Date : 2020-05-29
Diamond MT is specialized in xylylene (parylene derivatives) deposition for use in all the industries where it is applicable. The basic parylene monomer is Parylene N (poly-para-xylylene). The derivatization of new varieties can be achieved by the addition of functional groups to Paryelene N main-chain phenyl ring and its aliphatic carbon bonds. Parylene N’s modification by a functional group such as Chlorine and Fluorine leads to Parylene C (poly(2-chloro-para-xylylene)) and Parylene F, respectively. Derivatization results in a set of new material properties: %crystallinity, melting temperature, resistivity, mechanical and electrical properties.
Conformal coatings of parylene derivatives is achieved by the CVD process also named as Gorham after the scientist who achieved 100% yield under vacuum deposition conditions. CVD process results in the formation of polymers with high molecular weight. The parylene film formation process takes place by the polymerization mechanism (chain growth type) , . CVD process is done under vacuum and is achieved in 3 steps in different parts of the CVD instrument:
Parylene thin films find applications in numerous products and processes. Some of the application areas can be listed as follows:
While there is diverse types of applications parylene conformal coatings are mostly used for PCB and electronic device encapsulation/sealing purposes. MIL-I-46058 is a military standard that covers Parylenes and their testing for use as a protective layer on PCB’s. The required thicknesses are 0.0006 ± 0.0001 inch (15.24 ± 2.5 μm) .
Keys for success:
- Expectations from the conformal coating must be in alignment with the area of application. The type of parylene derivative used must comply with the standards of the application and it may or may not conform to all the standards out of this application area. Therefore, we offer to discuss your requirements with a conformal coating professional at Diamond MT.
- Masking of the substrates must be done before the parylene coating process. Removal of parylene is relatively hard and may harm the whole device if the masking areas are not defined before the process. It is vital for our clients to indicate the masking areas once they contact our professionals at Diamond MT. Masking assures selected assembly components are not covered by the applied parylene film, which would inhibit their functionality. For most applications, use of conventional masking materials and techniques obstructs parylene deposition on designated PCB keep-out regions. However, masking for MEMS/nano medical devices is more challenging and requires advanced masking solutions.
- Substrate surface cleanliness: The surface where the interface between the conformal coating and the substrate will be formed is of high importance. The cleanliness of this surface has a great impact on the final results of the conformal coating process and the coatings durability. The surface energy is changed by organic residues and dusts resulting in either uncoated areas or delamination of the coatings. At Diamond MT we provide professional surface cleaning services ensuring the long lasting results for your components. Alternatively, the surface can be cleaned by the client before handing over the substrates. However, this approach may result in organic deposition (carbon residues) and we suggest surface cleaning and preparation to take place just before the coating process.
- Trained professional operators: Diamond MT ensure optimal masking and coating process for complex structures. We can work on your topographical substrates to ensure the best coverage and do trials before working on the final product.
- Cooperation: We understand that some organizations have highly trained professionals who are experts in conformal coatings. If you are one of these lucky organizations we will be happy to coat your substrates and fully cooperate with the professional. If you do not have a conformal coating professional our experts are in your services and will be happy to guide your through the selection of materials and coating processes till the final product is obtained.
About Parylene Conformal Coating - Conformal Coating Services
Diamond MT is proud to offer Global Subcontract Parylene Conformal Coating and Liquid Conformal Coating Services: Offering ALL types of conformal coating including acrylic coating, epoxy coating, silicone coating, urethane coating, and parylene coating. Facilities Available in the United States, Europe, and Asia Conformal Coating systems available including selective spray robots and dip machines Low, Medium, or High Volume Capacity Cleaning and Ionic Contamination Testing also available 100% Inspection of completed product Dedicated Services within customer facilities, as required Qualified with automotive, aerospace, medical, electronics, and defense companies globally.
Parylene is considered by many to be the ultimate conformal coating for protection of devices, components and surfaces in electronics, instrumentation, aerospace, medical and engineering industries. Parylene is unique in being created directly on the surface at room temperature. It is chemically stable and makes an excellent barrier material, has excellent thermal endurance, as well as excellent mechanical properties and high tensile strength.
- There is no liquid phase involved. Coatings are truly conformal, of uniform controllable thickness, and are completely pinhole-free at thicknesses greater than 0.5µ.
- Parylene coating completely penetrates spaces as narrow 0.01mm.
