Which Conformal Coating Is Right for Me?

The effectiveness of polymeric conformal coatings in protecting printed circuit boards (PCBs) from damage due to environmental factors, such as dust, corrosion, moisture and temperature fluctuations, has been well-documented. While conforming to the physical configurations of the exposed face of the PCB, the conformal coating also:

• Acts as an insulator
• Secures circuitry and components against electronic shorts from contact with moisture, mist, salt spray or other contaminants
• Generates reliable mechanical protection from thermal shock or vibration

While conformal coatings are the optimal choice for preserving the functionality and performance of specialized electronics operating under extreme conditions, it’s crucial to note that not all conformal coatings are equal. Prior to selecting the most suitable coating type for the task, it is essential to:

• Selectively consider the properties of each coating material – acrylic, epoxy, Parylene, silicone and urethane
• Determine precise performance requirements
• Define the operational environment of the assembly to-be-protected

Types of Conformal Coating  

Of the many existent varieties of conformal coating, four liquid-applied materials are utilized most often. These are acrylic, epoxy, silicone and urethane. Liquid coatings rely on the following application methods:

• Brushing – applying the wet coating material to the substrate by brush
• Dipping – manually immersing the material-to-be-covered in a liquid bath of the coating substance, limited to materials that do not cure quickly by moisture, oxidation or light
• Robotic coating – automated, programmed coating application
• Spraying – hand spray application, using an aerosol can or spray booth

To be effective, application of liquid coating thoroughly covers all assembly surfaces. Flashing and curing should leave no surface defects, which commonly include:

• Air bubbles
• Cloudy/hazy surfaces
• Irregular levelling (orange peel)
• Voids/breaches/gaps in the coating where the assembly’s surface topography is not completely flat

In contrast to liquid coating methods, Parylene employs a chemical vapor deposition (CVD) process for applying thin films. Through chemical-vacuum polymerization in the vapor phase, the chemically inert powdered Parylene dimer is transformed into a gaseous form at the molecular level within a vacuum environment.

The vapor deposition process produces consistently high-quality conformal films that are pinhole-free, penetrating even the smallest surface crevices. As a durable and transparent polymerized film successful in the nanometer range, Parylene coats all regions of a component with exceptional performance ratings.

Selecting the Appropriate Conformal Coating    

Conformally coating PCBs is only a useful solution when operators recognize:

• The type of coating that most effectively provides the insulative and protective properties necessary for the particular coating assignment
• How film application process impacts its function, regarding the PCB’s expected uses
• Performance longevity requirements

Improper conformal coating will disrupt functional performance, leading to PCB assembly breakdown. Selecting the appropriate coating and application reduces the risk of failure.

Among liquid coatings, consideration of these material and performance properties is essential to appropriate selection:

• Acrylic resins are easy to apply, remove and re-work. With moderate surface elasticity and protective qualities, they also provide good abrasion resistance and high dielectric strength.
• Although epoxy conformal coatings offer dependable hydrophilic polar protection, water migration beneath epoxy films remains possible. Swelling within the coated region can result from the migration, despite the unquestionable durability and hardness of epoxy’s outer layers. Excess water permeation can stimulate film-peeling which leaves the assembly unprotected and subject to corrosion and performance degradation. Use in operating conditions characterized by the presence of water and condensation is not recommended.
• Silicone conformal coatings have high dielectric strength and low self-leveling properties, with good resistance to chemicals, salt spray and ultra-violet light. Very flexible, silicone films protect substrates through a wide temperature range, but are the least compatible with other coating types. Single-component silicone compounds are often chosen for electronics subjected to extreme fluctuations in temperature.
• Urethane resins are usually two-part compounds, categorized by superb abrasion, chemical and moisture resistance and good dielectric properties. For rework, specialized stripping solvents, such as MS-114 Conformal Coating Stripper, is required.

In contrast to liquid coatings, CVD-applied Parylene is free of air bubbles and pinholes. Parylene offers uniform film thickness, which helps assure true conformance to substrate contours. With outstanding dielectric properties, thermal expansion is minimal. The coatings resist chemicals, corrosives, moisture and solvents, protecting PCB’s function and performance through most operational conditions. Parylene can be applied in much thinner coats than liquid coating materials, making it ideal for MEMS and nano applications.

Conclusion  

Liquid coatings, acrylic, epoxy, silicone and urethane, along with vapor-deposited Parylene, are the primary conformal coating materials. Each coating type possess distinct advantages and disadvantages, dependent upon their chemical and mechanical properties, deposition method, interaction with substrate materials and re-workability.

Ease of application and initial affordability are important considerations when applying conformal coating, but not as essential as the assembly’s operational and functional requirements. Protecting the board with the appropriate material is crucial. In addition to environmental conditions, potential rework/repair needs also figure into coating selection.