Overall 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 (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 chemical-vacuum chamber, parylene's methodology 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 the Parylenes' C, N, AF-4, or a similar variants. After being situated in a loading boat, the dimer is inserted into the vaporizer for further processing. The powdery dimer is heated within a temperature range of 100-150º C. inside a closed-system vacuum chamber, converting it to a gaseous form at the molecular level. The vapor is then heated to a higher temperature, reaching 680º C (1255º F), without variation. Throughout the process, the deposition system must maintain reliable, consistent levels of heat. These upper range temperatures compel sublimation, splitting the molecule into a monomer. This condition effectively eliminates parylene's double-molecule structure, causing a single molecule vapor to be formed.
While parylene's vapor-deposition polymerization process produces exceptionally reliable conformal coating, it is very time-consuming. The monomer gas needs to be 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, leading to the cold-trap, the concluding phase of parylene's specialized coating technique. In cold-trap, radically lower temperatures, between -90º and -120º C, cool the coated materials, while removing any remaining parylene from the surface; the residual materials are unnecessary and can interefere with parylene's value as conformal coating.
Regarding the time element, parylene typically exhibits a deposition rate of approximately .2/mils per hour, slow in comparison to the liquid application technique employed by most competing coating substances. Thus, machine runs for parylene can last over 24 hours, and tend to encompass smaller production batches. Parylene's application process is rather different and, in consequence, 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 competitors' 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 its vapor monomeric quality.
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 the pinholes, pudding, bridging, run-off, thin-out along substrate surfaces, and tin-whisker problems virtually eliminated by parylene. In addition, liquid chemistry coatings lack the precision-application of parylene, limiting their use for a wide range of specialized aerospace/military, consumer, medical, and associated MEMS/ nanotechnologocial products.
Moreover, acrylic conformal coatings lack parylene's resiliency when exposed to solvents, offering considerably less protection. The same can be said for temperature standards: acrylics' continuous operating temperature top-out at 125ºC; urethane conformal coatings have a maximum operating temperature of 200ºC. Parylenes can match this, and also remain operable at -200ºC.
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 ruggedization products and processes.
Parylene's vapor-deposition polymerization process produces a uniform thickness conforming completely to the substrate, generating assured 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 pr oducts.