NASA Conformal Coating

NASA-STD 8739.1 is the Workmanship Standard for Polymeric Application on Electronic Assemblies, which describes NASA’s technical requirements, procedures, and documenting requirements for staking, conformal coating, bonding, and encapsulation of printed wiring boards and electronic assemblies. Included are requirements which establish the responsibility for documenting, fabrication, and inspection procedures to be used for NASA work including supplier innovations, special processes, and changes in technology. NASA-STD 8739.1 was initially released in August of 1999, with improvements and minor changes to the standard in recent years. The current up-to-date revision is NASA-STD 8739.1A with Change 2.

NASA-STD 8739.1A with Change 2 is important because it defines the processes and quality requirements needed for mission hardware and mission-critical ground support equipment. There is a certification needed for both operators and inspectors of electronic assemblies which will be used by NASA. I personally attended a five day training course and after two years, a two day retraining course at the Goddard Space Flight Center in Greenbelt, Maryland in which I was issued a diploma that certified me as a Level B instructor which allows me to train operators and inspectors employed at Diamond-MT and any sub-tier contractors of DMT.

NASA-STD 8739.1A with Change 2 is very similar to IPC J-STD-001 when it pertains to the application and inspection of conformal coating. Coating thickness tolerances and what is acceptable in regards to bubbles and scratches in the coating are nearly identical to IPC J-STD-001 with the exception that the NASA standard places strong emphasis on prerequisite vision testing which covers near vision, far vision, and a color test and on not negating the stress relief built into components. i.e.: no coating bubbles or foreign debris is allowed to bridge between the underside of a component or its leads and the circuit card surface.

Outgassing occurs when previously adsorbed or occluded gases or water vapor are released from some material. With respect to protective conformal coatings, outgassing encompasses the discharge of gases previously confined within a high-frequency printed circuit board (PCB) or similar assembly material, often resulting in functional difficulties.

For instance, outgassing can negatively impact the performance of PCBs used in medical implants or other monitoring devices, disrupting sterile and well-contained healthcare environments, challenging appropriate device performance and patient health. In aerospace systems, outgassing byproducts may condense, interfering with the function of systems’ optical elements, solar cells, thermal radiators and similar components. Industrial and scientific vacuum processes can also be impeded by outgassing.

Outgassing’s Potential Impact
Low levels of outgassing are associated with even high-quality/performance materials and systems; however, excess outgassing can stimulate dysfunction for components where reliable performance can be critical to such factors as patient health or mission/process success. Higher operating temperatures increase the incidence of outgassing, leading to the release of gases and moisture from cracks and impurities in component surfaces; defective assembly adhesives, lubricants, and sealants are also common sources of outgassing.

Also known as offgassing, the outgassing phenomenon often affects such emission related factors as:

• phase transitions of a substance into a gas as during evaporation or sublimation, as well as
• desorption, characterized by either discharged gaseous products of slow chemical reactions, or
• seepage from cracks or internal volumes.

Appropriate preproduction preparation and manufacture will substantially diminish outgassing, as will baking individual components/complete assemblies and cleaning their surfaces to eradicate volatiles. Applying conformal coating postproduction reduces outgassing tendencies during assembly performance. Two coating substances displaying low outgassing properties are parylene and arathane 5750.

Parylene: Near-zero Outgassing of Volatile Chemicals
Compared to liquid conformal coatings – acrylic, epoxy, silicone and urethane -- the prevalence of parylene outgassing is very low. Unfortunately, complications caused by outgassing can emerge for any electronic equipment intended for use in high-vacuum environments, ranging from aerospace-based equipment to medical implants, conditions where malfunctions are potentially very dangerous.

Outgassing is often a function of thermal stability. Defined as the fractional increase in the length- or volume-per-unit rise in temperature, a coating’s coefficient of thermal expansion (CTE) indicates the degree of coating expansion when heated. Lower levels demonstrate a lesser prevalence for coating expansion, and thus a commensurately lower tendency toward outgassing. Depending on the parylene type (N, C, D), parylene CTE measures between 35° C - 70° C/ppm, the lowest levels among conformal coatings under most circumstances.

Nevertheless, the frequency and intensity of parylene-covered component outgassing grows significantly at temperatures between 150° C – 220° C and 400° C – 510° C. The result can be cross-contamination of coated surfaces, inhibiting development of a direct bond with the reactive monomer, disrupting parylene performance. In these rare circumstances, substrate and assembly components under the film can outgas, impeding PCB function.

Despite these issues, parylene surpasses NASA outgassing requirements in virtually every case, with minimal generation of by-products, catalysts, fillers or hardeners during use. One of parylene’s most basic properties -- homogenous, pinhole-free covering – also supports outgassing protection. Outside of vacuum environments, prolonged exposure to direct sunlight may degrade parylene films to the degree that outgassing increases measurably.

Arathane 5750’s Outgassing Properties
A liquid urethanic elastomer previously called uralane, arathane 5750 is composed of two elements, parts 5750 A and 5750 B (LV). These components are mixed at a ratio of 9 parts A to 50 parts B (LV). When completed, arathane 5750 displays a tensile modulus that diminishes as the temperature rises. For instance, modulus levels (measured as E’MPa) are 103.5 at - 100° C; they decline markedly to 101 when temperatures are raised to 100° C.

However, arathane’s CTE remains relatively stable under the same conditions; that is, raising the temperature from - 50° C to 100° C has virtually no effect on it’s CTE, which exhibits a measured consistency in the vicinity of 200 linear CTE (25° C ppm) throughout. Nevertheless, these CTE levels significantly exceed those for the parylenes (35° C - 70° C ppm), indicating a heightened prevalence toward outgassing for arathane 5750, in comparison. Soft and flexible at normal operating temperatures, arathane can crystallize during prolonged exposure to colder temperatures, making its surface more brittle, a condition that can also increase outgassing.

