What can be Coated: A Comparison of Liquid and Parylene Coatings

Conformal coatings are used to protect printed circuit boards (PCBs) from dust, humidity/moisture, mildew/mold, temperature cycling and other elements whose prolonged contact might interfere with assembly function. Coatings also enhance electrical clearance-tolerance, while safeguarding PCB components from contamination (particulate or otherwise), corrosive materials, and mechanical stress.

The liquid coating materials – acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – are applied by brushing the coating material onto the designated substrate surface by hand, dipping (immersing) the PCB in a bath of coating materials, or spraying the coating onto the substrate by manual/automated means.

Liquid coatings conform to external PCB formations, generating increased dielectric resistance, insulation and operational integrity.

In contrast to liquids, Parylene (XY) uses a chemical vapor deposition (CVD) application method, wherein powdered Parylene raw material, called dimer, is transformed into a gas and deposited onto the targeted surface as a vapor. Parylene coats to a greater degree than the external, surface-covering provided by liquid materials; vapor deposited Parylene penetrates deep within substrate surfaces in a truly conformal manner, completely covering any irregular aspects of the PCB’s topography. This property provides Parylene a greater functional versatility in comparison to liquid coatings.

Coating Thickness

When asking what can be conformally coated, one must consider the material type and its properties. Liquid coatings are mainly used for PCBs, whereas Parylene is adaptable to a wider range of applications. Each conformal coating material exhibits a range of unique performance properties that determine its product-uses. Relevant factors include the required coating thickness necessary to assure reliable performance, which varies by coating type:

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

Liquid Coatings

Acrylic: AR is commonly used as moisture protection for PCBs. Easily applied, it is fungus resistant, with desirable electrical and physical properties. Acrylic typically cures in 30 minutes or less, making it an optimal choice for assignments with limited turnaround time. Easy to remove as apply, AR coatings offer quick repair. However, the low solvent resistance that enhances repair combines with low abrasion resistance to restrict AR’s use in harsh operating environments. A limited temperature range further interferes with larger-scale functionality.

Epoxy: Rugged epoxy coatings generally exist as two-component compounds, with dependable resistance to abrasion, humidity, moisture and solvents. ER provides ongoing performance in harsh environments. However, these coatings may shrink during their long curing process, lessening their coverage area. ER is also exceptionally difficult to remove and solvents capable of removing the coating can also dissolve the PCB. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a hot knife or soldering iron.

Silicone: With temperature resistance to 200°C, SR provides reliable high-temperature service for assemblies containing higher heat-dissipating components or have exposure to high temperature requirements. In addition to good thermal endurance, silicone adheres better to PCB components than other wet coatings and has high corrosion/humidity resistance, enhancing its use for PCBs.

Nevertheless, low cohesive strength renders SR coatings susceptible to abrasion. Removal requires use of strong solvents; only localized repairs are recommended. Generally, silicone needs to be applied in thicker layers than other coatings, restricting its use for microelectromechanical systems (MEMS)/nano applications or components that cannot handle higher stress loads.

Urethane: UR coatings can be either single or double-part substances, or take the form of UV-curable, and water-borne systems. All offer outstanding dielectric properties and reliable, difficult-to-penetrate chemical/humidity/mechanical resistance, for extended periods. They generally require longer curing and adhere less well than other coatings. Bond strength is limited; UR coatings covering larger areas can flake and peel, as well as lose their consistency at high temperatures. Urethane is difficult to remove, requiring use of a soldering iron, and repaired coatings are difficult to restore.

Parylene

Parylene: In comparison to liquid coatings, chemically inert type XY’s unique deposition process generates the thinnest effective coating application available. With excellent substrate coverage, Parylene coatings are:

• Truly conformal and pinhole-free
• Superior barriers, including moisture, gas and dielectric
• Able to withstand extreme temperature cycles

The CVD process applies Parylene as a gas uniformly to virtually any surface and shape, including ceramics, ferrite, glass, metal, paper, plastics, resin, and silicon, far exceeding the capacities of liquid coatings. Newly developed MEMS/nano devices/structures have operating components in the nanometer-range, integrating their functions onto a single micro/nano-chip. Parylene’s adaptability for MEMS/nano technologies considerably outstrips liquid coatings.

Parylene coating’s exceptional properties and ultra-thin nature render them exceptionally adaptable for MEMS/nano components, expanding their applications far beyond those of liquid coatings.

Conclusion

When selecting a coating, the most critical consideration should be its material properties and required functionality application.

While liquid coating types are easier to apply than Parylene, none consistently display Parylene’s product and performance versatility. In comparison to liquid coatings, Parylene best tolerates the rigors of specialized and frequently severe operating conditions. Parylene maintains optimal performance functionality through the widest range of functional environments and for the greatest number of devices, including MEMS/nano applications. As they currently stand, liquid conformal coatings simply lack Parylene’s capacity for providing flexible, micron-thin, pinhole-free coating resiliency, which uniformly covers circuit board topography while protecting assembly functional performance.