Parylene Dielectric Properties: Designing for High Voltages
Mallory McGuinness | September 11, 2020
For decades, product designers have used conformal coatings, such as acrylics, silicones, Parylene, epoxies, and polyurethanes, to protect PCBAs from contaminants, moisture, and other common threats. Coatings used in electronic circuits fulfill critical functions, including environmental protection and electrical insulation, the latter allowing for a reduction in PCB conductor spacing.
If you are developing a product that incorporates electronics that needs to operate reliably, knowledge of conformal coating electrical properties and their exact values is necessary to optimize for the most reliable coatings for your application.
Conformal Coating Dielectric Strength
Coatings should be able to withstand an applied voltage without breakdown. At the breakdown voltage, the coating fails to provide the intended benefit. The insulative properties left (if any) are negligible at this point. Therefore, an essential electrical property is the dielectric strength, defined as the maximum voltage at or below which no breakdown occurs.
At both high and low voltages, dielectric considerations are particularly important for the reliability of electronic circuits.
Dielectric Constant and Dissipation Factor
In general, the higher the dielectric strength a coating can maintain throughout exposure to various environments and variables, the better. Said differently, ideal electrical insulating coatings have low dielectric constants and low dissipation factors, as well as the ability to retain these low values over a wide temperature and humidity range.
Parylene Electrical Properties
Parylene’s electrical properties are among those of the best. The figure below illustrates the breakdown voltage as a function of film thickness:
Coating Materials for Electronic Applications, James Licari
Parylene is Biocompatible and Can Stand up to Sterilization
Parylene is chemically inert and biologically stable, meaning it can survive exposure to chemicals found in the body without reactions. It is also non-toxic.
In a medical environment, medical instruments must remain free of contamination. Parylene can withstand sterilization by autoclave, E-beam, gamma-ray, and more, protecting against moisture, chemicals, and bodily fluids. Over the long-term, Parylene helps prevent premature device failure.
High Flexibility, Low Friction Protection
Parylene serves as a dry lubricant and facilitates its intended purpose and medical procedure. This property is essential for medical devices, where more friction may mean a more complicated, lengthier procedure. With Parylene coatings, discoloration and contamination can be reduced on a variety of medical products.
Dielectric considerations are particularly important for the reliability of electronic circuits.
As you can see, each Parylene type has different dielectric properties, so there is a suitable Parylene for virtually any application. Additionally, the insulating properties of the Parylene coating increase with thickness, creating the possibility to fine-tune electricity-blocking properties.
Better Dielectrics Come from Better Processes
As a general rule, a polymer’s dielectric strength decreases as impurities or imperfections in the coating increase. Clean, dry coatings may exhibit high insulation resistances but have inadequate dielectric breakdown voltages because of pinholes or voids.
For this reason, chemical vapor deposition (CVD) lends itself to the excellent electrical properties of Parylene. With the CVD process, the dimer is heated into a vapor that deposits deep into the crevices of printed circuit board assemblies, providing consistent, even 3D coverage free of pinholes and imperfections.
Since every surface on a product is coated with uniform thickness, Parylene is an excellent choice you can use to supplement other forms of insulation.
To learn more or talk to one of our experts about how to design a better product, contact us today. Making a small investment of your time now could yield decades of reliability.
There is a suitable Parylene for virtually any application.
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