Polymerization

Parylene is often applied to substrates or materials where there is no room for any voids in the protective coating. These materials are likely to be placed in harmful chemicals, a moisture packed environment, or even the human body. These are often mission critical devices which can not allow any environmental factors to alter their performance. Whenever these devices need this stringent level of protection from the elements, parylene is the only logical choice.

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 an intermediate liquid stage. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids, as shown below:

Here the orange layer is parylene, with completely uniform coverage of the substrate (illustrated in green), leads (in grey), and component (in black). This all encompassing coverage is one of parylene’s greatest competitive advantages against other liquid conformal coatings. Let’s look at what a liquid coating’s coverage may look like:

Despite the poor editing, it is easy to see that on the sharp edges the coating is substantially thinner than at other areas. There is also pooling on the sides where the leads meet the connector. Factors like this are inherent with liquid coatings, simply because they are a liquid.

Parylene's unique vapor-phase polymerization differs considerably from the application processes of other coating materials. More complex, the technique deposits the substance directly onto the material being coated in a manner that penetrates deeper into the substrate surface. Implemented in a specialized vacuum chamber, parylene's application process does away with the intermediate liquid deposition procedure common to competing coatings.

Chemically, the various parylenes' main-chain phenyl group generates reliable molecule-to-molecule interaction, while possessing high levels of in-plane electronic polarization capabilities. Because these molecular layers are room temperature and chemical vapor deposited (CVD),

• no specialized surface treatment is required, resulting in
• a chemically stronger consistency than conventionally assembled monolayers.

Thus, parylene's molecular layers are particularly valuable for microelectomechnical systems (MEMS) and nanotechnology driven applications, which emphasize simplifying component manufacturability.

Unfortunately, compared to other coating options, more time is generally needed to assure parylene's superior conformal coating of targeted substrates, a consideration that needs to be an integral component of manufacturing strategies. For instance, the fact that coating thicknesses from 0.10 micron to 76 microns can be applied in a single operation enhances the quality of completion of manufacturing processes. In general, however, slower production time leads to:

• smaller product batches,
• delayed delivery to customers or retail markets, and
• more cost to both manufacturer and customer.

Parylene Deposition Rates and Process Duration
Parylene's application process is rather different and, in consequence, slower and more expensive than the traditional wet chemistry coating methods used for acrylic, silicone and other substances. The parylene process is multifaceted, involving several steps. Unlike many competing application processes, parylene deposition is not line-of-sight. Rather, the gaseous monomer uniformly encapsulates all sides of the object being coated simultaneously.

While CVD generates the truly conformal nature of parylene coating, it can be considerably slower to implement. In addition, appropriate cleaning, activation and masking precede CVD coating in the deposition chamber. Deposition rates for parylene conformal coating vary, often significantly, according to:

• the surface material of the selected substrate,
• the type of parylene coating applied,
• its deposition rate,
• the character of the coating project (large/small batch, etc.), and
• the assembly's required surface thickness.

Conclusion
Biologically and chemically inert, parylene responds well to the CVD process. Not requiring the liquid phase application standard from most competing coating materials, parylene provides an authentically conformal covering. The excellent barrier strength is characterized by a completely pinhole-free and uniform thickness. Coating application is controllable at thicknesses > 0.5µ, while effectively penetrating product spaces as narrow 0.01mm, making parylene highly relevant to MEMS/nano uses.

Confirmation of these standards is as important to Diamond as it is to our valued customers, and contributes to production time. However, the attention paid to quality throughout all stages of the parylene coating process engenders enhanced delivery of client expectations. It simultaneously generates substrate coatings guaranteed to maintain component protection and performance for the duration of its expected operational life.

Parylene (XY) conformal coatings are applied to substrate materials through a specialized chemical vapor deposition (CVD) process that completely eliminates the liquid phase of wet coatings. No initiators or catalysts are involved in CVD polymerization, which synthesizes truly conformal protective film in-process. This is in stark contrast to wet coating materials such as acrylic, epoxy, silicone and urethane, which are synthesized prior to application via, brush, dip or spray methods. Wet during application, liquid-coated substrates requiring further drying and curing.

With parylene CVD deposition, a gaseous monomer uniformly encapsulates all exposed substrate surfaces; no curing is necessary. Temperature levels influence outcomes of CVD throughout the process, from sublimation through pyrolysis and cold trap procedures.

Sublimation – Dimer Transformation
In contrast to liquid coating materials, parylene initiates CVD as a solid, stable powdered crystalline dimer di-p-xylylene. CVD’s first phase -- sublimation – begins with positioning dimer in an aluminum foil cup – the “boat” -- situated at opposite end of the deposition chamber, where actual coating will ultimately take place, Final coating thickness is determined by the volume of dimer placed in the boat.

CVD processing begins with radiant heater cycling within the unit.

Regarding process temperature, dimer is heated to levels between 120° - 150° Centigrade (C) under vacuum conditions, transforming the substance into a vapor. A pressure safety interlock cycles the radiant heater/vaporizer on-and-off according to coating project requirements, to regulate safe operating performance. As dimer changes from solid-to-vapor, its molecules move down the tube because of the reduced pressure at the opposite end.

