Parylene for Military Sensors

Today, security systems rely on different types of advanced, intelligent and connected sensor technologies. Application areas are diverse: radar systems, vision, night vision (IR-cameras), acceleration- orientation-location detection (accelerometers, gyroscopes, GPS), chemicals (neural toxins, other toxic gasses, liquids, materials), wearable sensors (body temperature, relative humidity, location detection), barometric (under water), air flow (aerospace, missiles) and they are brought together for multifunctionality on PCB’s which carry many sensor at a time. Sensors used in military applications pose stringent requirements such as robustness under severe environmental conditions and require longevity of sensing functions. Some of the environmental conditions that are harsh on sensors can be listed as:

Humidity /Moisture: in naval operations exposure to salty water, exposure to rain can damage the circuits of sensors due to corrosion and oxidation of parts.

Temperature: (i) Abrupt changes in temperature under service conditions such as in 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, (ii) very hot temperatures during summer and (iii) extreme cold during winter can affect the functions of a protective layer used to seal sensor packages or parts used in a sensor (eg. dielectric layers).

Chemicals: Oxidative gases, acidic, 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 standardize the products for use in military applications by department of defense. Today, they are also used in 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 XY 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 (Also, related content is listed in Table below) [3].

PARYLENE AS A CONFORMAL COATING IN MILITARY SENSORS:

Among other conformal coatings Parylene is a versatile material which is proven to conform to planar and complex geometries filling 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 based conformal coatings are deposited at room temperature which puts it in an advantageous place to be used in sensor technologies when processes allow for low thermal budgets. In many 3D IC’s, MEMS based systems or where CMOS integration is required thermal treatments above ≥300 °C’s can irreversibly damage the device functions due to the interdiffusion of species of the selected materials (eg. Low melting temperature Alloys, non-optimal diffusion barriers, and so on). Parylene C, among other types (N and F) 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 temperatures in excess of 200 °C are also achievable for long term thermal exposure it would a be good selection as a seal or dielectric material. Parylene C was shown to protect its mechanical properties over a very wide temperature range (-200 °C to 200°C). They are also preferred in a wide range of applications such as packaging of sensors as a chemical barrier due to its chemical durability which is critical to maintain their functionality [3]. Parylene C coatings provide very good barrier properties against common chemicals (water, acetone, ethylene glycol, DMSO, 1-propanol) and exhibit a low permeability for hours of operation exposed to these chemicals [4]. In another study, Parylene C was tested using the MIL-STD-202 Methods 106 and 302 for the insulation resistance during a temperature/humidity cycle with circuit board test patterns on a PCB (described in MIL-I-46058C). The test uses a range of humidity levels and temperatures (25°C, 50 % RH) to more severe conditions (65°C, 90% RH). Resistance readings are taken during different steps. The results have shown that for a 2.5 μm thick parylene the resistance range in between 10^-14 to 10^-12 Ohms in 10 days. Results showed one order of magnitude higher values for the insulation resistance compared to the suggested value in the MIL-STD-202 [5].

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 sub-micron coverage, a low dielectric constant and higher temperature operation is required Parylene F is the attractive choice of selection.

 

Table: Parylene Types and Properties

Property	Unit	Parylene N	Parylene C	Parylene F –AF4
Molecular structure
THERMAL  PROPERTIES
Melting point	°C	420 (400)[6]	290	≤ 500[6]
Durable Heat Resistance	°C	80	100	350
Literature reported values			20 days at 85 °C and 85 %RH [7] 2 hours 120 °C/100 %RH [7]
MECHANICAL/PHYSICAL PROPERTIES
Tensile Strength	psi	6500	10000	7800
Yield Strength	psi	6300	8000	7600
Friction		0.25	0.29
CHEMICAL RESISTANCE
			SWELLING: benzene, chloroform, trichloroethylene, toluene. SAFE: Methanol, 2-propanol, ethylene glycol, water.[4]	No oxidation [6]
Water Absorption (MWTR)		0.01%/24 hour	0.06%/24 hour	0.01%/24 hour
Water Transmission 90-95 RH, 40 °C	g/m²-24hr-25 μm	217 [3]	15.5 [3]
Oxygen Transmission  (@Temperature 25.0 °C)	cc-mm/m²-24hr-atm	15.4	2.80	34.7
Advantages		ü Constant dielectric coefficient at all frequencies ü High dielectric strength ü Less wear (low friction coefficient.)	ü Low gas permeability ü High Chemical Resistance	ü Sub-micron coverage [8] ü High thermal resistance ü UV-resistive ü High-density

 To learn more about different types of conformal coatings and how they help protect military electronics, download our whitepaper now:

Sensors & Parylene

 

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.

[6]         P. K. Wu, G.-R. Yang, J. F. McDonald, and T.-M. Lu, “Surface reaction and stability of parylene N and F thin films at elevated temperatures,” J. Electron. Mater., vol. 24, no. 1, pp. 53–58, Jan. 1995.

[7]         C. Hassler, R. P. von Metzen, P. Ruther, and T. Stieglitz, “Characterization of parylene C as an encapsulation material for implanted neural prostheses,” J. Biomed. Mater. Res. B Appl. Biomater., vol. 93B, no. 1, pp. 266–274, 2010.

[8]         W. R. Dolbier and W. F. Beach, “Parylene-AF4: a polymer with exceptional dielectric and thermal properties,” J. Fluor. Chem., vol. 122, no. 1, pp. 97–104, Jul. 2003.