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Conformal Coating Crevice Penetration Using Parylene


By Sean Horn, Diamond-MT Parylene Coating Services P l


arylene (XY) polymers provide robust, dielec- tric, micron-thin conformal coatings for a considerable range of electronic devices, most


prominently printed circuit boards (PCBs). XY’s unique chemical vapor deposition (CVD) applica- tion method synthesizes in-process, depositing gaseous parylene deep into a substrate’s surface. CVD occurs on a molecule-by-molecule basis,


conforming to all underlying contours, regardless of shape or position — to the nanometer, if neces- sary. Pre-synthesized liquid coatings lack many of parylene’s performance properties, having far less ability to successfully and conformally penetrate crevices in the substrate. Additional CVD advantages include:


Room-temperature processing, allowing damage-free coating of even the most delicate components.


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Vaporous XY flows anywhere there is air, coating under parts, inside small crevices and semi-sealed areas.


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Sub-surface penetration seals crevices and irregular, tightly confined areas, while forming an additional layer of protection.


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Micro-thin films coat exceptionally confined topographies, without forming bridges, dripping or running like liquid coating materials.


Although there are two basic kinds of pary-


lene dimer — C and N — other formulations exist, including D, F and HT. Each displays specified performance metrics, characterized by different electrical and physical properties.


Diagram showing the contrast between a liquid and parylene coating.


Types of Parylene Of parylene types, Type HT has the highest


penetrating ability, as well as the lowest coeffi- cient of friction and dielectric constant. It is also


expensive to make and used essentially for special- ized purposes, limiting its application for a wide range of conformal coating assignments. Of the more common XY varietals, Parylene


N is the base structure of the parylene group, offer- ing a higher dielectric strength than C, F or D. In addition, it penetrates crevices and other cracks or gaps in substrate surfaces more capably than the others. Type N does this because enhanced molec- ular activity occurs during CVD, in comparison to the remaining XY varieties, allowing more efficient coating of relatively deep recesses, blind holes, tubes and small openings. Parylene C is used more frequently than any


other XY variety. It offers better performance across most metrics, generating exceptional pro- tection from corrosive gases, due to low chemical, moisture, and vapor permeability. Depositing quickly on most substrate sur-


faces, C’s lesser throw capability can reduce crevice penetration activity. It provides less crevice penetration than Type N. With chlorine atoms added to its benzene ring, Parylene D film has greater thermal stability than N, F or C. Crevice penetration capacities are lower in comparison to other XY varietals. With a chemical structure of four fluorine


atoms on the aromatic carbons, Type F provides good crevice penetration. Its dimer is also very ex- pensive, limiting its use for many conformal coat- ing projects. CVD processes convert powdered parylene dimer into a vapor. The resultant coating substance


Continued on next page


May, 2019


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