Conformal Coatings for Oil and Gas Sensors

The oil and gas industry utilizes sensors in extremely demanding high temperature and high pressure (HTHP) environments, such as in downhole drilling (>200°C and 30 kspi). IThe US natural gas supply is predominantly found in reservoirs below 15,000 ft, presenting environmental drilling challenges due to temperature, pressure and gasses. Extending the lifespan of drilling equipment and enabling sensor recalibration at such depths could lead to substantial cost savings in these HTHP environments. Sensor packaging plays a critical role in safeguarding equipment under these conditions.

Common Oil and Gas Sensors:

• Temperature Sensing
• Pressure Sensing
• Seismic Sensing
• Level Sensing
• Hydrocarbon Sensing
• Flow Measurement – Liquid and Gas
• Corrosion Detection
• Vision based Non-contact Sensing

Studies conducted on electronic sensors showed that the use of a conformal coating is beneficial for improving their lifetime if the device will be exposed to a corrosive environment such as sulfur or chlorine and thermal cycles[1], [2]. It was shown that urethane (UR) can provide an excellent protection against sulfidation when used in a high-sulfur environments during its service life at 60°C [3]. 27 μm UR and 17 μm Parylene C was studied for the corrosion of metallizations under various environments including H₂S, Cl₂, NO₂, SO₂ at 50°C. It was reported that a uniform thickness coverage along the edges of the terminals may result in better performance with both coating types. Additionally, it is advised that focusing on terminal coverage as well as the flat surface thickness of the conformal coatings will result in a uniform protection [2]. Polyurethane conformal coatings can withstand thermal shock cycles of -65°C to 125°C. On the other hand, Parylene variants (Parylene C, Parylene N, Parylene D, Parylene HT^® or other variants) withstand higher temperatures with better conformal coating properties and better corrosion protection which makes them very competitive as conformal coatings for HTHP environments.

Both UR and Parylene are versatile materials and are suitable for different applications. For example, Thick UR coatings on electronics with uniform coverage can be used in the protection of electronic parts which require thermal and corrosive resistance in the vicinity of 125°C’s. UR provides protection against impact due to its high hardness.

Parylene provides a pinhole-free protective coating at lower thicknesses (a few microns). Among the Parylene variants, Parylene HT (350°C) has the highest thermal durability which is followed by Parylene C (100°C) and Parylene N (80°C). Also, Parylene HT has the lowest dielectric constant (2.17 @ 1 MHz) among the three which makes it more attractive. It can be used as a dielectric/insulating layer where high temperature exposure is required. Because Parylene variants are deposited via chemical vapor deposition (CVD), they have a very dense packaging which prevents corrosive gas penetration for a long time compared to other types of conformal coatings. The removal of Parylene is very difficult, and for drill hole sensing protective purposes, they are suitable as durable coatings.

Polyurethanes have high chemical resistance and they can be removed using stripping agents, on electronic parts urethane conformal coatings are offered as an easy to apply solution. UR can be applied by spaying, dipping or brushing. Both coatings can be used to prevent damage due to mechanical shock. However, due to UR’s thicker coating, it may be more prone to breakage compared to the thinner and more flexible Parylene. In the oil and gas industry, where vibrations and shocks are common in drill hole wells, the preference leans towards the use of Parylene. The coating is known for resisting room temperature chemical attack and is insoluble in all organic solvents up to 150° C [4].

References

[1]        E. Dalton, “Conformal Coating Protection Of Surface Mount Resistors In Harsh Environments,” 2012, Accessed: Apr. 27, 2020. [Online]. Available: https://www.smta.org/knowledge/proceedings_abstract.cfm?PROC_ID=3435.
[2]        T. Richardson, “Effectiveness of Conformal Coat to Prevent Corrosion of Nickel-palladium-gold-finished Terminals,” IPC APEX EXPO Conf. Proc., p. 8.
[3]        B. Hindin, K. Avenue, J. Fernandez, and S. Monica, “TESTING OF CONFORMAL COATINGS USING THE FLOWERS-OF-SULFUR TEST,” p. 9.
[4]        “Parylene Conformal Coating Specifications & Properties,” p. 12.
[5]        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, doi: 10.1109/JMEMS.2013.2273032.
[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, doi: 10.1007/BF02659727.
[7]        J. J. Licari, Coating Materials for Electronic Applications: Polymers, Processing, Reliability, Testing. William Andrew, 2003.
[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, doi: 10.1016/S0022-1139(03)00100-3.