Parylene Coating Blog by Diamond-MT

Parylene’s Optical Properties and Performance

Posted by Sean Horn on Fri, Sep 06, 2019 @ 08:00 AM

The polymer parylene (XY) is a reliable protective conformal film that safeguards the visual clarity and color of printed circuit boards (PCBs), similar electronic assemblies and other products.  XY optical clarity seldom diminishes to the extent either the coating or the underlying substrate becomes visually indistinct, although over-exposure to ultraviolet (UV) light may eventually interfere with optical perception.  However, in the majority of cases, colorless parylene generates advantageous optical properties for a wide range of uses -- including artwork/museum artifacts, cameras/sensors, computer touchscreens, healthcare/medical devices, light-emitting diode systems (LEDs), and optoelectronic components maintaining consistent aerospace, scientific, and telecommunication operations.optical-enhancements_600x300

Parylene’s advantageous optical properties are partially a consequence of its application method, via chemical vapor deposition (CVD).  

  • Using extreme heat, CVD transforms powdered XY dimer into a gaseous consistency, 
  • which penetrates the targeted component, 
  • providing a truly conformal, uniform and resilient coating both above and beneath substrate surfaces.  

Numerous barrier, dielectric, and insulative benefits result, supplied by micron-thin coating layers, with a normative thickness range of 0.0005 to 0.002 inches (0.013 – 0.051 mm.).  One valuable outcome is outstanding optical clarity, making parylene films very appropriate for coating lenses, photosensitive components and other devices requiring visual transparency.  For instance, XY’s optical clarity maintains the visual integrity of the museum/gallery level artwork/culturally important archival items, LED performance, and medical implant receptors.

CVD for Optical Clarity                          

CVD can be administered to improve parylene’s optical performance, by lowering peak chamber pressure during polymer deposition.  The resultant film transparency enhances optical clarity. For biomedical purposes, highly-reproducible microtextured membranes develop from the creation of multidimensional polymeric platforms during CVD.  These supply enhanced optical clarity at the micron/sub-micron level, useful for investigating cell-environment relationships that can lead to better healthcare solutions. 

In the process, CVD transforms whitish, powdered XY-dimer from solid-to-vapor-to-clear, colorless coating.  This synthesis is a strong element in the development of parylene’s superior optical clarity. It should be noted that while

  • thinnest parylene films (<1 um.) offer unblemished optical clarity, 
  • increased coating thickness can slightly mar visual acuity. 

XY layer application rarely exceeds .50 mm., assuring these minor visual alterations remain mostly undetectable, for practical purposes; the coating’s dependable visual perception will be retained.  

In addition, the type of parylene used for coating will impact optical clarity, according to film thickness.  Parylene C, for instance, is optically clear while parylene N displays minute surface haze at thicknesses > 5 μm.  Visually insignificant, these optical variances persist across XY types, as coating thickness increases, and should be registered:

  • prior to CVD to determine appropriate material/thickness strategies, and 
  • throughout the coating process to ensure application proceeds as planned.

The Impact of Parylene Type/Ultraviolet Light on Optical Clarity                

Prolonged exposure to ultraviolet (UV) light initiates coating degradation for parylene types N, C, and D.  In comparison:

  • parylene F is affected but at a much slower rate, 
  • while type AF-4 exceeds all other XY materials resisting UV degradation.

Both cost significantly more than N, C, and D, at a level of 400% for F, and 1500/2000% for AF-4. 

The products of degradation persist for all XY types, regardless of film thickness, after UV-absorption.  Film yellowing – which can substantially significantly weaken an XY film’s optical clarity – is the major consequence.  The degree of yellowing is a factor of:

  • the UV-resistance level of each XY-type, 
  • cumulatively intensifying with the length of exposure, 
  • regardless of coating thickness. 

In addition to diminished optical clarity, parylene coatings will finally oxidize from extended UV exposure, 

  • generating main-chain scission, 
  • molecular level breaks within the XY material, 
  • eventually rupturing the coating surface.  

Thicker films will sustain surface integrity longer, but yellowing will commence in due course, and coatings will crack.

Another potential drawback of parylene optical clarity is the very fact of their exceptional visibility also makes them susceptible to reverse engineering.  That is, 

  • proprietary or similarly exclusive design can readily be detected by anyone looking beneath the XY film, 
  • allowing easier identification and replication of the assembly’s unique functional elements.

This is not a condition rectified by applying thicker parylene layers, which will do little to significantly obscure the coating’s overall visual clarity.  Additions to the XY conformal film are necessary -- generally as supplementary pigmented liquid epoxy or urethane coating completely covering the unit -- to prevent fraudulent component replication.  Epoxy and urethane offer surface resilience comparable to parylene, masking the covered assembly while increasing removal difficulty.  

Despite these moderate shortcomings, parylene conformal coatings offer excellent optical clarity in the vast majority of cases.

Tags: parylene, parylene properties, optical performance