Meeting NASA Low Outgassing Requirements in EMI Shielding Applications
Design factors for satellite, high altitude, and space-based applications vary dramatically from those of land or ship-based programs. These factors include: low payload capacity, low operating temperature, and meeting material limits.
Outgassing standards, established by NASA, set a limit on the release of gasses that can possibly interfere with sensitive technology within vacuum environments such as Low Earth Orbit (LEO). This is especially important for sensitive optical systems or camera lenses where the smallest bit of vapor or gaseous components can dramatically reduce performance.
Test and metrics
NASA outgassing requirements are often used interchangeably with ASTM E595 which establishes the test method for determining outgassing levels. During the E595 test, small samples of material are kept under vacuum and heated to 125°C for a 24-hour period. While the samples are heated, all gasses are channeled through a single release port where a chromium-plated disk is used to collect the volatile materials.
After the test, there are two key metrics that are collected and used in certifying a material to NASA Outgassing Standards.
While NASA does keep an extensive database of all materials that they have tested in house, certified labs often run ASTM E595 testing as well. For a complete list of Parker Chomerics products that pass traditional outgassing requirements (and associated NASA Data Reference Numbers), please see: Parker Chomerics NASA Outgassing Information.
Outgassing in EMI shielding solutions
The most common materials to release stored vapors and gasses are sealants, adhesives, and less heavily crosslinked elastomers and polymers. Conversely, metals and glasses with few impurities tend to have a very low level of outgassing.
While often true, many conductive elastomers made of silicone and fluorosilicone can meet these standards due to high quality raw materials and efficient processing.
General trends for outgassing in EMI Shielding:
- Many conductive elastomers, with a variety of conductive fillers, such as: silver, silver-plated copper, silver-plated aluminum, and nickel-plated aluminum pass NASA outgassing requirements (Parker Chomerics Conductive Elastomer Handbook for more information)
- Nearly all metal mesh EMI gaskets including those made of Monel®, aluminum, and Ferrex pass outgassing
- Copper foil tapes with conductive and non-conductive adhesive meet NASA requirements
- EMI shielding heat shrinkable tubing meets outgassing levels
Steps can be taken to reduce the amount of vapor or gasses that are released by materials. One such example is known as post-baking, sometimes referred to bake-off or bake-out. This process involves baking materials at elevated temperatures (and sometimes in vacuum environments) after they have been manufactured in order to lower vapor and/or volatile compounds.
It is important to note that some materials that pass NASA outgassing standards are only able to do so after post-baking for some amount of time. It is possible that some NASA post-baking occurred at temperatures above the maximum recommended operating temperature of these materials. This elevated temperature exposure can change the physical, thermal, or electrical properties of tested materials.
Parker Chomerics has a long history of supplying manufacturers with outgassing-compliant solutions to EMI Shielding problems for vacuum and space-based. For a complete list of Parker Chomerics products that pass NASA outgassing requirements (and associated NASA Data Reference Numbers) and more information, please see: Parker Chomerics NASA Outgassing Information.
This blog contributed by Ben Nudelman, market development engineer, Chomerics Division.