Expanded Details of Parylene Conformal Coating

Expanded Details of Parylene

Uniform thickness and true conformality – the established process guarantees precise control of thickness and inherent uniformity, especially critical in microelectronics applications; no bridging, thin-outs, puddlings, run-offs which are common problem with other coating materials. Since it is based on a gas, Parylene can penetrate spaces which typical conventional coatings can not cover: small recesses, crevices and holes and even the edges and the inside spaces of very fine tubes.

Pin-hole Free – tough coatings as thin as 0.1 microns can be achieved without any voids. Chemical, fungus and bacteria resistance. Parylene resists attack from exposure to most acids, bases and solvents. It is an excellent inhibitor to the growth of fungus and bacteria.

Superior barrier properties –  Parylene provides exceptional corrosion protection from moisture, salt spray, corrosive vapors and other hostile environments. Its water vapor transmission rate has been found to be significantly lower than most conventional coatings. With respect to migrating ionic species, Parylene coatings have been proven to act as barrier to extractable metals which otherwise will contaminate substrates.

Impressive mechanical strength – since it has high tensile and yield strength, Parylene conformal coating is used for encapsulating microcircuits because it increases the pull strength of wire and lead bonds, face bonded chips and conductor bridges and therefore contributes significantly to device integrity. Since its specific gravity is low, the Parylene layers are typically lighter than most other functional coatings.

High dielectric characteristics – its extremely high dielectric strength combined with its electrical stability in various media provide unique insulating property. The dielectric constant and dielectric losses are low and unaffected by absorption of water vapors. Its volume and surface resistivities are advantageously high because of the purity, low affinity to moisture and in particular its freedom from trace ionic impurities present in conventional coatings.

Thermal stability – Parylene coatings remain stable at continuous temperatures as high as 130 deg.C in air, or 220 deg.C in the absence of oxygen. It has good mechanical properties from -200 to 275 C.

Stress-free – since the polymerization of the film takes place on the substrate surface at room temperature, there is no thermal or mechanical stress introduced during application, hence original performance parameters of coated subjects are basically unaffected.

Particle immobilization – assures circuit integrity, preventing mobility of loose solder, wire particles or other mobile debris left from manufacture. Pressed powder parts, ferrites, ceramics, corrosive metals, glass and epoxy particulates can be positively stabilized.

Dry film lubricant – inherent excellent dry lubricity, as indicated by coefficient of friction measurements make Parylene a valuable asset as a dry film lubricant, particularly as a coating for surgical instruments. Compared to fluoropolymers, Parylene conformal coating has also the ability to provide wear and abrasion resistance.

Sterilization – due to their thermal and chemical resistance, Parylene coatings can survive the conditions of many common sterilization techniques (e.g. autoclave, radiation, ethylene oxide).

Film Description

Thin, transparent Parylene film is unique in that it is applied to substrates in an evacuated chamber by a process called gas-phase polymerization. The dry, powdered raw material, or dimer which is a compound of two identical monomers is converted by heat to a dimeric gas and ultimately to a monomeric gas, after which it is deposited on substrates at room temperature. The coating grows as a uniform, conformal film (poly-para-xylylene) on all exposed surfaces including edges and in crevices. Parylene deposition has no liquid phase or gaseous byproducts and no solvents or environmentally restricted materials are required.

There are four primary variants of the polymer: Parylenes N, C, D and HT. Each has its own molecular form that results in unique characteristics. The optimum selection of a Parylene variant depends on the exact nature of the intended application. Other qualities:

  • Parylene film has useful dielectric and barrier properties, as well as extreme chemical inertness and freedom from the pinholes typical of liquid coatings. It resists organic solvents, inorganic reagents and acids. Because Parylene is not liquid at any stage in the process, it does not pool, bridge or exhibit meniscus properties.
  • Because of the thinness of its coating, Parylenes mechanical dampening and loading effects are minimal. The material can be coated on diverse substrates including glass, metal, paper, resin, plastics, ceramic, ferrites and elastomers, as well as powdered and granular substances.
  • Parylenes static and dynamic coefficient of friction values are in the range of 0.25 to 0.33, its dry-film lubricity being an important attribute for some coating applications.

Current Uses

Today, the use of Parylene ranges from the common to the arcane and encompasses markets from deep-space vehicles to automobile engines to heart pacers to military electronics. In every case, the selection of Parylene is based on the importance of one or more of its basic properties. The breadth of applications can be attributed to the availability of automated deposition equipment, increased familiarity with the polymer across technical disciplines and continuing improvements in coating efficiency. Following is representative of current applications:

Electronic sensors benefit from Parylene coatings by providing environmental protection without physically loading delicate transducer surfaces. Examples include various industrial components and automotive sensors.

Metering devices. The use of remote circuitry that reports on utility consumption is growing. These circuits must operate dependably under demanding environmental conditions, which Parylene coatings provide as long-term protection against moisture, solvents and other contaminants, including metered gases.

Aerospace applications for Parylene make use of its unique properties for space hardening, protection against condensation resulting from extreme temperature changes and restricting outgassing. For example, the material can protect high voltage-control units used on communication satellites from arcing and damaging corona discharge.

Military electronics. Parylene coats circuits for aircraft communication and navigation equipment, satisfying requirements similar to those for aerospace applications. Military field computers that are transported, stored and used under rigorous weather conditions must be ruggedized for dependable operation; Parylene is a key component in that process in places where alternate liquid and spray coatings have not proven satisfactory for moisture protection.

Electronic-access systems circuitry for hotel security is protected from condensation and corrosion by Parylene film. This is particularly important in coastal areas where the combination of salt air, high humidity and heat quickly degrade unprotected electronics.

Medical devices used in surgical procedures are selectively coated with Parylene to improve their lubricity and to provide selective electrical insulation. The coating also protects and lubricates hypodermic needles and can create a biocompatible barrier on implantable devices and prosthetic hardware.

Silicone keypads. Many systems, such as cellular telephones and specialized computer equipment, use silicone rubber keypads for data input. Parylene coating often is used for silicone keypad surfaces, protecting printed legends and sealing surfaces against finger oils and other contaminants.

Ferrite cores are coated with Parylene to serve several purposes. The coating provides dielectric insulation to avoid electrical loss without appreciable change to physical dimensions. Parylene film consolidates the surface of ferrites and pressed-metal components, which eliminates dust and contamination.