As a rule, parylene systems are vacuum systems. Accordingly, they always comprise a vacuum-tight and pressure-tight vacuum chamber and a vacuum pump. At approximately 0.02 to 0.1 mbar, their operating pressure is not particularly low. To ensure a good coating quality, foreign molecules should be removed as thoroughly as possible. For this reason, sophisticated sealing, omission of gas-emitting components and a powerful vacuum pump are required.
Between the vacuum chamber and the vacuum pump, a cold trap is installed in which the residual parylene monomer polymerises which is extracted from the vacuum chamber. If monomer were to get into the vacuum pump and polymerize there, it would damage the pump.
Parylene Coating Systems
The Properties of Parylene
Parylenes are benzene derivatives. The basic form parylene N consists of a benzene molecule on whose benzene ring the hydrogen atom is replaced at two corners by a CH2 group. The prefix "para-" (abbreviated "p-") indicates that these two CH2 groups are attached to opposite corners of the benzene hexagon.
Parylene N is a pure hydrocarbon.
However, in the parylene molecule one or several hydrogen atoms can be replaced by halogen atoms. Halogens are for example the chemical elements fluorine and chlorine. Theoretically, a multitude of parylene derivatives can be formed using these variations. But the only types which are of significance in practice and commercially used are parylene N, parylene C, parylene D, F-VT4 and parylene F-AF4.
All of them can be used in parylene systems with similar parameters. The resulting coats also have similar properties. However, if the already excellent properties of parylene N with regard to dielectric, thermal and barrier properties are not sufficient, the alternative parylene types can be used.
Characteristics of The Parylene Types
Parylene N
Basic version, consists only of the atoms hydrogen and carbon. Not the most widely used type. Extremely good crack penetration. Optimum dielectric properties and strength; therefore preferred for coating electronic components and assemblies. Lowest friction coefficient; therefore often used in catheters.
Parylene C
Most widely used product with excellent barrier effect. High moisture protection, also due to particularly strong hydrophobic properties. High elasticity, can therefore be used for plastic and elastomercoats. High coat thickness growth (up to 10 µm/h).
Parylene D
Widely used for many years because of its increased temperature stability, but is highly hydrophobic at the same time. Used to protect electronic components in aerospace applications.
Parylene F-VT4
Increasingly replaces parylene D in high-temperature applications since it can take even higher thermal loads.
Parylene F-AF4
By far the highest temperature resistance of all parylene types. Furthermore, this type is extremely resistant to exposure to aggressive radiation, in particular UV exposure. Because this variant is the most expensive, it is used only if these specific properties are absolutely required.
Coating All Parts of The Contour
Contrary to coats applied in liquid form, the gas-phase monomers also reach the parts of the component which are inaccessible for liquid coatings.
Good For Health and The Environment
Since parylenes are chemically highly inert and do not contain any foreign materials, they are classified as non-toxic and not harmful. Parylenes meet all requirements regarding food safety and biocompatibility. Furthermore, they are not harmful to potable water and to the environment. Parylenes comply with the European Directive RoHS 2002/95/EC (restriction of the use of certain hazardous substances in electrical and electronic equipment).
The raw material is always the pure dimer. As a rule, parylene coating processes do not carry out any modification by means of additives, stabilizers or alloys. Accordingly, the values in the table generally apply to the parylene types by all manufacturers. Nevertheless, there are differences in quality. The precondition for excellent parylene coats is an extremely pure dimer.
