The Early Stages of Parylene VDP on a Substrate Which is Permeable to Monomer.
This class of substrates includes Parylene itself (as experienced in restarting a coating process or overcoating), as well as rubber and plastic materials in general, including especially the cured epoxy gel coat of a printed wiring board.
Upon exposure to gaseous monomer under typical deposition conditions, availability of monomer from the gas and diffusivity of monomer within the substrate are sufficient that a growth zone of a few thousand Angstroms depth will be developed essentially instantly (<1 second). The chemistry, however, is a little slower. In the case of a Parylene substrate, if free radical chain ends are present, even if they are peroxides as a result of momentary exposure to atmospheric oxygen, the chains will continue to grow by the reaction with incoming monomer. In the absence of such radicals, which is usually the case when the substrate is other than Parylene, chain propagation must await initiation events. Because the initiation chemistry is third order in monomer, the region in which initiation occurs is one-third the depth of the zone in which the bulk of the polymer is formed by propagation.
Nevertheless, new parylene chains are initiated and grow in length in the midst of the substrate polymer, offering a unique mechanism for adhesion via chain entanglement which is unavailable to any other polymer system. One of the requirements for this entanglement adhesion mechanism to be successful is that the substrate polymer have good physical strength right out to the growth interface. A 500 Angstrom layer of undercured material or mold release deposit on the substrate might defeat the adhesion process and produce a blistering coating.
Parylene VDP on a permeable substrate starts out faster than on an impermeable substrate, and quickly develops into steady state growth. In the early stages, when both types of surfaces are present in applications such as circuit board coating, the thickness of the Parylene layer on the permeable substrate gets ahead of that on an impermeable substrate, a differential which after steady state growth is achieved over both substrates is never closed.
An additional feature in the case of non-Parylene substrates is that the composition in the vicinity of the original substrate interface moves gradually from totally substrate material to totally Parylene over a distance of several growth zone depths bracketing the original interface position.