Conformal Coating Types: Acrylic, Urethane, Silicone and Parylene

Conformal coatings, utilized in aerospace, automotive, commercial, defense, industrial, and medical sectors, are applied as thin film layers typically ranging from 5 to 130 microns or 0.0002 to 0.0051 inches. Their remarkable thinness is a key attribute. These coatings protect printed circuit boards (PCBs) and other electronics from operational issues caused by unintended contact with:

• Chemicals
• Corrosive liquids
• Current-leakage
• Dirt/dust
• Moisture
• Physical shock
• Temperature extremes
• Vibration

Non-conductive and dielectric, polymeric conformal films are applied through liquid or gaseous methods, precisely fitting substrate contours with protective coating. Conformance to surface topographies core objective and strength of these coatings. However, each conformal coating material type – acrylic, urethane, silicone and Parylene – has unique properties that dictate its use for specific coating projects.

Comparing Conformal Coatings: Choosing the Right Material Type for Your Needs

Acrylic
Acrylic resin (AR) is the go-to conformal compound for safeguarding PCBs against moisture. Known for its affordability and easy application, AR boasts high fluorescence and low glass-transition temperatures, along with outstanding humidity resistance and reliable dielectric properties. It dries swiftly in just thirty minutes, making it a convenient choice. AR-coatings, as one-part systems, can be easily reworked or removed. Liquid acrylic meets the following requirements:

• MIL-I-46058C – application of insulated coatings for PCBs
• IPC-610 – appropriate coating-thicknesses for conformal materials
• UL 746C – use/performance criteria/quality-control for electrical equipment polymers

AR is easily applied by standard wet methods — brush, dip, spray or robotic. The ease of these processes can be deceptive, leading to uneven coating if appropriate control is not exercised, especially with brush-coating. Ensuring the coating thickness aligns precisely with specifications is crucial to optimize the film’s protective and insulating functions. Any deviations in thickness can lead to subpar coating performance, potentially resulting in issues like moisture ingress on the PCB. Optimal AR performance is achieved at coating-thicknesses between 0.001in – 0.003 in.

Easy application, cleaning, rework and removal generate low production costs. Lower resistance to abrasives/chemicals/solvents limits uses, disadvantages balanced by good acid/base protection, reliable surface elasticity and overall component protection. AR coatings offer additional advantages, making them ideal for a variety of simpler conformal coating applications, including:

• Easy UV-inspection
• Low-glass-transition temperatures
• Minimal shrinkage during operation
• Post-application flexibility
• Resistance to most static/voltage discharges

Urethane
Applied through liquid methods at a thickness ranging from 0.001 to 0.005 inches, very-hard urethane (UR) offers reliable tin whisker mitigation. UR forms a robust coating that effectively shields the tin surfaces of assemblies, preventing the emergence of potential short-circuit hazards. With exceptional resistance to abrasion and various forms of mechanical wear, UR’s protective strength prevents the infiltration of external elements, a common culprit behind assembly failures.

Urethane offers good moisture, humidity and chemical resistance, along with dependable dielectric properties, enabling effective operation even under harsh chemical exposure and providing exceptional mechanical wear. Most UR coatings allow for reliable inspection without fluorescent or free-isocyanate content. However, complete UR curing demands several hours, potentially extending to 30 days at room temperature. Its outstanding durability and solvent resistance make urethane challenging to remove or rework, with the introduction of mechanical reworking techniques leading to increased production costs and downtime.

Moreover, the effectiveness of most urethanes as coatings diminishes at temperatures exceeding 125°C. Susceptible to cracking under extended exposure to heat, urethane may falter in environments with high levels of vibration and heat. Also, outgassing oil-modified or alkyd chemistries disrupt coatings’ long-term performance, limiting coating applications.

Silicone
Due to their material properties, conventional silicone (SR) conformal coatings necessitate a thicker liquid application compared to other wet films, ranging from 0.0197 to 0.0827 inches. SR offers excellent moisture protection, with continuous operating temperature range, extending from -40ºC to 200ºC; some silicones are rated as high as 600ºC for exceptional temperature applications. These properties are particularly valuable for applications where significant temperature variations lead to excessive moisture. Unlike other liquid coating materials that deteriorate swiftly in such conditions, these properties endure effectively over extended periods.

Also oleophobic, SR is inert biologically and chemically. Thick, rubbery layers are easily and smoothly applied; silicone cures quickly, in about one hour at room temperature. Other useful SR properties include:

• Flexibility, providing dampening/impact protection,
• High dielectric strength
• Low surface energy for better wetting

Silicones exhibit low resistance to solvents, a key factor for assemblies needing post-coating work. This reduces labor time while maintaining the functional integrity of the conformal film.

The unique properties of SR enhance its effectiveness for coating tasks that are challenging for traditional liquid coatings. However, these properties can hinder the bonding ability of silicone, resulting in delamination. Repairing chemically-resistant silicone may require mechanical methods. The thick, rubbery films of SR are not suitable for precise clearance tolerances or delicate solder joints that cannot withstand the stresses caused by the denser SR film layer.

Parylene
Unlike liquid coatings, Parylene’s chemical vapor deposition (CVD) method applies gaseous Parylene deep within substrate surfaces molecule by molecule. The coating offers superior dielectric, non-conductive insulation without the need for curing. Truly conformal, Parylene coatings provide:

• Chemical inertness
• Excellent uniformity regardless of assembly topography
• Low dielectric constant,
• Minimal added mass/outgassing
• Environmentally responsible processing

Coatings are flexible, and of uniform controllable thickness. Pinhole-free at thicknesses greater than 0.5 microns, and completely penetrating spaces narrow as 0.01 mm, Parylene coating remains adherent and intact, preserving dielectric/insulation properties. Unlike liquid coatings under severe temperatures, it won’t decompose at upper-range temperatures or become brittle.

Parylene coating is pinhole-free at thicknesses exceeding 0.5 microns and seamlessly penetrates spaces as narrow as 0.01 mm, ensuring adhesion and integrity while maintaining dielectric and insulation properties. Unlike liquid coatings vulnerable to extreme temperatures, Parylene resists decomposition at high temperatures and does not become brittle.

In the nanometer range, Parylene coatings resist chemicals, corrosives, moisture, and solvents. Thermal expansion is minimal, ensuring PCB function and performance in various operational conditions.

Specialized equipment and materials are necessary for CVD, leading to increased costs and slower batch production. The process of removing Parylene involves abrasion-processing, posing a risk of damaging board surfaces.

Conclusion

Each coating material has distinct advantages and disadvantages that influence its usability. Selecting the appropriate conformal coating material type and application technique can significantly mitigate the risk of failure. While liquid coatings such as acrylic, silicone, and urethane can cover a PCB entirely, they may result in air bubbles or uneven surface leveling (resembling an orange peel) during the curing process. PCBs with uneven surfaces are especially prone to coating gaps, regardless of the liquid material utilized. Although Parylene’s CVD process overcomes these issues, it is slower and more costly. A comprehensive coating strategy should consider these factors to ensure the successful execution of a project.