Optical Clarity of Parylene at Increased Thickness
Posted by Sean Horn
Friday, May 11, 2018 7:30
@ 7:30 AM
Generally applied at micron-thin coating layers, parylene (XY) offers numerous barrier, dielectric, insulative and similar protective benefits to printed circuit boards (PCBs) and related electronic assemblies. One property of parylene applied in its normal range of 0.013 – 0.051 mm. (0.0005 to 0.002 in.) is exceptional optical clarity, which makes it suitable for coating lenses and other devices requiring visual transparency, like photosensitive components.
Exhibiting minimal absorption in the visible spectrum, XY is transparent and colorless, providing optically advantageous characteristics, whose benefits can be enhanced when appropriately strategized during film application. Parylene’s unique chemical vapor deposition (CVD) process applies powdered-XY dimer to substrates in a gaseous form, which penetrates targeted surfaces, effectively adding an operational underlayer to its overall external conformal protection. The resultant optical clarity is sufficient to maintain the visual integrity of museum/gallery level artwork and culturally important archival items. XY coatings also enhance visual clarity and performance of light-emitting diode (LED) systems.
Properly managed, CVD can also add to the film’s normal visual clarity. Reduction of peak chamber pressure during polymer deposition can improve many aspects of film quality, including adhesion, coating uniformity, and transparency. With respect to adapting current CVD methodologies for physiological and biological systems’ applications,
- fabrication of multidimensional polymeric platforms can create highly-reproducible microtextured membranes,
- with added optical clarity at the micron/sub-micron level,
- to further study fundamental cell-environment interactions
- useful to improved healthcare.
XY-dimer materials initiate CVD as a white/off-white powdery solid, transformed to a clear, colorless coating. This property assures preserve excellent-to-superior optical clarity over time, with an accurate, well-defined view, especially in thin film form (<1 um.). Thinnest parylene films maintain true optical clarity, a quality that can diminish slightly as coating thickness increases; this change is most frequently almost imperceptible, since XY layer application rarely exceeds .50 mm., sufficiently minute to retain dependable visual perception through the film.
However, the same property also means XY-coated devices are visible to anyone investigating beneath the film, including someone interested in reverse engineering the item. In these cases — because thicker parylene layers will do little to interfere with visual clarity – additions to the coating may be required. To prevent potential incidents of fraudulently replicating proprietary or similarly exclusive design, additional pigmented liquid coatings – generally epoxy or polyurethane — can be added to completely cover the unit, concealing its components. These wet films possess surface resilience similar to XY, making their removal difficult, while obscuring the covered assembly.
Differences in the relation between optical clarity and coating thickness are evident to a minor degree, depending on parylene material type. For instance, parylene C is optically clear while parylene N exhibits a slight haze at thicknesses > 5 μm. These differentials remain visually insignificant, as thickness increases across XY types, but should be noted.
In addition, parylene types N, C, and D degrade after prolonged exposure to ultraviolet (UV) light, a condition that also affects parylene F at a much slower rate. Parylene AF-4 is far superior to the others in resisting film degradation but, along with F, is substantially more expensive to use than N, C, and D (4 times more for F, 15/20 times for AF-4).
Whatever the XY-type, evidence indicates the products of degradation absorb UV light, regardless of thickness. Yellowing results, increasing incrementally with length of exposure, and according to the UV-resistance level of each XY-type. Film yellowing will significantly diminish the coating’s optical clarity, regardless of its thickness.
Optical clarity is not the only parylene property negatively impacted by extended UV exposure. XY films ultimately oxidize under these conditions, causing main-chain scission, the source of molecular level breaks, and the development of larger-scale rupture throughout the coating surface. Thicker films will sustain surface integrity longer, but yellowing will eventually commence, and coatings will crack.
In most cases, however, colorless parylene generates advantageous optical properties for a wide range of uses — including LEDs, cameras/sensors, artwork/museum artifacts, healthcare/medical devices, computer touchscreens, and opto-electronic components supporting reliable aerospace, scientific, and telecommunication functions.
XY is recommended when protective conformal film is needed to safeguard products’ visual clarity and color. Unless subjected to UV or other elemental over-exposure, parylene coatings seldom are so thickly applied that their optical clarity declines so much covered surfaces are visually obscured.
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