- No initiators or catalysts are involved in the polymerization, so the coating is very pure and free from trace ionic impurities.
- Room temperature formation means the coatings are effectively stress-free.
- Parylene is chemically and biologically inert and stable and make excellent barrier material.
- Parylene is unaffected by solvents, have low bulk permeability and are hydrophobic. Coatings easily pass a 100hr salt-spray test.
- Parylene has excellent electrical properties: low dielectric constant and loss with good high-frequency properties; good dielectric strength; and high bulk and surface resistance.
- Parylene has good thermal endurance: Parylene C performs in air without significant loss of physical properties for 10 years at 80°C and in the absence of oxygen to temperatures in excess of 200°C.
- Parylene is transparent and can be used to coat optical elements.
- FDA approval of parylene-coated devices is well-documented. The coatings comply with USP Class VI Plastics requirements and are MIL-I-46058C / IPC-CC-830B listed.
- Parylene coatings are completely conformal, have a uniform thickness and are pinhole free. This is achieved by a unique vapor deposition polymerization process in which the coating is formed from a gaseous monomer without and intermediate liquid stage. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
- Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases.
- Parylene coating has excellent mechanical properties, including high tensile strength.
- Parylene is stable over a very wide temperature range (-200 ‘C to +200 ‘C), allowing the chamber items coated in Parylene to be put in an autoclave.
Here is a brief list of some of the items that can be parylene coated:
|Printed Circuit Boards||MEMs||LEDs|
|Ferrite Cores||Metallic Blocks||Optical lenses|
|Molds||Motor Assemblies||Power Supplies|
|Test Tubes||Probes||Fiber Optic Components|
|Pace-makers||Bobbins||Plus Many More...|
Relatively easy to understand, the parylene deposition process can be difficult to implement, particularly with respect to controlling coating-thickness and otherwise ensuring a successful coating cycle.
Because coating type and required surface thickness vary according to substrate material and coating-project, deposition rates fluctuate. Processing can require less than an hour or more than 24 hours, at a deposition rate of about .2/mils-per-hour. While this slower rate of substrate covering generates parylene\'s superior conformal coating, compared to other coating options, it also adds to its cost. Mastering the parylene coating process helps assure these production expenditures are diminished.
Parylene\'s complex and specialized vapor-phase deposition technique ensures the polymer can be successfully applied as a structurally continuous film, entirely conformal to the characteristics of the selected substrate. To correctly master the process, assure each incoming order possesses all pertinent information affecting parylene application. This will include drawings, specifications, and special instructions that distinguish the order from others, allowing creation of customized solutions for the particular item.
Parylene deposition process completely eliminates the wet deposition method used by such other coating materials as epoxy, silicone, or urethane. It begins in a chemical-vacuum chamber, with raw, powdered parylene dimer placed in a loading boat, and inserted into the vaporizer. The dimer is initially heated to between 100º - 150º C, converting the solid-state parylene into a gas at the molecular level. The process requires consistent levels of heat; the temperature must increase steadily, ultimately reaching 680º C, sublimating the vaporous molecules and splitting it into a monomer.
Drawn by vacuum onto the selected substrate one molecule-at-a-time in the coating chamber, the monomer gas reaches the final deposition phase, the cold trap. Here, temperatures are cooled drastically to levels sufficient to remove any residual parylene materials pulled through the coating chamber from the substrate, between -90º and -120º C.
Mastering the parylene coating process requires detailed attention to these procedures, prior to commencement of deposition and coating:
Thorough inspection of incoming items to-be-coated, verifying their quantity and condition. Preparation procedures enacted as necessary. For instance, cleaning/cleanliness-testing, or similar unique processes, are commenced, followed by masking of connectors and electrical components. Accumulated substrate contaminants diminish adhesion, so assuring appropriate levels of surface cleanliness is integral to parylene coating. Depending on the substrate surface, cleaning may be enacted manually, or through application of batch, inline, or ultrasonic methods. Most materials--glass, metal, plastic, etc.--require treatment with A-174 silane to effect appropriate surface modification before parylene application. Typically, doing so employs either manual-spray, soaking, or vapor-phase technology, applied after the masking operation, A-174 silane\'s molecule forms a unique chemical bond with the substrate\'s surface, sufficient to improve parylene adhesion.
Masking is exceptionally labor-intensive. Exceptional care is required to ensure every connector is effectively sealed, so gaseous parylene molecules do not penetrate their surfaces. All tape, or other covering materials, must thoroughly encompass the keep-out regions, without gaps, crevices or other openings, to ensure connector function is retained after coating.
