Top 5 Myths of Parylene
Posted by Sean Horn
Friday, June 8, 2018 7:30
@ 7:30 AM
Although parylene (XY) is a well-recognized and often used conformal coating, misconceptions about what it is and can do are common. These mistaken beliefs interfere with true understanding of parylene’s uses. Five of the most consistent misconceptions – and appropriate corrective information – should clear things up.
- Parylene Is Too Expensive for Normal Use.
Compared to liquid coatings, parylene’s higher processing cost is no misconception.
- Material expenditures for raw XY dimer (di-p-xylene) range between $200.00 to $10,000,00 per pound, far more costly than liquid substances.
- The unique demands of the parylene chemical vapor deposition (CVD) application process is similarly costly; production batches are generally small and time-consuming to complete.
However, these costs can be made more affordable. While dimer is expensive, XY coatings frequently are effective at thicknesses measured at the micron/nano level, rather than in millimeters, lowering material costs. Parylene’s more dependable performance, relative to liquid coatings, means fewer operational problems and less assembly failure, an important cost consideration whether XY films are used for medical implants, aerospace technology or weapons’ systems. Parylene’s performance reliability and exceptional operational versatility for critical devices cannot only be life-saving, but also cost-saving, despite initial higher price during production. Dimer and process costs are high, but their appropriate selection leads to longer-lasting, better coating quality and more reliable performance, XY’s best recognized and respected characteristics.
- Every XY Type is the Same
Parylene dimer comes in two primary types — C and N formulations, but other types — D, F and AF-4 – are available; each has a specified utility distinguished by different electrical and physical properties. The most popular type — C — offers better performance across most metrics, generating exceptional protection from corrosive gases, due to low chemical, moisture, and vapor permeability. Depositing quickly on most substrate surfaces, C’s lesser throw-capability can reduce crevice-penetration activity. Among Parylene N’s key advantages are:
- low dielectric constant/dissipation values,
- encouraging use in high frequency applications.
- a melting point (420°C) higher than Parylene C/D, and
- a more active molecular structure, enhancing permeation of exceptionally minute surface crevices/fissures,
- offering superiority for coating extremely complex surfaces.
Parylene D withstands temperatures as high as 125°C, but lacks sufficient biocompatibility for wide use in medical devices. Very durable, D provides reliable film protection at 134°C, for 100,000 hours, through persistent exposure to 100°C in atmospheres dominated by oxygen. Type F parylene offers good throw-capability and crevice penetration. Slower deposition limits its uses, impeding current commercial uses. Due to higher oxidative resistance and UV stability than other XY types, AF-4 reliably protects outdoor assemblies from pollutants, salt and water.
- Parylene Noodling is a Defect
Before CVD, XY is configured as a covalently-fused, monomer-based linear chain that resemble strands of pre-cooked spaghetti-like noodles, when viewed microscopically. Held together by chemical cross-lengths, they may be elongated, but are never clustered together or precisely straight, ranging between a few nanometers to several tens-of-nanometers in length. This basic alignment of parylene’s physical structure is its standard morphology. In this format, noodles are NOT a defect, but characteristic of XY’s normative shape and a source of the applied coating’s strength. However, defective noodling can occur through inadequate application or deposition onto a surface unprepared for adhesion:
- Unclean substrate surfaces,
- overheating or
- improper coating selection relative to assembly material/purpose
- may result in disordered structural configuration,
- resembling a platter of chaotically-coiled noodles,
- wrapped around each other.
This noodling is deficient, assuming a random shape, rejecting parylene’s typical pinhole-free uniformity. Disordered adhesion is the outcome, compromising the conformal film’s structural integrity and adhesion. As an integral structural XY-component, you can’t avoid the presence of parylene noodles, but you can control them.
- De-wetting is a prominent parylene post-application problem.
De-wetting is a major failure mechanism of liquid coatings, disrupting their protective conformal protection. It occurs when coating material does not wet substrate surfaces upon application, causing deterioration of the conformal film. Solder also fails to adhere to components. In contrast, parylene is NOT pre-synthesized or dispensed during application in a wet format. No wet processes/liquid materials are used. Applied by CVD rather than the wet methods used by liquid coatings, XY is converted from a solid to a gas, with no wet stage. CVD’s free radical polymerization technique forms resonance-stabilized XY diradicals that penetrate substrates, entering and filling pores/voids under and above the targeted surface. Directly-synthesized in one-step processing, CVD sequencing bypass de-wetting, freeing XY of this coating defect.
- Parylene Has Poor Metal Adhesion.
As with parylene cost and noodling, this is not entirely a mistaken belief. Relying on mechanical adhesion, XY doesn’t actually adhere to the surfaces it coats; rather it sticks to itself, filling in the space surrounding the targeted surface, securing existing cracks, pits or other imperfections. Metal surfaces have far fewer of these surface defects; their material smoothness leaves XY less of a substantive base of coating adherence. This is especially true for noble metals, noted for their excellent conductivity — gold, silver, or platinum are capable of resisting acid attack, chemical action and corrosion, making them valuable components for many assemblies. Treating the noble metal surface with the compound A-174 silane improves XY adherence, making it viable for metal coating. Easy to apply, silane can be:
- selectively sprayed onto the metallic surface for specifically targeted areas of the assembly, or
- if an entire assembly requires conformal coating,
- soaked pre-CVD, or
- applied during parylene’s CVD application phase.
Silane treatment assures effective parylene adhesion to metal substrate components, negating this misconception.
If you have additional questions about parylene, its benefits, problems and uses, contact us. Our expert staff can answer your questions, clear up your misconceptions and help you find the right coating for your next project.