New coating option for component protection

  • 30-Jun-2008 06:17 EDT
corr_nocorr.jpg
A Parylene HT-coated board (top board) shows no sign of corrosion after 144 h of salt-fog testing via ASTM B117-(03). The uncoated board does shows signs of severe corrosion.

The family of Parylene conformal coatings (N, C, and D) has been used as a rugged, reliable protection solution for a wide range of aerospace applications for more than 35 years. However, while these standard formulations still meet the coating requirements of most applications, some of the leading-edge technology in aerospace devices often need a little more.

Specialty Coating Systems recently introduced a new Parylene variant, SCS Parylene HT. According to Alan Hardy, Automotive, Electronics, and Military Market Manager, this new formulation is not intended to replace existing formulas, but adds capabilities to the Parylene family that make it attractive for many high-reliability aerospace applications.

“Parylene HT was developed by replacing the alpha hydrogen atom of the Parylene N dimer with fluorine,” he said. “It possesses increased dielectric capabilities and superior thermal and UV stability, for a variety of aircraft and space electronics.”

Hardy explained that Parylene HT has a smaller molecular structure than any other Parylene formula. Because of this, Parylene HT opens the door for more opportunities where components are in nano dimensions and coatings must penetrate very small areas, providing complete coverage without compromising operational capabilities. It also provides thermal stability up to 350°C (short term up to 450°C), both assets when configuring instrumentation incorporating MEMS (microelectromechanical systems) and multiple-layer circuit devices for rugged applications, such as those in cockpit and engine management and control systems.

Two capabilities that are truly unique to Parylene HT are long-term UV stability and the fact that it has the lowest dielectric constant and dissipation factor of all Parylenes. With increased use of LEDs in aircraft lighting both external and in cockpit displays, all components exposed to UV rays can now have the same long-term protection as any other component.

“All other Parylenes perform poorly in applications that require prolonged exposure to ultraviolet light, ultimately breaking down and degrading,” said Hardy.

As for the low dielectric constant and dissipation factor, Hardy stated that because of the increasing use of RF and other forms of wireless devices entering into both control-to-human and control-to-control cross-device signal interface, these features allow Parylene HT coating to provide dielectric insulation for true and undistorted signals.

Some factors are equal for all Parylene formulas. All Parylenes provide barriers or moisture, chemical, and biological agents. Since they are applied via a vapor-deposition process, where the coating molecules basically “grow” to an ultra-thin barrier completely covering all components, they create no added weight. Parylenes actually strengthen delicate leads and connections by 10 times, claims the company. This gives enhanced durability wherever vibration is present, such as in landing-gear control assemblies. Additionally, Parylene coatings are RoHS-compliant and have been shown to suppress the formation of metallic whiskers for tin whiskers mitigation in lead-free solder applications.

As a result of the vacuum deposition, there is no potential for trapping of air bubbles underneath or in the coating. Other industry-standard coatings that are sprayed, dipped, or brush-applied have the potential for trapping air bubbles in or underneath the coating. When this occurs and the coating is exposed to high altitudes or space, air bubbles could open and expose the circuit to the elements and a possible short circuit.

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