Best Coating for Dielectric Strength

Dielectric strength, also known as voltage breakdown strength (Vbd) is a measure of a conformal coating’s electrical insulation effectiveness. Higher values are often key to success for design engineers, and when it comes to conformal coatings, those with hydrophobic properties and lower extractible ionic impurities are less likely to both attract water and render the ions less mobile, enhancing dielectric strength.

Conformal coatings protect and prolong the life of printed circuit boards (PCBs). Design engineers need to ensure adequate protection for normal operation and to withstand operational extremes. To accomplish this, conformal coatings are often utilized. Coatings increase dielectric strength between conductors, enabling smaller, denser PCB design. They also protect circuitry and components from abrasion and exposure to solvents.

When considering the choice of a coating, the objective is to avoid reaching the coating’s breakdown voltage, the minimum difference in charge between two points in an electrical field, causing the insulating conformal film to become electrically conductive. A primary function of all conformal coatings is maintaining sufficient insulation and avoiding dielectric breakdown.

Another important electrical measurement is dielectric constant, which measures the coating’s response to electric fields. A dielectric constant measuring more than 3.0 indicates the molecular response to the alternating field and current which might be sufficient to vary circuit speed, an undesirable outcome for higher frequency assemblies.

Conformal Coatings’ Inherent Electrical Properties

Conformal coatings provide a protective, nonconductive dielectric covering for PCBs, adding security from the incursion of external elements that cause corrosion and contamination. The four most common liquid types of conformal coating are acrylic, urethane, silicone and epoxy. Accompanying these is a fifth distinctive coating known as Parylene, applied using a vacuum vapor-phase polymerization process.

Liquid coatings are applied to PCBs by wet techniques, incorporating saturated-brush, immersion (dipping the component in a tank of liquid film material) or spray methods. These methods are generally inexpensive and simple to complete.

Parylene’s vapor deposition polymerization (VDP) method converts powdered or granular Parylene dimer into a monomeric gas, which permeates deep within the surface of electronic assemblies, providing a more comprehensive coating than the surface treatment offered by liquid coatings. More reliable as a process, VDP can be slower and more expensive to implement than wet application methods. However, Parylene can be cost competitive if properly planned and designed for.

Table 1 lists dielectric strength and dielectric constants for Parylenes N and C. A more comprehensive listing of electrical properties can be found at scscoating.com.  For varying frequencies, the dielectric constants for the Parylenes range from 2.17 – 3.15. Understanding how these properties change with frequency is key for design engineers since uniform performance relies on uniform properties across frequencies.

Compared to liquid coatings, Parylene’s lower dielectric constants show its ability to resist powerful electric fields. Its higher dielectric strength indicates superior capacity for providing dielectric protection to critical electronics. A distinct benefit of Parylene is a greatly diminished tendency to degenerate when subjected to substantial electrical activity.

In contrast with Parylene, liquid coatings usually exhibit a dielectric constant above 3.0, indicating possible circuit speed fluctuations that could impact the performance of high-frequency assemblies. Lower dielectric-constant materials like the Parylenes, on the other hand, are made up of weakly bonded molecules, forming a reliable buffer between a PCB and its operating environment. Polarized by electrical charges, Parylene resists electrical conduction, enhancing its value as a coating for high-speed electrical components, in a manner superior to liquid coatings.

Since liquid coatings generally exhibit higher dielectric constants and have lower dielectric strength, they are more likely to break down during prolonged contact with intense electrical activity. Parylene conformal films seldom conduct current or suffer dielectric breakdown.