Conformal coatings are used to protect printed circuit boards (PCBs) from dust, humidity/moisture, mildew/mold, temperature extremes, 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 resins – acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) -- are applied by wet methods:
• 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, parylene (XY) uses a chemical vapor deposition (CVD) application method, wherein powdered parylene 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 XY penetrates deep within substrate surfaces in a truly conformal manner, completely covering any irregular aspects of the PCB’s topography. This property provides XY a greater functional versatility in comparison to liquid coatings.
When asking what can be conformally-coated, one must consider the material type and its properties. Liquid coatings are mainly used for PCBs; 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, epoxy — 0.025 – 0.127 mm. (0.001 to 0.005 in.).
• Silicone — 0.051 – 0.203 mm. (0.002 to 0.008 in).
• Parylene — 0.013 – 0.051 mm. (0.0005 to 0.002 in.).
Acceptable performance thickness determines a coating’s uses. Appropriate film-thickness for one coating may be either too wide or narrow for specific product purposes.
Acrylic: AR is commonly used as moisture protection, conformally-coating PCBs. Easily applied, it is fungus resistant, with desirable electrical and physical properties. Low-cost acrylic typically cures in 30 minutes or less, making it an optimal choice for assignments with limited turnaround-time. As 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 2-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; solvents capable of removing the coating can also dissolve the PCB itself! 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. The need to apply silicone in generally thicker layers than other coatings restricts 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, also losing their consistency at high temperatures. Urethane is difficult to remove, requiring use of a soldering iron; repaired coatings are difficult to restore.
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 coated-surfaces are:
• exceptionally resilient, and
• dielectric, with
• superior barrier properties, that
• withstand temperature extremes,
• while preventing leakage.
The CVD process can be applied 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 wet coatings’.
Monolayer, liquid coating processes like spraying or dipping cannot accommodate MEMS/nano requirements, severely limiting their uses for these evolving technologies. In contrast, parylene coating’s 0.1-micron operating thicknesses render them exceptionally adaptable for MEMS/nano components, expanding their applications far beyond those of liquid coatings.
More costly than other coating types, parylene is not well-suited to prolonged outdoor exposure; it is difficult to remove.
When selecting a coating, the most critical consideration should its material properties and required electronics’-functionality following film-application.
While wet coating types are all easier to apply and less expensive than XY, none consistently display parylene's product/performance versatility. In comparison to liquid coatings, parylene best tolerates the rigors of specialized and frequently severe operating conditions. XY maintains optimal performance functionality through the widest range of functional environments and for the greatest number of devices, including developing MEMS/nano applications. As they currently stand, wet application conformal coatings simply lack parylene’s capacity for providing flexible, micron-thin, pinhole-free coating resiliency, uniformly covering board topography as they protect assembly functional performance.
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