Optical Clarity of Parylene at Increased Thickness

Generally applied at micron-thin coating layers, Parylene 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 mm to 0.051 mm (0.0005 in 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, Parylene 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-Parylene 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. Parylene coatings also enhance visual clarity and performance of light-emitting diode (LED) systems.

When managed effectively, chemical vapor deposition (CVD) can enhance the visual clarity of films. By reducing peak chamber pressure during polymer deposition, various aspects of film quality such as adhesion, uniformity and transparency can be improved. Adapting existing CVD techniques for physiological and biological applications involves creating intricate polymeric platforms. These platforms yield reproducible microtextured membranes with exceptional optical clarity at micron/sub-micron levels, facilitating in-depth study of cell-environment interactions.

Parylene dimer materials begin the the CVD process in the form of a white or off-white powdery solid, transitioning into a clear, colorless coating. This characteristic ensures the preservation of excellent-to-superior optical clarity over time, offering a precise, well-defined view, especially in thin film form (<1 µm). The thinnest Parylene films uphold genuine optical clarity, a trait that may diminish slightly as the coating thickness increases. However, this alteration is typically barely noticeable, given that the application of Parylene layers seldom surpasses 0.50 mm. This thickness is minute enough to maintain consistent visual perception throughout the film.

However, this feature also means that Parylene coated devices are visible to anyone inspecting beneath the film, including potential reverse engineers. In such scenarios, as thicker Parylene layers do not impair visual clarity much, additional coatings may be necessary. To prevent instances of fraudulent replication of proprietary designs, pigmented liquid coatings such as epoxy or polyurethane can be applied to fully conceal the unit and its components. These coatings, similar in surface resilience to Parylene, make it challenging to remove them and obscure the underlying 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 Parylene 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 HT^® at a much slower rate. Parylene HT is far superior to the other variants in resisting film degradation.

Regardless of the Parylene type, evidence suggests that the degradation products absorb UV light, leading to yellowing that intensifies with prolonged exposure. The degree of yellowing correlates with the UV-resistance level of each Parylene type. This yellowing significantly reduces the optical clarity of the film, irrespective of its thickness.

Extended exposure to UV rays does not just affect the optical clarity of Parylene. Over time, these rays cause Parylene films to oxidize, leading to main-chain scission, which creates molecular breaks and eventually larger ruptures on the surface. While thicker films can delay the onset of surface damage, yellowing will eventually occur, causing the coatings to crack. Despite this, colorless Parylene offers excellent optical properties for various applications, such as LEDs, cameras, medical devices, touchscreens, and opto-electronic components used in aerospace, scientific and telecommunication technologies.

For products requiring protective conformal film to preserve visual clarity and color, Parylene is the top choice. Unless excessively exposed to UV rays or other elements, Parylene coatings are typically applied thinly enough to maintain optical clarity without visually obscuring covered surfaces.