Top 5 Myths of Parylene

Although Parylene (XY) is a well-recognized and regularly 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 common misconceptions – and appropriate corrective information – should clear things up.

1. Parylene Is Too Expensive for Normal Use.

Compared to liquid coatings, Parylene’s higher processing cost is not a misconception.

• Material expenditures for raw XY dimer (di-p-xylene) range between $200 to $10,000 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 can be 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 lifesaving but also cost saving, despite initial higher price during production. Dimer and process costs are high, but appropriate selection leads to longer-lasting, better coating quality and more reliable performance, XY’s best recognized and respected characteristics.

2. Every XY Type is the Same

Parylene dimer comes in two primary types — C and N formulations, but other types — D, Parylene HT^® and ParyFree^® – 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
• Higher melting point (420°C) than Parylenes C and D
• A more active molecular structure, enhancing permeation of exceptionally minute surface crevices/fissures
• Superior 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.  Parylene HT, replaces the alpha hydrogen atom of the N dimer with fluorine. This variant of Parylene is useful in high temperature applications (short term up to 450°C) and those in which long-term UV stability is required. Parylene HT also has the lowest coefficient of friction and dielectric constant, and the highest penetrating ability of the four variants.

3. 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 a 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  ordered structural configuration, resembling a platter of chaotically-coiled noodle 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, the presence of Parylene noodles cannot be avoided, but it can be controlled.

4. 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.

5. 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 SCS’ AdPro family of adhesion technologies or A-174 silane improves XY adherence, making it viable for metal coating. Easy to apply, adhesion promotion can be:

• Selectively sprayed onto the metallic surface for specifically targeted areas of the assembly, or
• Soaked pre CVD
• Applied during Parylene’s CVD application phase

The use of an advanced adhesion promotion technology, such as SCS’s AdPro, assures effective Parylene adhesion to metal substrate components, negating this misconception.

If you have additional questions about Parylene, its benefits, problems and uses, contact SCS. Our team of experts can answer questions, clear up misconceptions and help find the right coating for your next project.