Parylene (XY) polymer conformal films are recognized for their exceptional range of desirable functional properties for coating printed circuit boards (PCBs) and similar electronics. Beneficial properties include biocompatibility, chemical/solvent resistance, dielectric/insulative reliability, and ultra-thin pinhole-free film thicknesses between 1-50 μm. They also generate complete surface conformability, regardless of substrate configuration, exceeding the coating capabilities of liquid conformal materials, such as acrylic, epoxy, silicone and urethane.
Despite these advantages -- which allow XY to effectively encapsulate virtually all surfaces with durable protective conformal film – one question about its performance cannot be satisfied:
“Can parylene be used as a standalone enclosure?”
XY can effectively enclose any object or form fitting in the deposition chamber; however, it cannot standalone. Parylene requires a surface to adhere to before it can be successfully applied as conformal film.
For a device or object to standalone structural independence is required, to support
- maintenance of an upright posture, either horizontal or vertical, and
- ongoing functionality, without peripheral assistance or subsequent addition.
In this regard, a standalone object sustains and preserves its operational purpose, as a separate entity, exclusive of external support or power.
An enclosure is a physical construction that circumscribes an object, completely surrounding its structure, defining its internal spatial limits by its own material boundaries.
By this definition a standalone enclosure is a manmade (basement, courtyard) or natural (cavern, tree trunk) structure that requires no secondary aid to maintain and protect its position. This basic condition eliminates XY’s standalone abilities. Much has to do with parylene’s chemical vapor deposition (CVD) method of film application.
While CVD thin-film deposition techniques generate a versatile platform for a wide range of parylene coating applications, its procedural requirements also terminate any chance of using XY to create independent standalone enclosures. Completely excluding the liquid phase of pre-synthesized wet coatings, CVD polymerization synthesizes truly conformal protective film in-process. It does so by
- transforming powdered, solid parylene dimer into a gas at the molecular level,
- through heating the material to 100º - 150º C;
- further heating to 680º C sublimates the vaporous molecules,
- splitting each into a monomer,
- directed by vacuum into the coating chamber,
- where they deposit deeply and homogeneously into the substrate surface,
- creating a truly conformal, pinhole-free film both below and above the targeted surface.
These procedures are followed by rapid cooling, to between -90º and -120º C, helping to solidify the coating while removing residual XY materials from the substrate. The result is uniform film thickness conforming completely to the substrate, regardless of substrate topography, with excellent chemical, dielectric barrier and moisture protection, among many other performance benefits.
CVD processes can compel parylene to provide complete and conformal encapsulation of three dimensional objects. XY does this by attaching to the targeted surfaces, including those already assuming a standalone enclosure format.
But if the question is:
“Can parylene be used as a standalone enclosure?”
The answer is no.
To work at all – adhere to a substrate and provide ultra-thin conformal protection – XY must undergo CVD conversion, from a powdered dimer to a vaporous state. Parylene only acquires its coating capacity after infiltrating a substrate’s surface in a gaseous form, providing protection both below and above the substrate’s surface. As a vapor, XY can completely cover – encapsulate -- the flattest surface or larger structures of virtually any shape. In doing so, it encloses the targeted surface and form.
However, it cannot standalone precisely because it must be a vapor to provide conformal coating and, as such, simply lacks the physical constitution to provide a stable stand-alone molecular structure and shape. After it solidifies as a protective film, it has already surrounded, and infiltrated the surface of, the selected substrate. As such, it is already part of something else, and does not standalone, despite enclosure of the object.
Parylene vapor must form around something to generate conformal coating; standalone is not a possibility. If no target is provided to receive parylene CVD application, XY will remain in a gaseous form, adhering to whatever is available or free-float, without a material shape of its own. While it is adaptable for enclosing pretty much any physical configuration, including standalone structures, parylene lacks the material consistency to become standalone by itself.
In the future it may become possible to either:
- create a free-standing parylene structure enclosing nothing but air, or
- separate an applied XY coating from a standalone object, so that it too stands on its own.
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