Parylene Coating vs Conformal Coating

Conformal coatings consist of various polymeric materials used to protect the function and extend the life of electrical and mechanical circuitry, parts and related components. They safeguard parts and products from environmental contamination during use, insulating the substrate while doing so.

To achieve this goal, ultra-thin layers, usually just a few millimeters thick, are meticulously applied onto specific substrates. Common conformal coatings like acrylic, epoxy, silicone and urethane are typically administered through dipping, brushing or spraying methods, although some applications are now employing selective coating or robotic dispensing techniques. Notably, Parylene stands out as it is uniquely applied using a vapor deposition process.

Operational security generated by conformal coating includes protection from:

1. Contamination caused by exposure to harsh physical environments, extreme temperatures and humidity.
2. Abrasion during performance and contact with acids, solvents or, in the case of medical equipment, bodily fluids.
3. Conductor electro-migration, corrosion, dendritic growth, or short circuits to electronic assemblies and circuitry.

Additionally, coatings offer stress relief and insulation to ensure continuous functionality.

This level of protection has been adapted to a wide array of product applications, spanning aerospace and military technologies, various consumer electronics, demanding automotive electrical and mechanical uses, industrial applications, microelectromechanical systems (MEMS) and medical devices that operate inside or are attached to the human body. Without conformal coatings, many everyday products we rely on would not perform as effectively and would require more frequent replacement.

Parylene: The Distinctive Conformal Coating

Parylene offers superior coverage for substrates, providing the thinnest yet effective coating compared to other conformal coatings. By utilizing a vapor deposition process that deeply permeates the substrate surface, it ensures the highest levels of protection for various products. Parylene exhibits exceptional resilience, enduring extreme temperatures and physical stresses. It maintains remarkable uniformity, offering excellent pinhole coverage to prevent any leakage. With outstanding dielectric properties, Parylene is the preferred choice for a wide range of electrical components. However, the reliance on a vacuum application method increases its manufacturing cost compared to other coatings. Additionally, Parylene may be susceptible to contamination.

Other Conformal Coatings

Acrylic, epoxy, silicone and urethane each offer the following unique properties:

• Acrylic:  Acrylic coatings dry quickly after application using brush, dip or spray methods. They maintain their size during curing and consistently show strong resistance to fungus and humidity. Nevertheless, they tend to degrade more easily at elevated temperatures than alternative polymers and have limited abrasion and stress-relief properties. The cost benefit of acrylic coatings compared to other options has diminished in recent times.
• Epoxy: Epoxy resins offer impressive abrasion and chemical resistance, a stark contrast to acrylics. Their application process mirrors that of acrylics and demonstrates commendable humidity resilience. However, the prevalent issue of film shrinkage during polymerization somewhat diminishes the efficacy of their exceptionally robust coatings. Furthermore, exposure to extreme temperatures significantly compromises their stress resistance capabilities.
• Silicone: Silicone offers impressive thermal resilience, withstanding temperatures ranging from -55°C to +200°C. It features low dissipation factor, excellent PCB adhesion, and high resistance to heat, humidity, moisture and UV light. The versatile silicone conformal coating can be tailored to meet specific product requirements, offering options from elastomeric, stress-relieving coatings to highly durable, abrasion-resistant surfaces. Silicone coatings have low toxicity, are easily repairable and cater to a wide range of needs.
• Urethane: These coatings boast excellent dielectric properties, strong chemical resistance, low moisture permeability and good temperature flexibility at lower temperatures. While they are durable and exhibit reliable solvent resistance, their bond strength is constrained. Urethane coatings covering extensive areas tend to flake and peel, significantly compromising their excellent abrasion resistance capability. Furthermore, their high-temperature resistance and repairability are limited.

Conclusion

Conformal coatings, including Parylene, acrylic, epoxy, silicone and urethane, are crucial for safeguarding products from environmental factors, ensuring longevity and reliability. Different coatings provide unique properties catering to various industry needs in aerospace, electronics, automotive, and medical devices. Understanding these coatings’ characteristics is vital for enhancing product performance and durability in diverse environments.

For example, NASA Standard 8739.1a specifies precise coating thicknesses for various materials in space flight systems to ensure circuit and component operation in harsh conditions. When compared to other coatings, Parylene’s thinner layers offer similar or superior protection, meeting these stringent criteria effectively.

• Parylene: 0.013 – 0.051 mm (0.0005 to 0.002 in)
• Silicone: 0.051 – 0.203 mm (0.002 to 0.008 in)
• Acrylic, Urethane, Epoxy: 0.025 – 0.127 mm (0.001 to 0.005 in)

Parylene presents consistent advantages in coating thickness and temperature for various conformal coating projects. While other types of conformal coatings are more cost-effective and easier to apply than Parylene, they do not match its versatility. Parylene excels in a wide array of applications, from aerospace and military to medical, consumer goods and automotive industries, as well as in MEMS and other advanced technologies. Among available conformal coatings, Parylene stands out for its ability to withstand specialized and harsh environments while ensuring optimal functionality to the highest degree of reliability.