Parylene deposition- Conformal ultra-thin coating
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Parylene is the trade name for a chemical vapor deposited (CVD) based coating material, commonly used as electrical insulation, moisture barriers, corrosion/chemical damages protection and more. Parylene coating occurs as a result of polymerization of p-xylylene monomer which forms a flexible isotropic transparent layer. Parylene conformal coating has been utilized in the industry for a various of applications, among them biomedical implants, implanted medical electrical devices, space crafts, satellites, and military applications, corrosion protection for metallic surfaces and microelectronics and semiconductor industry etc.
Parylene conformal ultra-thin coating is also used for the strengthening of Nano and micro scale features and nanomaterials and sensors protection from rough and harmful environments. Parylene also holds lubrication properties similar to PTFE, and for that reason the coating is commonly utilized for friction reduction of, e.g., guiding catheters, medical acupuncture needles and MEMS systems. Parylene transparent quality makes it an excellent covering layer for optical equipment and elements.
Parylene is resilient to most chemicals and biologicals agents and is both inert and durable. Bacteria and fungi do not form biofilms over Parylene coated surfaces and the Parylene layer does not dissolve in any organic solvent at room temperature. The Parylene coating has superb mechanical characteristics, including high tensile and yield strength.
Ultra-Thin to thick Parylene layers (0.1-250 microns) can be deposited on almost on any surface, substrate or structure. One of the main attraction of the Parylene covering procedure is that the deposition process is carried out at room temperature, allowing the conformal coating over materials which are sensitive to even slight elevation in temperature. Parylene coating penetrates through even the smallest pores and cracks down to at least 10 µm deep.
Benefits of Parylene
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Ultra-thin pinhole-free, uniform thickness coating,
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Available from 400 Nanometer to ten of microns thickness
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Process is carried out at room temperature
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Conformal barrier layer
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Moisture and chemical protection
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Hydrophilic coating adhesion tie-layer
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FDA approved material for use in human implantable devices
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Lubrication properties similar to PTFE
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Widely used as Waterproofing Electronics
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Have high Dielectric Strength
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Good thermal endurance to a very wide temperature range (-200 ‘C to +200 ‘C)
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Items coated in Parylene can be put in an autoclave
Parylene deposition process
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Parylene conformal coating is achieved through a chemical vapor deposition (CVD) and the process typically consists of three-stage taking place in different parts of the system. Parylene raw material comes as a powder in the form of solid dimer precursor. The most commonly used dimers are: Parylene C, Parylene N, Parylene D, Parylene AF-4, while other less common types are available. In the first stage, the Parylene solid dimer is being loaded into a vaporizer chamber. The parylene raw material is than heated to temperature ranging between 100-170º C, causing the dimer to evaporate. At this point, the vapor is transferred under vacuum into a preheated furnace and goes through a thermal decomposition into monomers. Still in gas phase, the p-xylylene based monomer flows into the reaction chamber, where the sample is placed under vacuum condition. In the reaction chamber, molecule by molecule, the Parylene monomer starts to polymerized onto the requested substrate at to create unified Parylene conformal coating in the room temperatures reaction chamber.
Parylene removal process
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In order to perform tests and validations, sometimes Parylene coating removal is required. Parylene removal can be achieved by performing the following processes, which can be considered based on the coated sample properties:
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Chemical Removal: As mentioned before, Parylene polymers are known for their excellent chemical resistance. Parylene is practically insoluble in any organic solvents at temperature up to 150C and has an can withstand most chemical attacks at room temperature without any harm. Therefore Parylene cannot be dissolved off at room temperature by any solvent. That being said, Tetrahydrofuran (THF) can be used in order to temporarily weaken the adhesion between the Parylene coating and the sample, and by softening the coating the Parylene layers can be lifted off, like a crust, with tweezers. While this is seems like a crude method, which is not fitted for every task, this procedure can be used for small volume inspections. The main problem is that THF It is toxic, highly flammable, and as consequence severe safety measures must be taken when removing Parylene by THF.
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Thermal Removal: Parylene has demonstrate high heat-resistance qualities, from 290C for parylene C, upto temperatures reaching 500C for parylene D, N, and F. This is why thermal removal is considered as an intense method for the removal of parylene conformal coating. As the temperature increases, the Parylene polymer starts to break in to its monomers and evaporate. This can be performed by inserting the coated substrate into a furnace, or by using soldering tip in the proximity of a certain area. This method is only effective for restricted area repairs or temperature stable substrates, even though it is not considered a clean procedure. Components and parts can be damaged during this process, suffer from discoloration on the substrates and more.
