Used for aerospace. automotive, commercial, defense, industrial and medical applications, conformal coatings are applied in film layers generally 30-130 microns (micrometers/μm) thick, or 0.0012-0.0051 inches (“). Conformal films’ exceptional thinness is their greatest asset. Coatings safeguard printed circuit boards (PCBs) and similar electronics from performance malfunction generated by unwanted contact with:
- corrosive liquids,
- physical shock,
- temperature extremes, and
Non-conductive and dielectric, polymeric conformal films are applied through liquid or gaseous methods, precisely fitting substrate contours with protective coating. This ‘conformance’ to surface topographies is their essential purpose and strength, although each coating material – acrylic, urethane, silicone and parylene -- has unique properties determining its use for specific coating projects.
The most commonly-used conformal compound, acrylic resin (AR) provides PCB-moisture protection. Inexpensive and simple to apply, acrylic displays high fluorescence-level and low glass-transition temperatures, with excellent humidity resistance and reliable dielectric/insulative properties; it dries in about one-half hour. One‐part systems, AR-coatings are easily reworked/removed. Liquid acrylic meets these important professional performance standards:
- MIL-I-46058C, application of insulated coatings for PCBs,
- IPC-610, appropriate coating-thicknesses for conformal materials, and
- 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. Coating-thickness should precisely match the assignment’s spec-sheet, assuring the film achieves optimal protective/insulative function; inaccurate thickness causes lackluster coating performance, including moisture-ingress on the PCB. Optimal AR performance is achieved at coating-thicknesses between .001”-.003”.
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 display further benefits recommending them for a wide range of less complex conformal coating assignments:
- easy UV-inspection,
- low-glass-transition temperatures,
- minimal shrinkage during operation,
- post-application flexibility, and
- resistance to most static/voltage discharges.
Applied at 0.001” - .005” thickness via liquid methods, very-hard urethane (UR) provides dependable tin whisker resolution. UR generates coating sufficiently thick and strong to protect assembly’s tin surfaces, preventing development of additional short-circuit threats. Exceptionally resistant to abrasion and other forms of mechanical corrosion/wear, UR’s coating strength inhibits penetration from external elements, a major cause of assembly failure.
With good moisture/humidity/chemical resistance and dependable dielectric properties, urethane operates effectively through persistent exposure to harsh chemicals, offering exceptional mechanical wear. Most UR-varieties support reliable inspection, without fluorescent or free-isocyanate content.
Complete UR-cure requires at least several hours, and may need 30 days at room temperatures. UR’s exceptional durability/solvent resistance makes it difficult to remove or rework; introduction of mechanical reworking techniques increases production cost/downtime.
In addition, high levels of operational heat/vibration can distress UR-films; most urethanes lose coating-effectiveness at temperatures more than 125°C. Prone to cracking during prolonged thermal exposure, urethane can fail in high-vibration/high-heat environments. Also, outgassing oil-modified or alkyd chemistries disrupt coatings’ long-term performance, limiting coating applications.
Because of their material properties, typical silicone (SR) conformal coatings require thicker liquid-application than other wet films – between 0.0197”-0.0827”. SR offers excellent moisture protection, with continuous operating temperature range, extending from -40ºC to 200ºC (-40ºF to 392ºF); some silicones are rated as high as 600ºC (1112 ºF) for exceptional temperature applications. These properties combine usefully for applications (aerospace, artic, under-sea) where extreme temperature differences produce excessive moisture. Other liquid coating materials fail under these conditions, often within hours or days.
Also oleophobic, SR is inert biologically and chemically. Thick, rubbery coats 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, and
· low surface energy for better wetting.
Silicones have weak resistance to solvents, important for assemblies requiring work after coating -- reducing labor-time without compromising the conformal film’s functional integrity.
These properties enhance SR’s utility for coating assignments untenable for other liquid coatings, but interfere with silicone’s bonding-ability, leading to delamination. Rework of chemically-resistant silicone can necessitate mechanical processing. SR’s thick, rubbery films are unsuitable for tight clearance tolerances or solder joints unable to support stresses generated by the denser SR film-layer.
Unlike liquid coatings, parylene’s (XY) unique chemical vapor deposition (CVD) application method deposits gaseous parylene deep within substrate surfaces on a molecule-by-molecule basis, generating superior dielectric, non-conductive, insulation. They require no curing. Truly conformal parylene coatings offer:
- chemical inertness,
- excellent uniformity regardless of assembly topography,
- low dielectric constant,
- minimal added mass/outgassing, and
- safe environmental-impact processing
Coatings are flexible, and of uniform controllable thickness. Pinhole-free at thicknesses greater than 0.5µ., and completely penetrating spaces narrow as 0.01mm, XY-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.
Successful in the nanometer range, XY-coatings resist chemicals, corrosives, moisture and solvents; thermal expansion is minimal, ensuring PCB function/performance through most operational conditions.
Specialized equipment and materials are required for CVD, causing higher cost and slower batch production. Parylene removal requires abrasion-processing, which can damage board surfaces. XY-coatings are not recommended for longer-term, outdoor exposure.
Each coating material possesses specialized advantages/disadvantages that impact their use. Choosing the appropriate coating type/application method significantly reduces risk of failure. Liquid coatings like acrylic, silicone and urethane can entirely cover a PCB, but may leave air bubbles or irregular surface-levelling (orange-peel) during curing; PCB surfaces with irregular topography particularly suffer from gaps in the coating, regardless of the liquid material used. CVD-processing eliminates these problems for parylene, but processing is slower and more expensive. Coating strategy must incorporate these factors for successful project implementation.
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