What is Parylene Coating
Parylene is the abbreviation for the polymer group named poly (para-xylylene). These polymers consist of differently substituted para-xylylenes. The initial state for the coating is given by the so-called dimers. A dimer is a molecule composite which consists of two identical sub-units, namely the monomers.
Why are Parylene coatings so unique?
- Parylene is an organic polymer which consists (in its basic form as parylene N) only of the atoms of hydrogen (H) and carbon (C). Parylene is hydrophobic and resistant to almost all chemicals. This applies to other polymers such as PTFE, too, but the special properties result from the exceptional manufacturing technology.
- Parylene have an economic importance only as thin coatings. The polymer is formed by polymerization of the gaseous monomer on the cold substrate surfaces. All liquid coatings contain gas inclusions and the coating tends to contract locally, also with a low surface tension. This results in forming of gaps, edge loss of the coating and inhomogenous film thicknesses. By the polymerization of parylene directly from the gas phase, the molecules are fitting to one another. There exists no pores, but a constant film thickness at molecular scale.
- Parylene polymerizes on cold surfaces. There is no thermal load at the substrates. Almost any material can be coated by Parylene.
- Parylene coatings have excellent barrier properties against almost all substances. Parylene form a very high and especially reliable protection against chemicals, environmental influences and ageing.
What is the performance of Parylene?
Parylene coatings are superior to all other coating methods in the following properties:
- Continuity of the coating thickness
- Coverage of edges and spikes
- Penetration into extremely thin columns
- Tightness with minimum layer thickness
- Barrier properties against permeation of gases and liquids
- Protection against moisture
- Protection against electrical breakthrough
- Protection against oxidation
- Ageing resistance and protection against material ageing
In addition to the basic type and halogen-free Parylene N, the types Parylene C, Parylene D, Parylene F-VT4 and Parylene F-AF4 are used commercially. All Parylene types have the properties mentioned before.
But if special requirements are needed, e.g. regarding high-temperature stability, electrical properties or barrier properties, so the Parylene type should be chosen with the most suitable properties.
Parylene are derivatives of benzene. The basic form of Parylene N consists of a benzene molecule. At the benzene ring, at two corners, each hydrogen atom is replaced by a CH2 group. The prefix "para" (abbreviated "p") indicates that these two CH2 groups are attached to the opposite corners of the benzene hexagon.
Parylene N is therefore a pure hydrocarbon.
But one or more hydrogen atoms can be replaced by halogen atoms at the Parylene molecule. Halogens are the chemical elements fluorine, chlorine, bromine and iodine. Thus, a variety of parylene derivatives can be formed theoretically. Only the types parylene N, parylene C, parylene D and parylene F-VT4 are of practical importance. In addition, there is the Parylene F-AF4.
Advantages & Properties
Parylene coatings offer a wide range of benefits. Parylene coatings …
- are perfectly conformal: that means that the coating is adapting also on complex substrate contours such as sharp edges or holes.
- are "pinhole-free“, starting at layer thicknesses of about 0.5 microns.
- are chemically insoluble and resistant to a wide range of chemicals.
- exhibit very good barrier properties to moisture and chemicals.
- own a high dielectric strength.
- possess dry lubricating film properties (low friction coefficient).
- are hydrophobic - contact angle of H2O between 92 ° and 98 °.
- are transparent between 90 and 96 % in the range of the visible wavelength (depending on the type of parylene).
- are biocompatible - The parylene types C and N can be certified according to USP Class VI, ISO 10993 and FDA.
The following outlines the Parylene Coating system;
- The deposition takes place by polymerization through a vacuum assisted coating process, the so-called chemical vapor deposition – CVD.
- The “dimer” (solid [2,2]-p-cyclophane), which is present in a powder-like shape, will be sublimated within the the vapourizer.
- The thermal decomposition of the relatively inert „dimer-gas“ is carried out using a pyrolysis tube with temperatures of about 650 °C, resulting in the formation of highly reactive monomers.
- The process pressure is dependent on the used dimer-type and the dimensions/construction of the deposition system - common values are between 0.02 and 0.1 mbar. Via diffusion the reactive monomers enter into the vacuum chamber in which the coating material is placed on a rotating frame.
- The reactive monomers preferably polymerize on cold surfaces, and form a thin layer of poly(para-xylylene) or rather Parylene. Since not all of the monomers polymerize inside the chamber, it is necessary to work with a cold trap which is installed behind the chamber to avoid a coating/damage of the vacuum pump.
During the process, the trap can be easily filled with liquid nitrogen, so that the residual monomers will polymerize in the cold trap.
Types of Parylene
Parylene N - Basic Parylene Type
- consisting only of atoms of hydrogen and carbon and is therefore halogen-free
- Very good crack penetration.
- Very good dielectric properties and dielectric strength (low dielectric constant)
- Lowest friction coefficient*, often used for equipment of minimal invasive surgery
- FDA approval
Long-term temperature resistance**: 60 ° C
Parylene C - Most Common Parylene Type
Most common parylene type
- One chlorine atom on the aromatic ring
- Lowest permeability of humidity (H2O) and other gases and thus excellent barrier properties and good corrosion protection.
- Relatively high deposition rate, thus the most economical deposition process.
- Good mechanical properties
- FDA approval
Long-term temperature resistance**: 80 ° C
Parylene D - Mechanical Properties
Good mechanical properties
- Two chlorine atoms on the aromatic ring
- Low permeability of humidity (H2O) and other gases
- Highest temperature resistance of chlorinated types
Long-term temperature resistance**: 100 ° C
Parylene F-VT4 - Thermally Higher Load than D
Since thermally even higher loads than Parylene D, it displaces this increasingly
- Four fluorine atoms on the aromatic ring
- Good dielectric properties
- Good crack penetration
Long-term temperature resistance**: 200 °C
Parylene F-AF4 - Highest Temperature Resistance
Best crack penetration.
- Four fluorine atoms on the aliphatic groups
- By far the highest temperature resistance of all Parylene types - up to 350 °C long-term temperature stable**.
- Resistant to UV radiation
- Very low coefficient of friction*
- FDA approval
- Excellent dielectric properties and dielectric strength (low dielectric constant)
- The most expensive version, therefore only used when these special properties are necessarily required.
* On steel without lubrication ** 11,4 years
When do you need Parylene?
- When electronic components have to fulfill highest safety and durability requirements (Aerospace).
- When components are exposed to extreme environmental conditions.
- When very worthy pieces should be kept "forever" (antiques, jewelry).
- When seals should "never" become brittle.
- When diffusion should be prevented by walls (pipes, containers).
- When implanted components should not react with fluids from the human body (Medicine).
- When sensors should be protected against aggressive media.
- When metals have to be protected against oxidation.
- When good lubricity (and low abrasion) is important (catheters, cannulae)
- high performance and certifications according to US MIL and US NBC (Nuclear Biological and Chemical Contaminations) secure a wide range of applications in military / aerospace applications. Parylene are also UL listed (Underwriters Laboratories) for safety-related applications.
- FDA Approval for food contact and USP (United States Pharmacopoeia) Class VI certification allow unrestricted contact with foodstuffs, pharmaceuticals and living body tissue, as well as in medicine and pharmacy.
All kinds of sensors
Probes and Endoscopes
Ampoules / bags
Implants, Corona Stents
Radar / Detectors
Rotors / Stators / Motors
Monitoring and Control System
Battery / Cell Systems
Radar / Detectors
Aviation / Automotive Lighting
Bottles / Containers