Parylene for Military Sensors

Security systems rely on different types of advanced, intelligent and connected sensor technologies. Application areas are diverse, including radar systems, vision, night vision (IR-cameras), acceleration/ orientation/location detection (accelerometers, gyroscopes, GPS), chemicals (neural toxins, other toxic gases, liquids), wearable sensors (temperature, humidity, location detection), barometric (under water), air flow (aerospace, munitions), and they are brought together for multifunctionality on PCB’s which carry many sensors at a time. Sensors used in military applications pose stringent requirements, such as robustness under severe environmental conditions, and require longevity of sensing functions. These environmental conditions include:

• Humidity/moisture: Exposure to salt water, rain and humidity can damage the circuits of sensors due to corrosion and oxidation of parts.
• Temperature: Abrupt changes in temperature, such as climates where the temperature raises and drops quickly during the day (diurnal temperature variation) or thermal shock due to immersion hot and cold environments sequentially and repeatedly. Very hot temperatures during summer and extreme cold during winter can affect the functions of a protective layer used to seal sensor packages or parts used in a sensor.
• Chemicals: Oxidative gases, acidic, and basic liquids can penetrate through protective layers inevitably leading to the device failure.
• Mechanical impact and failure: Sensors may encounter friction, compression, tensile stresses and bending under service conditions. Protective layers, insulators, dielectric materials used in such systems must possess mechanical properties that can withstand such mechanical forces for the longevity of applications in the field.

Testing

Military standards are used to regulate products for use in defense applications, as well as some commercial applications. The design of the product and test limits to be achieved are established with use of such standards under replicated service conditions. Test methods for standard electronic and electrical component parts are determined by MIL-STD-202, where methods for testing of seal, mechanical, chemical and thermal properties are described [1]. Parylene testing for use as an sealant for PCBs is also covered by MIL-I-46058 C [2]. A comprehensive knowledge base on Parylene derivatives and their properties under different conditions as a conformal coating can be found in literature [3].

Parylene Properties and Applications

Parylene is a versatile material which is proven to conform to planar and complex geometries, fitting into the smallest crevices and gaps, resulting in a continuous, pinhole-free and transparent thin film that can be deposited as micrometers to millimeters thick. Parylene conformal coatings are deposited at room temperature, which is advantageous in sensor technologies when processes allow for low thermal budgets. In many 3D integrated circuits (ICs), MEMS based systems or where complementary metal-oxide semiconductor (CMOS) integration is required, thermal treatments above 300°C can irreversibly damage the device due to the interdiffusion of species of the selected materials. Parylene C shows an excellent thermal endurance in air without significant loss of physical properties for 10 years at 80°C, and in the absence of oxygen, Parylene can withstand temperatures in excess of 200°C. Parylene is preferred in a wide range of applications, such as packaging of sensors, as a chemical barrier due to its excellent chemical durability [3]. Parylene coatings provide very good barrier properties against common chemicals (water, acetone, ethylene glycol, DMSO, 1-propanol) and exhibit low permeability when exposed to these chemicals [4].

Parylene N has a very low friction coefficient and is resistant to water absorption therefore it is proven to be a good candidate for sealing and encapsulation applications where friction forces comes into action. When a low dielectric constant and higher temperature operation is required, Parylene HT^® is an attractive choice.

References
[1]         “MIL-STD-202 | Test Method Standard for Electronic and Electrical Component Parts | Document Center, Inc.” [Online]. Available: https://www.document-center.com/standards/show/MIL-STD-202. [Accessed: 17-Dec-2019].
[2]         “MIL-I-46058 C INSULATING COMPOUND ELECTRICAL (FOR COATING PRINTED CIRCUIT ASSEMBLIES).” [Online]. Available: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-I/MIL-I-46058C_11366/. [Accessed: 18-Dec-2019].
[3]         J. J. Licari, Coating Materials for Electronic Applications: Polymers, Processing, Reliability, Testing. William Andrew, 2003.
[4]         H. C. Koydemir, H. Kulah, and C. Ozgen, “Solvent Compatibility of Parylene C Film Layer,” J. Microelectromechanical Syst., vol. 23, no. 2, pp. 298–307, Apr. 2014.
[5]         R. Olson, “Parylene conformal coatings for printed circuit board applications,” in 1985 EIC 17th Electrical/Electronics Insulation Conference, Boston MA, USA, 1985, pp. 288–290.