Parylene

Unparalleled surface protection

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Parylene

Unparalleled surface protection

Diener electronic pursues the aim of offering the ideal surface solution for all applications. In many cases, a parylene coat is the ideal solution for the protection of valuable components, assemblies or devices.

Parylene is a group of polymers whose chemically exact name is para-xylylenes. In a process technology that can only be used for this material class, almost transparent protective layers can be produced.

The benefits of parylene

✓ contour-independently, constant coat thicknesses with deviations of under 1 µm
✓ from 1 µm, reliably "pinhole-free", i.e. without faulty spots
✓ thermally resistant (depending on parylene type)
✓ chemically inert (resistant to a wide range of chemicals)
✓ resistant to environmental influences and UV radiation
✓ very good barrier characteristics
✓ on surfaces cleaned and pre-treated with plasma excellent adhesion
✓ cover even complex contours, edges, cracks and cavities consistently

What are parylenes?

What makes them so unique?

In 1947, the chemist Michael Szwarc researched the reactions of xylole (also "xylene") at high temperatures up to 1000 °C. In this process, he discovered a transparent precipitation at the cold surfaces of his apparatus. This solid film was analysed as poly(p-xylylene).

As a more effective process for manufacturing this film, Union Carbide in 1955 introduced the method which is still used today: pyrolysis of the dimer paracyclophane. This opened the way to commercial application. Union Carbide also gave the polymer poly(p-xylylene) the catchier name parylene.

Parylene is an organic polymer which (in its basic form as parylene N) consists only of hydrogen (H) and carbon (C) atoms. Parylene is hydrophobic and resistant to almost all chemicals. This is also true for other polymers such as PTFE, but the very special characteristics are the result of extraordinary manufacturing technology. They are economically significant only as thin coatings. The polymer is formed by polymerisation of the gaseous monomer on the cold substrate surfaces. All liquid coatings comprise gas inclusions and have a tendency to contract locally even at low surface tension. This results in cracks, poor edge covering and variable coat thickness. Due to the polymerisation of parylene directly from the gas phase, molecule attaches to molecule, there are no pores, no poor edge covering, and a constant coat thickness on a molecular scale. Parylene polymerises on cold surfaces, which avoids thermal load on the substrate. Parylene is suitable for coating almost any type of material.

Parylene coats have extraordinarily good barrier properties against almost all substances and thus offer unparalleled high and above all reliable protection against chemical attack, environmental influences and ageing.

Additional information

Pre-product: Dimer

Chemically accurate term: paracyclophane or Di-p-xylylene

More practical term: parylene-dimer
 

After pyrolysis: monomer

Chemically accurate term: quinondimethane or Di-p-xylylene

More practical term: parylene-monomers
 

End product: polymer

Chemically accurate term:  Poly(p-xylylene)

Catchier term: parylene N

What are the benefits of parylene?

Parylene coats are superior to other coating materials in terms of the following characteristics:

✓ Constant coat thickness
✓ Covering of edges and peaks
✓ Penetration into extremely thin cracks
✓ Tightness of extremely thin coats
✓ Acts as a barrier against permeation of gases and liquids
✓ Protection against moisture
✓ Protection against electric breakdown
✓ Oxidation protection
✓ Ageing-proof, protection against material ageing
✓ Biocompatibility

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.

Parylene systems

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.

Schematic diagram of a parylene coating system with horizontal chamber

1) Vaporiser (resistance heating: temperatures typically 130-180 °C)

2) Pyrolysis tube (resistance heating: temperatures typically 550-650 °C)

3) Vacuum chamber (basic pressure approx. 0.01 mbar; working pressure between 0.02 – 0.1 mbar)

4) Rotating table (rotating substrate carrier)

5) Cold trap (e.g. liquid nitrogen: temp. approx. -196 °C)

6) Vacuum pump

7) PC control: setting and monitoring the process parameters

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