Further inspection assures masking is in compliance with customers\' specifications.
The diversity of adhesion promotion methods requires a similarly diverse list of raw materials. Establishing best-adhesion practices is only part of mastering the parylene coating process; once established, strict adherence standards need to be reliably enforced to ensure quality of the conformal coatings. Using industry best practices, such as substrate cleansing and A-174 silane application, appropriately combined with standard, repeatable processes, will ensure strong adhesion for parylene coating. Adhesion promotion methods are typically used prior to the actual coating process, however some can be integrated during the process itself.
The parylene coating is applied through the deposition process described above. Once coating has been deposited, masking materials are removed; extreme caution must be exercised not to damage the thin layer of applied parylene.
An important consideration of appropriate parylene thickness is total required clearance. While an enclosure-PCB has few clearance issues, in many cases even an additional millimeter of parylene coating can be sufficient to generate dysfunctional mechanical abrasion, damaging the parylene surface and reducing its conformal qualities.
Regarding dielectric strength, items whose required levels of dielectricity are higher will need a thicker coat of parylene. Balancing dielectric strength with clearance generally requires quality testing to determine their correct ratio. The end-item customer may not always provide these specifications; learning how to determine dielectric/clearance ratios without this data is integral to mastering the parylene deposition process.
The coating process must generate a conformal covering explicitly meeting the customer\'s precise specifications. If changes are necessary, making them to order and on time are essential elements of mastering the parylene coating processes. A final inspection ensures successful completion of all process phases, and that the final product complies with the customer’s drawings and specifications.
The raw material, parylene dimer, is rather expensive ranging from $200-$10,000+ per pound. Because parylene is applied through a vapor deposition process, everything, including items that do not need to be coated like inner diameter of the chamber, gets coated. This makes parylene an inherently inefficient process and wasteful with materials, which escalates the end cost to the customer.
Masking and otherwise prepping an article for parylene coating can be a labor intensive affair. Because parylene is applied as a vapor, it literally gets everywhere that air can. Our operators and quality inspectors must take this into account prior to coating to ensure that every one of the customer’s coating free areas are just that.
One major issue that often comes up for several of our high volume manufacturers is the limited throughput of parylene. Runs of the parylene machine can take anywhere from eight to over twenty-four hours. As a result of the limited chamber space, there is a fixed amount of product that can be processed during one coating cycle. This, coupled with the high capital cost of new equipment, can wreak havoc with our internal and our customer’s delivery schedules.
One final disadvantage of parylene to consider is the poor adhesion to many metals. Parylene has always had poor adhesion to gold, silver, stainless steel and other metals. Many printed circuit board manufacturers use gold in their products because of its conductivity. While there are some adhesion promotion methods that will greatly improve adhesion to these metals, they are either material or labor heavy and can increase costs significantly.
Date : 2020-05-29
At Diamond MT, we offer parylene coatings of different polymeric varieties (N, C, and F) as listed in the following Table. The basic parylene molecule is the Parylene N (poly-para-xylylene) monomer. Modification of the Parylene N monomer by a functional group such as Chlorine and Fluorine leads to Parylene C (poly(2-chloro-para-xylylene)) and Parylene F, respectively. The derivatization of new varieties can be done by the addition of functional groups to Paryelene N main-chain phenyl ring and its alip
hatic carbon bonds. Derivatization results in a set of new material properties: crystallinity, melting temperature, resistivity, mechanical and electrical properties. New derivatives with enhanced properties can be used under different service conditions.
Dielectric constant (@ 1 MHz)
Dielectric constant (@ 1 kHz)
Dielectric constant (@ 60 Hz)
(@ 1 MHz)
(@ 1 kHz)
(@ 60 Hz)
Durable Heat Resistance
(@Temperature 25.0 °C)
Parylenes are pure organic polymers that contain trace amount of impurities or zero percent chloride, sodium, potassium ions . Parylene thin films offer complete coverage of surface even between closely spaced features that is useful in microelectronic applications. Due to their multiple advantages they have a wide application area in various industries.
Parylenes are transparent in the visible region of the solar spectrum. They exhibit a high absorption in the UV- region, λ˂ 350 nm. This high absorption results in their photodegradation process through chain scission and formation of macroradicalar species . Accordingly, the photodiscoloration of Parylene C was reported to be higher compared to that of Parylene N. For all cases, parylene conformal coatings are advised to be used indoors without direct exposure to sun. Parylene F on the other hand is less susceptible to photodegradation. A problem interfering with wider scale adaptation of Parylene F is difficulty synthesizing dimer. Low availability of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.
One of the advantages of parylene conformal coatings is that they are stress free, this aids in protecting the underlying application from generated stresses due to the coating process. They are light and mechanically strong with high tensile and yield strengths (table).
As discussed earlier Parylene N is the most basic form of parylenes, its advantages are its availability, constant dielectric coefficient at all frequencies, low friction and high dielectric strength. Parylene N is has a lifetime of 10 years at 60°C before failure, at 80°C it fails in a year, and can only withstand 24 hours at 120°C in air. Under vacuum parylenes can withstand longer at higher temperatures. . The decomposition of Parylene N takes place through diffusion of oxygen into the molecule and reacting with C in the polymer at the decomposition temperature. The reported temperatures for decomposition under air, N2 and vacuum are 175, 350, and 425 °C, respectively . Parylenes C and D follow similar Electrical properties of Parylene N were reported to be more favorable than that of Parylene C . Parylene C’s volume resistivity drops to 10-15 compared to 10-16 of Parylene C at 160 °C’s. Dielectric constant of Parylene C increases with temperature while Parylene N has a relatively stable dielectric coefficient between 20 -200 °C’s . As the thickness of Parylene C and Parylene N increases from 0.5 μm to 4-5 μm their breakdown voltage becomes closer and Parylene N shows a higher breakdown voltage at higher thicknesses. Parylene N was shown to be useful as a dielectric layer for use in microelectronic systems. In an advanced study, it was used as a conformal dielectric layer in 50 μm through silicon vias (TSV) .
Parylene C on the other hand is the most widely used type. It finds applications in microelectronics, microfluidic devices, medical implants (stents, needles, etc.), PCBs due to its diverse properties such as fast deposition rates, common CVD type deposition method. It complies with the FDA regulations with the biomedical use (ISO-10993 Biological Evaluations for Medical Applications).
Paryleen F is deposited using the Gorham method, with the cyclic dimer octofluoro-[2.2|paracyclophane]. Low availability and price of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.
Parylene N, C and F offer a low dielectric constant (k≈2-3) which makes them well suited for use as intra- and interlayer dielectrics electronic devices . The dielectric constant is relatively high for Parylene C due to the polar bonds C-Cl. Among them, Parylene F exhibits a very high melting temperature (≤ 500 °C) compared to parylene N (420 °C) and C (290 °C). And, the highest thermal durability is observed for ParyleneF. The decomposition of fluorinated parylene takes place by the decoposition of -CF2- functional groups into -CF- functional groups both in air and nitrogen at the decomposition temperature and no reaction products with oxygen was observed . It has the lowest dielectric constant (2.17 @ 1 MHz) among the three which makes Parylene F more attractive. It can be used as a dielectric layer in the semiconductor chips where high temperature exposure is required. Also, they were shown to be patternable in the micro-scale using Ar/O2 and N2/O2 discharges with straight sidewalls under right processing conditions . The drawback of fluorinated parylene is the difficulty in obtaining the commercial dimer which makes it an expensive alternative to other parylenes. Also, it has the best conformal coating property through entering the smallest crevices.
In conclusion, each parylene type offers different advantages under various types of service conditions (optical, thermal, mechanical, electrical, biomedical). The selection criteria must be based on the aimed end use of the product.
Date : 2020-05-29
A natural process, corrosion enacts chemical/electrochemical reactions that degrade and gradually destroy materials or components within a functional environment. The outcome can be dangerous and costly to repair.
For printed circuit boards (PCBs) and similar assemblies, corroded electrical contacts can cause potentially life-threatening mechanical malfunction of aerospace/automotive/industrial systems during operation; corroded medical implants may disrupt pacemaker function or lead to blood poisoning. PCBs can suffer electrolytic corrosion, when:
- electrical contacts within the assembly are subject to water or other moisture trapped between them.
- Applied electrical voltage causes development of unintended electrolytic cells, which can decompose chemical compounds, initiating corrosive reactions.
- However, contaminants such as chemical residue, dirt, oil and salt trapped between the substrate surface and the conformal film can also generate corrosion.
- Although corrosion generally starts from underneath a conformal coating, because of liquid/residue on the substrate surface, rapid changes in temperature can also crack or rupture the coating’s external layer, instigating corrosion response.
- Similarly, metal components within a PCB can produce oxides or salts within the operating environment, leading to corrosive dysfunction.
- Other assembly materials, like ceramics or plastics, can also suffer corrosion and subsequent degradation of their useful properties, performance expectations and structural integrity.
Considering the smaller size of PCBs, some corrosion mechanisms are less visible and therefore difficult to predict, but pits and cracks can develop, leading to wider spread physical disruption of assembly surfaces and interiors. Conformally coating PCBswith the non-critical, non-toxic layer material parylene (XY/poly-para-xylylene), can prevent corrosion in most cases.
Parylene provides substrates with ultra-thin, pinhole-free conformal protection characterized by excellent moisture barrier properties, as well as surface resilience/strength and insulation. Unlike wet coatings – acrylic, epoxy, silicone or urethane – which are applied by brushing/spraying the wet substance onto an assembly, or immersing it in a bath of liquid coating, parylene uses a chemical vapor deposition (CVD) application method. With CVD, a powdered poly-para-xylylene dimer is subjected to intense heat, transforming it into a gas, which penetrates the targeted surface internally, while also forming an external layer that conforms precisely to virtually any assembly shape. XY synthesizes in-process, basically growing on the deposition surface one molecule at a time. It does not require curing after application, as liquid coatings do.
Parylene outperforms wet coatings in most measures. It has a broad temperature range, can withstand most normal types of abrasion and is chemically inert, making corrosion unlikely. However, one should not think parylene is foolproof. Contamination generated by dirty surfaces can stimulate coating delamination and severe degradation of affected operating systems, as the parylene coating begins to disengage from the surface. To assure reliable XY-adherence to substrates, contaminants of any kind – chemicals, dust, oils, organic compounds, process residue, wax – must be removed, negating development of mechanical stress between coating/substrate.
Highly corrosion resistant, parylene coatings nevertheless adhere poorly to metals, a potential problem for use with PCBs; for instance, because of its conductive properties, many PCB manufacturers equip their assemblies with gold components. With XY coatings for metallic medical implants, formation of OH-dot radicals on the implant’s metallic surface may result from the body’s inflammatory response. In these cases, degradation processes start at metal/polymer interface and progress towards the outer, parylene surface.
However, adhesion to metal surfaces and subsequent corrosion resistance can be vastly improved by addition of a silane layer (A-174 silane) at levels of 2 μm to the parylene. A-174’s molecules form a unique chemical bond with the substrate surface, improving XY’s mechanical adhesion. Silane application is achieved by immersion, manual-spray, or vapor-phase processing, forming a chemical bond with the surface. In addition to metal, materials benefiting from A-174 silane treatment prior to CVD implementation include elastomer, glass, paper and plastic.
Research has repeatedly substantiated the corrosion resistant powers of appropriately treated parylene:
- Plasma-surface treatment methods have limited parylene delamination for medical implantables. In a study of XY corrosion protection of coated aluminum sheeting, pre-corrosive film delamination was rectified by deposition of an ultra-thin layer of plasma polymer (50 nm) on the substrate.
- Two-layer (organic silane 174 + parylene) coating of 2 μm successfully protects implant grade stainless steel surfaces against corrosion in body fluid.
- Pre-treatment of medical implants with silane A174 prior to parylene coating supports the film’s reliable corrosion protection, while improving XY’s biocompatibility and ultra-thin/continuous/inert film formation. Unlikely to trigger immune response, clear XY layers are highly resistant to corrosive conditions of bodily fluids, protecting against development/channeling of contaminants.
- Interface engineering (IE) improves parylene C’s corrosion protection of cold-rolled steel (CRS). Adhesion between XYC films and most smooth or nonporous substrates is minima; directly applied parylene C offers little corrosion-support to CRS surfaces. IE processes situate a layer of plasma polymer between XYC/CRS, stimulating interfacial bonding between the two materials, enhancing corrosion protection.
For reliable corrosion protection, pre-CVD treatments begin with cleanliness testing to check for contaminants, followed by thorough cleaning if they are detected. Connectors, electrical components and other keep-out areas need to be masked. Non-porous materials like glass, metal, paper and plastic generally require a pre-CVD application of A-174 silane adhesion promoter to minimize delamination and assure corrosion cannot begin.
To learn more about parylene, download our Parylene 101 whitepaper now: