How Parylene is Applied Compared to Other Conformal Coatings

The generic name Parylene describes a distinct collection of polycrystalline and linear organic coating materials with innumerable applications. The essential basis of today’s Parylene N, p-xylene, was inadvertently synthesized at England’s University of Manchester in 1947. The filmy residue resulted after high-temperature heating of compounds of toulene and the xylenes polymerized into para-xylene. The substance immediately demonstrated an exceptional capacity for generating the fine but resilient surface-covering that characterizes today’s range of Parylene conformal coatings.

Subsequent development led to such commercially viable coatings as the Parylenes N, C, and a number of other variants. Most Parylene materials possess properties similar to Teflon^TM (polytetrafluoroethylene, PTFE) but offer a wider range of better-protected applications for consumer, industrial, medical and military uses. The highly specialized application process inherent in the use of Parylene generates the superior conformal coatings that distinguish it from competitors.

Vapor Phase Polymerization

Compared to the application processes of other coating materials, ParyIene’s unique vapor-phase polymerization technique is more complex, depositing the substance directly onto the substrate or material that is being coated. Implemented in a specialized vacuum deposition system, Parylene’s deposition mechanism does away with the intermediate liquid deposition procedure common to competing coatings.

At the outset, a raw dimer in solid state is used, comprised of Parylenes C or N, ParyFree^®, Parylene HT^® or other variants. The dimer is inserted into the vaporizer for further processing, and after the system is brought down to the appropriate level of vacuum, the dimer is heated within a temperature range of 100-150ºC, converting it to a gaseous form at the molecular level. The vapor is then heated to a higher temperature in the pyrolysis section, reaching 680º C (1255º F). Throughout the process, the deposition system variables (temperatures, pressures, etc.) must be carefully maintained. Upper range temperatures compel sublimation then the splitting the molecule into a monomer gas. This condition effectively eliminates Parylene’s double-molecule structure, causing a single molecule vapor to be formed.

The monomer gas is vacuum-drawn onto the selected substrate at the extremely gradual rate of one molecule at a time. The procedure takes place in the coating chamber, at ambient temperature.
Regarding the time element, Parylene typically exhibits a deposition rate of approximately 0.2/mils per hour, slow in comparison to the liquid application technique employed by most competing coating substances. Thus, deposition runs for Parylene can last for hours in this batch process. Compared to processes requiring little or no masking, the application of Parylene can be slower and more expensive than the traditional wet chemistry coating methods used for acrylics, silicones and other substances.
Parylene’s vapor polymerization process eliminates problems common to liquid application processes. With the effects of gravity and surface tension eliminated, Parylene achieves superior coating of even the most complex structures, largely because of the physics of its deposition.

The Wet Chemistry Application of Non-Parylene Coatings

Non-Parylene conformal coatings like acrylic, epoxy, silicone or urethane rely on a liquid, wet chemistry coating technique. Typically, application of these conformal coatings involves either:

• Dipping the substrate into a liquid bath consisting of the coating substance
• Brushing the coating onto the substrate
• Spraying the wet coating material directly onto the substrate surface

While these procedures are quicker and less costly than Parylene deposition, they are also subject to a number of shortcomings such as pinholes, pudding, bridging, run-off, thin-out along substrate surfaces. In addition, liquid chemistry coatings can lack the precision-application of Parylene, limiting their use for a wide range of specialized aerospace/military, consumer, medical, and associated MEMS/ nanotechnologies. Moreover, acrylic conformal coatings lack Parylene’s resiliency when exposed to solvents, offering considerably less protection.

Application processes affect utility. For instance, to be effective, silicone must be applied far more thickly than other coatings, reducing flexibility while limiting its MEMS/nano uses. Urethane generates lesser heat (125ºC) and vibration protection, making it largely unsuitable for the ruggedization of products and processes.

Conclusion

Parylene’s vapor deposition polymerization process produces a uniform thickness conforming completely to the substrate, generating pinhole-free coverage. The result is excellent chemical, dielectric barrier and moisture protection. Parylene’s non-liquid application process derives a coating free from the edge-effect and meniscus problems common to competing conformal coatings. The vapor-phase deposition technique separates Parylene from competitors and is largely responsible for its superiority as a conformal coating for a wide range of products.