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Low Outgassing Conformal Coatings
Hermeticity and Outgassing
In an ideal hermetic system that is under vacuum and completely sealed, unless gas is injected, entry of gasses from the surrounding environment is restricted. However, in reality hermetic systems can still have some leakage through their sealings and via outgassing of internal components. Outgassing is the desorption of gases/vapors from the surfaces of components in the vacuum system. The outgassing rate is defined as the amount of gas desorbing from a unit area of surface in unit time.
Once a surface is exposed to an environment other than vacuum it absorbs gases and vapors to decrease the number of dangling bonds or surface energy. Therefore, all materials outgas under vacuum conditions. Especially, porous materials and high surface energy surfaces absorb the most thus outgas the most. The most common gases/vapors detected are H2O, C, carbon-based molecules and gases that are present in the preparation of the surfaces such as Ar, N.
In applications, where low amount of outgassing and low outgassing rate is required the question of material selection becomes of importance for a long-lasting service life of the product.
ASTM E595 Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment[1].
According to ASTM 595 acceptance and rejection of materials shall be determined by the user and based upon specific component and system requirements. Historically, TML of 1.00 % and CVCM of 0.10 % have been used as screening levels for rejection of spacecraft materials.
ASTM E1559 Standard Test Method for Contamination Outgassing Characteristics of Spacecraft Materials
ASTM E1559 Standard Test Method for Contamination Outgassing Characteristics of Spacecraft Materials covers a technique for generating data to characterize the kinetics of the release of outgassing products from materials deal with the outgassing of materials.
According to ASTM 1559 the presence and orientation of reinforcements affects diffusion rates. Outgassing from resin-rich surfaces predominates during the initial period [2]. Therefore, resin based conformal coatings are not suitable for vacuum applications. NASA examined conformal coatings and reported that the ones that are of low viscosity and 100% solids formulated, are most successful from the outgassing viewpoint [3]. Conformal coating types such as Parylene and Arathane® 5750 are known to exhibit excellent properties in terms of low to no outgassing.
Packaging and lifetime challenges in the electronics industry directs researchers and manufacturers to find new solutions to improve these aspects. Parylene therefore finds application in PCB’s as a protector layer, in the MEMS devices as a dielectric layer, optoelectronic devices as protective layers. Also, Parylene finds application in vacuum systems from medical and healthcare devices and spacecraft components. The materials used for space systems shall be selected based upon low outgassing criteria as mentioned above.
Parylene vs. Arathene 5750
Parylenes are pure organic polymers that contain trace amount of impurities or zero percent chloride, sodium, potassium ions. Parylene thin films offer void-free complete coverage of surface even between closely spaced sub-micron range features and crevices that is useful in microelectronic applications. A study, on a miniaturized vacuum tube system (VacuStor system) for an integrated finger stick blood collector use of Parylene was demonstrated to prevent outgassing and to act as a barrier coating. Vacuum shelf life of the VacuStor tube has been shown to extend beyond one year using Parylene barrier coating and container vacuum bag sealing [4].
Also, in another study 200 nm thick Parylene N was tested for the level of outgassing in vacuum on Mg nano-blades. The sample was heated from room temperature to 403 K in vacuum. In conclusion, the authors reported that Parylene N neither yielded new gas species nor noticeable pressure increases of background gas species during the heating [5]. A similar result was reported in an early study on 300 nm Parylene N conformal coatings by ED Erikson et al [6]. NASA Goddard Space Flight Center makes use of Parylene C as a sacrificial material in microfabrication due to It having little or no outgassing and being fully functional at cryogenic temperatures [7].
Arathene 5750 on the other hand is a liquid conformal coating that can be applied by dipping and spraying. Arathane 5750 is used for PCB’s and electronic components. Arathane 5750 shows minimal outgassing, it is repairable, has low modulus and complies to Mil spec MIL-I-46058C. Arathane requires the mixing of two components for use. Two or more coats must be applied for optimum protection of parts. Outgassing at 10-6 Torr is reported as: Total mass loss (TML), % 0.41 and collectible volatile condensable materials (CVCM), % 0.03. When cured it meets NASA outgassing properties (TML of 1.00 % and CVCM of 0.10 % ) critical for applications in outer space and high vacuum environments.
In conclusion, both materials are useful for vacuum applications. For critical applications parylene would be a better option because it offers a void free conformal coating filling in the smallest voids on the substrate due to the CVD process being used. For a faster processing time Arathane 5750 can be chosen.
References
[1] E21 Committee, “Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment,” ASTM International.
[2] E21 Committee, “Test Method for Contamination Outgassing Characteristics of Spacecraft Materials,” ASTM International.
[3] “A Compilation of Low Outgassing Polymeric Materials Normally Recommended for Gsfc Cognizant Spacecraft Introduction.” NASA.
[4] J. Gu et al., “Development of an integrated fingerstick blood self-collection device for radiation countermeasures,” PLoS ONE, vol. 14, no. 10, Oct. 2019, doi: 10.1371/journal.pone.0222951.
[5] Y. Liu et al., “A study of Parylene coated Pd/Mg nanoblabes for reversible hydrogen storage,” Int. J. Hydrog. Energy, vol. 38, no. 12, pp. 5019–5029, Apr. 2013, doi: 10.1016/j.ijhydene.2013.02.007.
[6] E. D. Erikson, T. G. Beat, D. D. Berger, and B. A. Frazier, “Vacuum outgassing of various materials,” J. Vac. Sci. Technol. A, vol. 2, no. 2, pp. 206–210, Apr. 1984, doi: 10.1116/1.572724.
[7] M. Beamesderfer, “Parylene C as a Sacrificial Material for Microfabrication,” 2005.
