NASA Inspection Criteria for Conformal Coating

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 printed circuit assemblies (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. Conformal coating materials used to accomplish this include 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. Conformally-coated products subjected to NASA inspection criteria during manufacture include:

• Assemblies and devices used for the aerospace industry, exposed to frequently exceptional atmospheric pressures and temperature conditions during aircraft/spacecraft flight
• 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 and 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

Conformal coating materials used to protect NASA systems include 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 gaseous Parylene molecules 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:

• Parylene: 0.013 mm – 0.051 mm (0.0005 in – 0.002 in)
• Silicone: 0.051 mm – 0.203 mm (0.002 in – 0.008)
• Acrylic, Urethane, Epoxy: 0.025 mm – 0.127 mm (0.001 in – 0.005 in)

These levels of coating assure reliable, safe performance of circuits and components under often-extreme conditions.

Change 2

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 in 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 and undergo oven-bake or vacuum-bake processes at the prescribed temperature and time duration, as specified by the assembly’s engineering document, for demoisturizing procedures per 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.

Summary

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.