Is Parylene Hydrophobic?

Hydrophobic Basics and Hydrophilicity

In the science of surfaces and thin-film protection, the term hydrophobicity is used to designate the capacity of both a substrate surface and its protective coating to repel water. In contrast, hydrophilicity represents a substrate and coating’s affinity for water adsorption.

• Hydrophobic coated surfaces: static water contact angle θ is >90°
• Hydrophilic coated surfaces: contact angle θ = <90°

One of the primary purposes of conformal coatings is to safeguard electronics from the impact of water/moisture incursion, which can negatively impact their operation. The inherent hydrophilicity of items such as printed circuit boards (PCBs) and medical implants relies on conformal coating hydrophobicity to ensure water does not violate the interior of electrical components, disrupting their intended performance.

Assured surface hydrophobicity is also essential to improved energy applications for transportation, defense and medical applications, environmental protection and solar energy harvesting.

Although some question remains why a differentiation of a slight 2° — from 89° to 91° — is sufficient to render transition from a hydrophobic surface to one hydrophilic, this definition maintains until one more accurate is supported by technical data.

Hydrophobic Properties of Parylene and Liquid Coatings

The wetting behavior of conformal coating materials is very relevant to their use. Parylene (XY) offers generally superior hydrophobicity in comparison to liquid coatings acrylic, epoxy, silicone, and urethane. Liquid coatings are pre-synthesized and applied to substrates by:

• Brushing the wet coating onto the designated surface
• Immersing (dipping) the component in a bath of liquid material
• Spraying the coating substance through either manual or automated methods

Of wet coatings, acrylic, silicone and urethane offer reliable water-based moisture protection under limited conditions.

In contrast, Parylene uses a specialized vapor deposition polymerization process (VDP). Solid, granular Parylene dimer is converted to a gas, cleaved in a pyrolysis process and deposited on targeted surfaces in a molecule-by-molecule fashion. The VDP process generates uniform conformal film on virtually any surface topography and material, penetrating even the smallest surface crevices. The process effectively generates an internal, as well as external protective layer, maximizing XY’s barrier protection.

XY typically provides effective barrier protection at film thicknesses ranging from 0.1 µm to 50 µm The exceptional uniformity of pinhole-free XY prevents leakage due to the vaporous coating penetrating deep into substrate surfaces during application, providing additional moisture protection. XY water barrier properties also restrict water vapor transmission rate (WVTR), the passage of water vapor through barrier film, measured in terms of area and time. Resultant ultra-thin films demonstrate exceptional hydrophobicity and are recommended for MEMS/nanotechnology uses.

Parylene’s distinctive properties include biocompatibility, dielectric/insulative advantages, and truly conformal pinhole-free film deposition and protection, all of which combine with low water absorption (swelling) to enhance their applicability to a wide range of purposes. Biomedically, applications include cardiac stents and pacemakers, infusion technology, electrosurgical and powered surgical tools, mandrels and forming tools and electrodes and stents in neural technology. Specialized biomedical applications have been developed in addition to Parylene’s extensive use for PCB protection and use for aeronautic/defense, automotive, communication, consumer and industrial products. Enhanced hydrophobicity also extends XY’s use for product ruggedization.

Permeation barriers for electronic devices are essential to assure their ongoing performance through a wide range of operational environments. For this purpose, Parylene’s versatility is largely based on its hydrophobicity – repelling water and moisture at the surface, with resistance to swelling in aqueous environments. Hydrophobicity is a fundamental quality that combines with XY’s many other advantages — biocompatibility, chemical inertness, diminished cytotoxicity, pinhole-free conformality – to expand its application to an extensive range of uses.

Parylene can offer superior moisture protection not only to water, but also liquid pollutants, high humidity, salt-air and persistent or acid rain. The same can be said for internal environments with high moisture/liquid content. For instance, XY’s ready adaptability for bio-implantable medical devices further demonstrates its moisture control capacities. Providing ongoing and reliable surface adhesion and moisture barrier protection, Parylene conformal films withstand permeation by water, water vapor and other forms of moisture, as well as incursion by most substances under a wide range of operating circumstances. VDP-generated Parylene coating provides a simple, versatile and dependable hydrophobic conformal coating.