The Workmanship Standards developed by the National Aeronautics and Space Agency (NASA) are essential to assuring reliable performance of the aeronautic, defense and space equipment it uses and monitors.
Conformal coatings have many applications for these purposes, particularly to provide protection for PCAs commonly found in computers or specialized electronic equipment which control their operations. Consisting of microchips and similar electronic components mounted on assembly panels, PCAs need conformal protection to generate insulation and environmental protection; the objective is to minimize degradation of their critical performance factors, maintaining long-term functionality. Major conformal coating materials are acrylic, epoxy, silicone, urethane and parylene.
Origin and Purpose of NASA 8739.1
NASA-STD 8739.1 is the Workmanship Standard for Polymeric Application on Electrical Assemblies, covering conformal coatings for PCAs used for defense and aerospace purposes. Originally released in August of 1999, NASA 8739.1 provides manufacture and performance standards for conformal coatings used in products that must function optimally under continual high-stress conditions, potentially hazardous to human life and mission success. Commitment to using appropriately designated and inspected designs, materials, processes, and personnel assures quality performance, streamlines failure-cause analyses and stimulates ongoing product/process evolution. Among conformally-coated products subjected to NASA inspection criteria during manufacture and use are:
- assemblies and devices used for the aerospace industry, subjected to frequently exceptional atmospheric pressures and temperature conditions during aircraft/spacecraft flight, and
- embedded systems for aeronautic military/battlefield uses requiring dependable functionality through extreme conditions for an often extended duration.
Because these are operating environments where excessive moisture or dryness, extreme temperatures, high levels of vibration, wind, or lack of atmosphere are the rule, NASA standards for conformal coatings are designed to provide suitable quality assurance, with guidelines that:
- minimize product/process defects,
- demand best-practice product design and manufacture,
- designate standards for progressive inspection procedures featuring detailed and specific acceptance/rejection standards to
- stimulate process/product improvement on an ongoing basis, and
- document necessary changes in product design and performance.
NASA 8739.1: Standards of Performance
NASA’s numerous outer space exploration projects include
- lunar spacecraft landings, as well as those on Mars, and Saturn,
- manned and unmanned exploration,
- such as the International Space Station, SkyLab, and the Space Shuttle program.
These aerospace missions require functional solutions far exceeding those acceptable for terrestrial use. Communications between earth command and spacecraft, radar/detection equipment, satellite electronics, and a variety of specialized treatments for interstellar functionality stipulate reliable and exceptional performance. Assuring conformal coatings provide the expected protection of assemblies and components is essential to safe project implementation, maintenance and mission completion.
Comprehensive inspection for meticulous component functionality is fundamental to assuring conformally-coated equipment is ready for use for NASA aerospace systems. Particularly important are mission-hardware and related mission-critical ground support technology.
Conformal Protection for NASA Systems
The basic conformal coating materials are acrylic, epoxy, silicone, urethane and parylene. Of these types, the first four are applied by liquid methods – brushing, dipping or spraying the material onto the substrate. Only parylene employs a chemical vapor deposition (CVD) process, wherein the gaseous parylene penetrates deep into the substrate surface in a vaporous form, rather than simply attaching to the surface, as with liquid methods. Because of the different compositional result, liquid methods generally require thicker coating films than parylene.
Whatever application process is used, each conformal coating material requires a specific thickness to function according to standard. As stipulated by NASA-STD 8739.1, these measures of conformal film are mandated for covering the designated circuit or component, (quantified in millimeters [inches]):
- Parylene -- 0.013 – 0.051 [0.0005 to 0.002].
- Silicone -- 0.051 – 0.203 [0.002 to 0.008].
- Acrylic, urethane, epoxy -- 0.025 – 0.127 [0.001 to 0.005].
These levels of coating assure reliable, safe performance of circuits and components under often-extreme conditions.
Like all NASA Workmanship Standards, 8739.1 is revised and updated as necessary, to reflect the evolution of aerospace systems and the requirements of conformal coatings protecting their PCAs and related components. The most recent revision is designated NASA-STD 8739.1A with Change 2, approved 2008; it stipulates that, prior to application, PCAs to be coated must be:
- Cleansed and demoisturized no more than 8 hours prior to application of the designated conformal coating.
- Oven-bake or vacuum-bake processes, at the prescribed temperature and time duration, as specified by the assembly's engineering document, are mandated for demoisturizing procedures under 8739.1A.
These processes assure the component will be sufficiently dry, to safely accept application of conformal coating. Further revisions of 8739.1 are developed as necessary to reflect evolution of industry requirements.
Additional Inspection Criteria for Conformal Coatings
Visual inspection of coating coverage employs an ultraviolet (UV) lamp sufficiently equipped to effectively compare fluorescent areas to uncoated portions. The objective is to determine if all the surfaces and electrical parts are adequately and conformally coated. Several factors generate reliable interpretation of conformal coating efficiency under UV light:
- Uniform surface coating is indicated by an even blue glow; nonuniform covering appears as blemishes and discolorations within the blue glow.
- Glowing rings with dark centers indicate bubbles.
- Voids and dry spots are identified by dark spots along the observed surface under UV light; minute dark spots signify debris or surface pinholes.
- A large dark area shows absence of coating.
- Intensely glowing areas denote bulges in the surface.
- Larger, elongated surface sections exhibiting intense glowing indicate runs within the coating.
- Straight dark lines indicate bristles or debris, especially in a brush coating.
- If necessary, higher magnification is suggested to identify and inspect bubbles or surface contaminants.
Inspection of operator workmanship includes use of the proper tools and techniques (UV light, appropriate instrument calibration, etc.). In addition, proper environmental conditions, such as facility cleanliness, product/process handling, and proper material storage/shelf life.
With the rise of digital technology, conformal coatings are currently being applied to a widely evolving range of advanced PCAs. NASA’s Workmanship Standards manage the design and production of equipment and technology intended for space flight and exploration. The Standards designate each component’s technical, procedural and documentation requirements, to provide complete and dependable production and performance guidelines. Although NASA 8739.1 is effective, inspection criteria for conformal coatings will require monitoring and improvement as these many uses proliferate.
For instance, liquid coating-materials use spraying, brushing, or dipping methods, alone or in a combination appropriate to assure dependable film adherence to the designated aerospace substrate. Even when applied in multiple layers, sometimes using overlapping techniques – for instance, spraying the coating onto the substrate following dipping processes – crevices, edges and points on the substrate surface may remain inappropriately coated, jeopardizing the component’s function (and potentially mission success).
In contrast, parylene’s CVD procedures are more expensive, but encompass surface inconsistencies far more comprehensively, resulting in a pinhole- and bubble-free film, completely covering crevices, without surface inconsistencies. Unlike liquid coatings, parylene is less susceptible to excessive filtering and material-runs (dripping or oozing). It also has a far lower incidence of surface scratches or other imperfections in the coating surface; these are acceptable ONLY when they do not expose a component’s conductive areas. Parylene’s properties are generally more amenable to NASA-STD 8739.1A requirements, suggesting its advantages in comparison to liquid conformal coatings, in most cases.
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