene HT have been tested to meet the requirements of IPX8, making them useful to waterproof consumer electronic applications.

As engineers consider the environmental compliance of their designs, ParyFree offers increased moisture barrier properties in a halogen-free coating.

For advanced electronics that rely on RF transmission, optically clear Parylenes have low dielectric constants and dissipation factors and high dielectric strengths, enabling electrical signal transfer without absorption or loss.

For nearly 50 years, engineers in the electronics, aerospace, defense, medical device, and transportation industries have relied on Parylene coatings to increase the reliability of their innovations. Use of the coating has continued to expand over the years into the energy and consumer electronics industries, among others. Today, Parylene coatings are applied in both small- and large-scale/mass production settings across the globe.

This article was written by Aaron Thomas, Director of Market Segments at Specialty Coating Systems (Indianapolis, IN). For more information, visit here .


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This article first appeared in the October, 2020 issue of Tech Briefs Magazine.

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