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Laser ablation (LA): A much more precise Parylene coating removal technique which performed by a bursts of short wavelength focused laser beam across a sample surface. the Laser beam causes the Parylene coating material to go thought either fusion or sublimation during irradiation. The fused material is removed, and as a result, causes an alteration of the surface topography of the targeted substrate. In general, Laser ablation method can be used to drill, cut, or engrave a variety materials with great precision and reproducible results. Laser ablation can be used to pattern Parylene coatings when dealing with a substrate with high density and difficult to protect areas. When it comes to microelectronics, miniature electrosurgical equipment and implantable medical devices, an accurate Parylene coating can be proof to be with high necessity. As this process in is restricted to local treatments, the heat dissipates and does not harm the overall structure. However, this method is less likely to be used for total removal of Parylene coating over a large area, and is not compatible with complex structures such as round shapes and hollow samples.
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Physical abrasion Removal: Parylene has a poor resistance to physical wearing and scratch. Since the polymer is a soft flexible material, it’s intrinsic physical properties make the usage of mechanical micro-abrasion processes as an highly effective an therefore widely used Parylene removal procedure. The most commonly used materials for physical abrasion Parylene removal are from sand (sand blast), aluminum oxide, various beads, sodium bicarbonate etc. The main issue of using such a rough and discriminating procedures is the roughening of the substrate surface and weakening of its structure. In some cases, the particles are hardly removed from cracks and cavities.
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We have developed a unique know-how and methods for the removal of Parylene coating down to the sub-microns targeted areas and up to the centimeters scale area. Our developed methods overcome the problems of damaging sensitive parts by excessive heat which occur during other commonly used techniques.
Once the parylene coated area is removed for client’s purposes, the specific area could recoated with other protective layer. However, the general recommendation is that the whole part would be recoated again with parylene.
Parylene Types
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Parylene N: Type N is the basic sort of the p-xylylene monomer, and as suggested by its name it is the most natural, with the highest crystalline form material; each molecule consists of only a carbon and hydrogen atoms. When compared to the different Parylene types, N’s has an relatively low-dielectric constant and for that resons is a very popular choice when high frequency applications requires Parylene coating. Parylene N has melting point of 420°C, which is much higher than either C and D. N gives a superb vacuum stability as well an highly conformal coating, and a total weight-loss of up to 0.30% at 49.4° C, and 10-6 torr.
Parylene C: The most commonly used Parylene monomer for isometric coatings, Parylene C is a poly para-xylene, produced from dimeric powder, with one chlorine group monomer. Even though Parylene C is a carbon-hydrogen combination similar to the Parylene N, it differs from it by the chlorine atom in place of one hydrogen atom of the phenyl group. Type C parylene conformal films generate exceptional protection from corrosive gases, due to low chemical, moisture, and vapor permeability. Parylene C generates high moisture resistant, its coating process is rapid, and occurs at room temperature by the Parylene’s extraordinary CVD process. Type C’s lesser has a lower cavity-penetration capabilities, while providing effective coating protection through continuous oxygen-contolled environments exposure for 100,000 hours at 100°C. Parylene C’s vacuum stability is verified with performance ratings of 0.12% total weight-loss, at the same standards as Parylene N (49.4° C/10 -6 torr).
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Parylene D: Parylene Type C and parylene D begin from an identical building block. Like parylene C, parylene D's synthesis varies from parylene N; it has two chlorine atoms instead of two hydrogen. In spite of the fact that it can endrure temperatures as high as 125°C, Parylene D provides adequate biocompatibility for wide uses in medical devices. Parylene D offers viable protective coating at 134° C through in similar condition as Parylene C.
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Parylene AF-4: This product provides a higher oxidative resistance and stronger UV stability than other Parylene monomers. The Parylene AF-4 is used for special applications such as satellites/spacecrafts and military equipment. However, its precursor demand a three-step production process, which increases costs, and it has limited yield.
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Parylene F: Type F is fluorinated, replacing the choline atoms by fluorine atoms on its phenyl ring; It has a significantly lower coating capacitation, lowering the coated surface’s electrical charge while operating. Parylene F exhibits a greater throw-capability than Parylene Type C, with an impressive higher crevice penetration-activity. It’s deposition rate is much lower in comparison to Parylene C (and other Parylene types. The F dimer currently not highly used by commercial application.
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