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Crevice Penetration of Parylene

Posted by Sean Horn

Friday, October 19, 2018 7:30

@ 7:30 AM

liquid vs parylene coatingParylene (XY) polymers provide robust, dielectric, micron-thin conformal coatings for a considerable range of electronic devices, most prominently printed circuit boards (PCBs).  XY’s unique chemical vapor deposition (CVD) application method synthesizes in-process, depositing gaseous parylene deep into a substrate’s surface.  CVD occurs on a molecule-by-molecule basis, conforming to all underlying contours, regardless of shape or position, to the nanometer, if necessary.  Pre-synthesized liquid coatings lack many of parylene’s performance properties, having far less ability to successfully and conformally penetrate crevices in the substrate

Additional CVD advantages include:

  • Room temperature processing, allowing damage-free coating of even the most delicate components
  • Vaporous XY flows anywhere there is air, coating under parts, inside small crevices and semi-sealed areas.
  • Sub-surface penetration seals crevices and irregular, tightly-confined areas, while forming an additional layer of conformal protection.
  • Micro-thin films coat exceptionally confined topographies, without forming bridges, dripping or running like liquid coating materials.

Although there are two basic kinds of parylene dimer — C and N — other formulations exist – including D, F and HT.  Each displays specified performance metrics, characterized by different electrical and physical properties.

Of parylene types, Type HT has the highest penetrating ability, as well as the lowest coefficient of friction and dielectric constant.  It is also expensive to make and used essentially for specialized purposes, limiting its application for a wide range of conformal coating assignments.

Of the more common XY varietals, Parylene N is the base structure of the parylene group, offering a higher dielectric strength than C, F or D.  In addition, it penetrates crevices and other cracks or gaps in substrate surfaces more capably than the others.  Type N does this because enhanced molecular activity occurs during CVD, in comparison to the remaining XY varieties, allowing more efficient coating of relatively deep recesses, blind holes, tubes and small openings.

Parylene C  is used more frequently than any other XY variety.  It 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; it provides lesser crevice penetration than Type N.

          With chlorine atoms added to its benzene ring, Parylene D film has greater thermal stability than N, F or C.  Crevice penetration capacities are lower in comparison to other XY varietals.

With a chemical structure of four fluorine atoms on the aromatic carbons, Type F provides good crevice penetration.  Its dimer is also very expensive, limiting its use for many conformal coating projects.

CVD processes convert powdered parylene dimer into a vapor; the resultant coating substance responds with reliable conformability to surface geometries, including:

  • crevices,
  • hidden areas,
  • points,
  • sharp edges, or
  • exposed internal surfaces.

Only XY offers this level of adaptability to highly complex coating problems, generating precise conformance to substrate topography without bridging or pooling.  Applied in micron-thin layers, parylene’s pinhole-free coverage penetrates and coats complex surfaces to a degree impossible with liquid conformal film materials.

Ultimately, CVD is responsible for XY’s exceptional crevice penetration.

  • Since the film is formed as a vapor from a molecular state, and
  • deposits on a molecule-by-molecule basis,
  • it readily adapts to the precise configurations of substrate topography,
  • accommodating and filling crevices and fissures in the surface,
  • whether they are a designed aspect of the board
  • or a production accident.

In any case, the gap is filled with a durable, micron-thin parylene conformal film, regardless of surface complexity.

To learn more about properties of parylene coating, download our whitepaper now:

Download our guide on Parylene 101


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