Conclusion
Translucent and repairable, arathane 5750 is a liquid conformal coating applied by dip, spray, and spread methods. Created specifically as a coating for PCBs and electronic components, arathane exhibits minimal outgassing when used for high vacuum or aerospace systems.

Parylene-coated devices are pinhole-free, insulating, and chemically/electrically stable, exhibiting little or no change in response characteristics during operation. Encapsulated parylene coated devices perform equivalently to hermetically-sealed units, even under pressurized conditions, further supporting outgassing protection. While arathane 5750’s low outgassing properties are useful, they do not match those of parylene.

NASA certification is essential to aerospace conformal coating applications, and thus is sought after by many firms in the industry. In addition to 8739.1, other NASA standards, such as ASTM E595 – a test developed by NASA to determine levels of coatings’ volatile content, appropriately screening low outgassing materials for use in space – are inspection and performance parameters applicable to aerospace uses.

The AS9100/9120 and ISO9001 qualification are international aerospace standards for quality assurance, development, production, installation, and servicing also relevant to quality workmanship for aerospace products and the firms that manufacture them; equally relevant are IPC-CC-830 and IPC--A-610 classifications, from the Association Connecting Electronics Industries, often considered equivalent to NASA 8739.1 for aerospace conformal coatings. Nadcap’s AC7120 Rev A - Nadcap Audit Criteria for Circuit Card Assemblies is also applicable.

Essential aerospace applications for conformal coatings include: PCAs and related circuit card assemblies, sensors of all types, microelectricalmechanical systems (MEMS), motor components, power supplies. elastomeric components and backplanes.

NASA’s 8739 Standards describe uniform engineering and technical requirements for processes, practices, and methods that have been endorsed as standard for NASA programs and projects, including requirements for selection, application, and design criteria for conformal coating and related encapsulation techniques; these are applied primarily to the manufacture and use of PCAs and electronic assemblies used in space flight components, but have uses for MEMS, sensor and other systems as well. The AS9100 and ISO9001 Standards also provide criteria for coatings similarly essential to their reliable performance during operation.

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-STD 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-STD 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.

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 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.

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. 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.

Choosing parylene gets you half way to protecting your company's products with the best possible conformal coating. To close the circle, unless you have invested in the proper equipment and training, you also need to choose the right service to apply the coating. The Parylene coating industry, while it is a fairly niche business, is still a competitive market, but the very few top-quality providers are relatively easy to identify. Here are some of the attributes that you should actively seek:

Liquid Coating Capabilities
While it might seem counterintuitive, look for a parylene coating service that can also apply liquid coatings. This way, you can use a single vendor for more of your conformal coating projects. More importantly, you also get the benefit of having an outside party that can tell you if there is a better choice than parylene for a given project. Too often, conformal coating services will only provide either a liquid coating service or a parylene coating service, but not both. When trying to evaluate which coating makes the most sense for your project, you will benefit tremendously from having an unbiased coating expert make the right choice with you.

Convenient Location
It's unlikely that a best-in-class parylene coating company is located next door or maybe even in the same state as your production facility. As such, your priority should be to find a facility that is well connected to road and air networks. That way, you can truck or air freight products in and out with as few connections as possible.

The Right Certifications
Even if the item that you are getting coated will not be used inside the human body, in combat or in space, look for a parylene coating service that holds the certifications that will allow their work to be used in these challenging settings. As Aristotle said, "Excellence, then, is not an act, but a habit," so the companies that have the habit of doing certified work are likely to carry those processes into your work -- whether or not you need it to be at certification level. Important certifications to hold include the following:

• Quality certifications. The ISO 9001 standard was once the gold standard for an organization that has quality control processes in place and is a good minimum. Organizations that meet the more stringent AS9100 standard, though, are held to the even more extreme requirements of the aerospace industry. Their higher quality can benefit you even if your projects stay on the ground.
• NASA-STD-8739.1. Before an item can be launched into space, its coating must meet NASA's quality standards. This standard not only describes what has to be done to create a compliant coating but how to document the work, since documentation is one of the best ways to know that the coating was done right (other than waiting and seeing).
• Bio-medical certification. While the USP Class VI certification applies to coating compounds (like Parylene) and not to services, the best Parylene coating companies have other certifications that show that their materials are safe for bio-medical applications.
• IPC Standards. The IPC is the electronics industry's trade association, and they publish a range of standards that define quality standards for conformal coatings of electronic items. Their Class 3 Standard for Coating Requirements is especially important since it helps to ensure that coated items can maintain high levels of performance even in challenging situations.
• Joint Certification Program Approval and ITAR Registration. A Parylene coating service that holds Joint Certification Program Approval meets US and Canadian military standards for quality and security. ITAR registration is a sign of trustworthiness, as it means the facility is trusted by the federal government to participate in arms exporting activities.

Experienced, Skilled Workforce
Applying Parylene coatings can be a complicated process and there are a lot of items that can be parylene coated. Look for a company with at least 10 years' experience in the field and with a dedicated workforce that is fully trained and highly skilled.

Rapid Turnaround Capability
Many of parylene's benefits come from its unique vapor-based deposition method. Unfortunately, vapor deposition is relatively slow and it is not uncommon for items to spend 24 hours just in the deposition chamber. Due to this, a two week turnaround time is a good goal to shoot for if you need non-expedited service. The best services have the capacity that gives them the ability to expedite when you need it. Typically, expedited services take five business days, all the way down to 24 hour turn arounds.