The value of polymeric conformal coatings for protecting printed circuit boards (PCBs) from functional retardants like dust, corrosion, moisture, and temperature fluctuations has been well-documented. Conforming to the physical configurations of the exposed face of the PCB, conformal coating:
- acts as an insulator, securing circuitry and components against
- electronic shorts from contact with moisture, mist, salt spray or other contaminants,
- while generating reliable mechanical protection from thermal shock/vibration.
Representing the best choice for safeguarding the function/performance of specialized electronics that must work under extreme operating conditions, not all conformal coatings are the same. It is important to:
- selectively consider the properties of each coating material – acrylic, epoxy, parylene, silicone and urethane –
- as well as the precise performance requirements and
- operational environment of the assembly to-be-protected,
- before choosing the coating type best for the job.
Types of Conformal Coating
Of the many existent varieties of conformal coating, four liquid-applied materials predominate. 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, and
- 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), or
- voids/breaches/gaps in the coating where the assembly’s surface topography is not completely flat.
Contrasted to liquid coating, parylene uses a chemical vapor deposition (CVD) process for film application. Vapor-phase, chemical-vacuum polymerization converts chemically inert, powdered parylene dimer into a gaseous form at the molecular level in a vacuum. CVD produces consistently high-quality conformal films that are pinhole-free, penetrating even the smallest surface crevices, tightening all areas on multi-layer elements with uniform coating, on a molecule-by-molecule basis. A durable and transparent polymerized film successful in the nanometer range, parylene coats all regions of a component, with exceptional performance ratings compared to liquid coatings.
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/protective properties necessary for the particular coating assignment,
- how film application process impacts its function, regarding the PCB’s expected uses, and
- 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/performance properties is essential to appropriate selection:
- Acrylic resins offer conformal coatings 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, despite the unquestionable durability/hardness of epoxy’s outer layers. Excess water permeation can stimulate film-peeling, leaving the assembly unprotected, subject to corrosion and performance degradation. Use in operating conditions characterized by the presence of water and mist is not recommended.
- Silicone conformal coatings have high dielectric strength, and low self-leveling properties, with good resistance to chemicals/salt spray/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/moisture resistance and good dielectric properties. For rework, specialized stripping solvents like MS-114 Conformal Coating Stripper is required.
In contrast to liquid coatings, CVD-applied parylene is free of air bubbles and pinholes and uniform film thickness 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. Even with these strengths, parylene does have its disadvantages. Parylene can be applied in much thinner coats than liquid coating materials, making it ideal for MEMS/nano applications.
Liquid coatings -- acrylic, epoxy, silicone and urethane -- and vapor-deposited parylene are the primary conformal coating materials. Each coating type possess distinct advantages and disadvantages, dependent upon their chemical/mechanical properties, deposition method, interaction with substrate materials, and re-workability.
Cost of applying coating also figures into coating selection. However, application costs alone can be deceptive. For instance, parylene, often the most expensive to apply, can also provide PCBs the most reliable, longest-lasting conformal protection, actually diminishing operating costs in the longer-term. These factors must be analyzed when deciding what conformal coating is right for your purposes.
Ease of application and initial affordability are important considerations, 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.
It may be advisable to seek professional assistance to develop solutions that coincide with your firm’s project, budget, and timeline.
To learn more about selecting the appropriate conformal coatings for your products, download our Basics of Conformal Coatings whitepaper: