5 Keys to Parylene Process

Parylene Process:

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) [1], [2].  CVD process is done under vacuum and is achieved in 3 steps in different parts of the CVD instrument:

  • Sublimation: The powdery precursor, dimer, is weighed and inserted into the sublimation chamber using a boat.
  • Pyrolysis: Heating the dimer results in the formation of monomers.
  • Deposition: Monomers travel to the deposition chamber and are deposited as a thin layer in a way to allow for a top layer to grow on them (chain growth). In the meanwhile, the monomers penetrate to the smallest voids resulting in a uniform, void-free conformal coating.

  • Applications areas:

    Parylene thin films find applications in numerous products and processes. Some of the application areas can be listed as follows:

  • PCB and electronic circuit encapsulation layers: Parylene derivatives exhibit excellent sealing properties and chemical durability. They comply and surpass the requirements of the MIL-STD-302 for electronic components when used as an encapsulation material on them. Proven results make them useful as an encapsulation material for PCB’s  [3]–[5], 
  • Intermediate bonding materials: Parylene is vapor deposited from a solid powder onto the substrate surfaces providing a seamless (eliminates air gaps) interface. Thus, thin films of parylene are void-free and are highly uniform in terms of thickness across the wafers diameter which is crucial for the overall uniformity of chips produced on the substrate [6],
  • Thin film membranes in sensors and actuators: Due to their large elastic compliance and low residual stress [7], [8] parylene derivatives are used as membranes,
  • Gate dielectric layers in electronics: Their relatively low dielectric constants in the 2-3 range make them useful as a low-K candidate as a gate dielectric in transistors [9],
  • Microfluidic channel layer for chemical and bio-sensors: Parylene C is an FDA approved biocompatible material which shows a very good chemical durability which makes it an interesting material for use in microfluidic devices, also it is well researched for patterning of the microfluidic channels [10]
  • 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) [11].key | meaning of key in Longman Dictionary of Contemporary English ...

    Keys for success:

    1. 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.
    2. Masking of the substrates must be done before the parylene coating processRemoval 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.
    3. 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.
    4. 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.
    5. 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 coatingepoxy coatingsilicone coatingurethane 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 Conformal Coating

    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.
    Parylene Coating Applications

    Parylene Coating Applications

    Here is a brief list of some of the items that can be parylene coated:

    Printed Circuit BoardsMEMsLEDs
    CathetersStentsMagnets
    PaperNeedlesSensors
    Ferrite CoresMetallic BlocksOptical lenses
    Implantable devicesValvesO-rings
    TubingSilicon WafersKeypads
    StoppersSealsMandrels
    MoldsMotor AssembliesPower Supplies
    BackplanesPhotoelectric CellsForceps
    Test TubesProbesFiber Optic Components
    Pace-makersBobbinsPlus Many More...
    The Parylene Deposition Process

    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.

    Pre-Deposition

    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.

    Parylene Coating Requirements
    Coating Requirements

    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.

    Parylene Disadvantages

    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.

     

     

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