A conformal coating is a thin, electrically nonconductive, and protective coating that will conform to the configuration of the board or other assembly. Conformal coatings are intended to provide electrical insulation and environmental protection, thus minimizing the performance degradation of electronic assemblies by humidity, handling, debris, and contamination. Reliable conformal coatings result from proper design, control of equipment, materials, work environments, and from careful workmanship by trained and certified personnel.
Design Considerations for Conformal Coating
Design considerations for conformal coating should include:
Have excellent dielectric properties that will meet the minimum circuit requirements in all anticipated environments
Not cause excessive stress to the PWA, its parts, or its electrical interconnections, or to the assembly during thermal or mechanical tests
Meet outgassing requirements
Have maintainability properties (repair and rework) compatible with the parts and board or other substrates
Be compatible with and adherent to all materials used in PWA's and electronic assemblies
Be hydrolytically and thermally stable as required
Meet offgassing requirements if assembly is to be placed in crew compartment
Be noncorrosive and curable under conditions compatible with the parts on the boards and assemblies, including their temperature limits
Approved Applications for Conformal Coating
Conformal coating is applied using methods designed to yield complete coverage without excessive filleting or runs. The preferred application methods are spraying and brushing.
Inspection of Conformal Coating
Coating coverage and location shall be determined by visual inspection. An ultraviolet (UV) lamp with adequate intensity shall be used to compare fluorescent areas to uncoated portions. A UV lamp shall be used to inspect the fluorescent cured urethane to ensure that all the surfaces and electrical parts are well coated. Inspection shall also be done using 4X to 10X power magnification.
Interpretation of Conformal Coating Under Ultraviolet Light:
An even blue glow indicates a uniform surface coating.
Dark spots indicate voids and dry spots.
Splotches of blue glow indicate nonuniform coating.
Small dark spots point to pinholes or debris.
Glowing rings with dark centers indicate bubbles.
Lumps are highlighted by small intensely glowing areas.
A large dark area shows absence of coating.
Large, elongated, intensely glowing areas indicate runs.
Straight dark lines indicate bristles or debris, especially in a brush coating.
Adhesion: The resin will not adhere to sharp points, sharp edges, teflon surfaces, or silicone surfaces. "Pull-Back" occurs at the edges of silicone surfaces that have squeezed out from under heat generating parts.
Operator Workmanship Inspection: The following steps are completed by the operator immediately after the conformal coating has been applied to the PWA. This inspection may be witnessed by the inspector if required.
A lighted magnifier having a 4X to 10X magnification shall be used to inspect the florescent cured urethane to ensure that all the surfaces and electrical parts are well coated. Higher magnification may be used, as necessary, to inspect suspected anomalies or defects (e.g. bubbles and bubble sizes) and contaminants.
Coating coverage shall be determined by the UV lamp. The wavelength range of the UV lamp shall be between 300 to 400 nanometers (nm), long wave (UV), and adequate in intensity to allow comparison of fluorescent areas to uncoated portions.
Thickness measurements shall be made on coupons processed at the same time and under the same conditions as the PWA.
Final Inspection: The inspector shall examine the PWA after the resin has become tack-free, for compliance to the following criteria. Both sides of a conformally coated PWA shall meet these requirements. Workmanship and adhesion requirements shall be verified by visual inspection using 4X to 10X power magnification. Higher magnification may be used, as necessary, to inspect suspected anomalies or defects (e.g., bubbles and bubble sizes) and contaminants.
Acceptance of Conformal Coating
The staking material shall adhere to the intended surfaces as per paragraph 9.2-1 of NASA-STD-8739.1.
The staking material shall be free from contamination as per paragraph 9.2-3b of NASA-STD-8739.1.
Jumper wires shall be staked a minimum of every 2.54 cm (1 inch) and every change of direction, outside of the radius of curvature as per paragraph 9.2-4 of NASA-STD-8739.1.
All axial lead solid slug tantalum capacitors shall be staked as per paragraph 9.2-4 of NASA-STD-8739.1.
The staking material shall be tack-free when cured as per paragraph 9.2-5 of NASA-STD-8739.1.
Staking material meets the hardness requirements as per paragraph 9.2-6 of NASA-STD-8739.1.
Rejection of Conformal Coating
Staking material used after shelf life expiration as per paragraph 6.7-lb of NASA-STD-8739.1.
Staking material bridges between the PWB and the bottom of the DIP’s or flatpacks as per paragraph 9.2-1 of NASA-STD-8739.1.
Staking material fills the stress relief areas as per paragraph 9.2-3a of NASA-STD-8739.1.
Staking material encloses the part lead as per paragraph 9.2-3a of NASA-STD-8739.1.
Rigid staking has been applied directly to glass bodied parts as per paragraph 9.2-3c of NASA-STD-8739.1.