Parylene Pyrolysis
Pyrolysis is essentially decomposition of a substance, in this case solid parylene dimer, brought about by exposure to high temperatures. Consisting of two parylene molecules, the dimer undergoes this transition at temperatures ranging between 650° - 700° C. This level of heat causes the molecules to split apart forming the reactive, vaporous monomer p-xylylene. This resulting monomeric vapor becomes the parylene conformal coating when it reaches the item to-be-coated in the deposition stage.

Cleaved into divalent radical monomers by these high temperatures, monomer molecules:

• enter the deposition chamber,
• making approximately 10,000 collisions with other similar vaporous molecules,
• eventually reconstructed as a single, long chain polymer on ALL exposed substrate surfaces within the chamber,
• regardless of topography;
• crevices and edges are covered,
• providing a uniform, pinhole-free coating
• that penetrates within substrate surfaces while simultaneously formulating above the surface.

Averaging about 680° C, this temperature initiates the polymerization described above, reliant on physisorption -- a function of deposition pressure and temperature. Because of surface energy at the interface of vaporous parylene and the substrate, the gaseous molecules adhere to its surface, depositing a conformal film. Physisorption’s kinetic properties are stronger at lower temperatures than higher.

Cold Trap Processes
Drawn one molecule at-a-time onto the selected substrate, monomeric XY gas reaches the final deposition phase, in the cold trap. Temperatures are cooled well below zero (0º C), between -90º and -120º C. This process allows for:

• liquid nitrogen or mechanical chiller cooling,
• which removes residual parylene materials pulled through the coating chamber from the substrate,
• preventing molecular back-streaming into the deposition chamber.

At temperatures below its freezing point, the monomer condenses as a crystalline solid. The freezing point of Parylene N registers at approximately -73° C after thermal measurement; for Parylene C, freezing point is higher, closer to -65° C.

Threshold Temperature
Common to all parylenes, the threshold temperature is that thermal level essentially negating film deposition. Also known as the ceiling temperature, further coating deposition is negligible. Physisorption diminishes incrementally as one nears the threshold temperature, slowing deposition until none occurs when ceiling is reached. Once physisorption begins, the p-xylylene intermediate needs to react with itself to assure polymerization.

Parylene threshold temperatures vary according to parylene type, and each type’s molecular weight. The greater the molecular weight, the higher the threshold temperature; consult the following table:

Additional Thermal Data
Combining high thermal stability with a low dielectric constant, CVD-generated parylene films also provide sustained substrate adhesion, characterized by minimal moisture absorption. Further beneficial thermal properties of XY protective coatings include reliable performance through an exceptional range of temperatures. Depending on the parylene type, XY can function at temperatures as low as -271º C, or as great as 450º C, representing total sustained operation within a thermal span of 721º C.

The following table provides additional significant temperatures pertinent to using parylenes C, D and N.

Overall the generic name parylene describes a distinct collection of polycrystalline and linear organic coating materials with innumerable applications. The essential basis of today's parylene N, p-xylene, was inadvertently synthesized at England's University of Manchester in 1947. The filmy residue resulted after high-temperature heating of compounds of toulene and the xylenes polymerized into para-xylene. The substance immediately demonstrated an exceptional capacity for generating the fine but resilient surface-covering that characterizes today's range of parylene conformal coatings.

Subsequent development led to such commercially viable coatings as the parylenes N, C, and a number of other variants. Most parylene materials possess properties similar to Teflon (polytetrafluoroethylene, PTFE), but offer a wider range of better-protected applications for consumer, industrial, medical and military uses. The highly specialized application process inherent in the use of parylene generates the superior conformal coatings that distinguish it from competitors.

Vapor Phase Polymerization
Compared to the application processes of other coating materials, paryIene's unique vapor-phase polymerization technique is more complex, depositing the substance directly onto the substrate or material that is being coated. Implemented in a specialized chemical-vacuum chamber, parylene's methodology does away with the intermediate liquid deposition procedure common to competing coatings.

At the outset, a raw dimer in solid state is used, comprised of the Parylenes' C, N, AF-4, or a similar variants. After being situated in a loading boat, the dimer is inserted into the vaporizer for further processing. The powdery dimer is heated within a temperature range of 100-150º C. inside a closed-system vacuum chamber, converting it to a gaseous form at the molecular level. The vapor is then heated to a higher temperature, reaching 680º C (1255º F), without variation. Throughout the process, the deposition system must maintain reliable, consistent levels of heat. These upper range temperatures compel sublimation, splitting the molecule into a monomer. This condition effectively eliminates parylene's double-molecule structure, causing a single molecule vapor to be formed.

While parylene's vapor-deposition polymerization process produces exceptionally reliable conformal coating, it is very time-consuming. The monomer gas needs to be vacuum-drawn onto the selected substrate at the extremely gradual rate of one molecule at a time. The procedure takes place in the coating chamber, at ambient temperature, leading to the cold-trap, the concluding phase of parylene's specialized coating technique. In cold-trap, radically lower temperatures, between -90º and -120º C, cool the coated materials, while removing any remaining parylene from the surface; the residual materials are unnecessary and can interefere with parylene's value as conformal coating.

Regarding the time element, parylene typically exhibits a deposition rate of approximately .2/mils per hour, slow in comparison to the liquid application technique employed by most competing coating substances. Thus, machine runs for parylene can last over 24 hours, and tend to encompass smaller production batches. Parylene's application process is rather different and, in consequence, slower and more expensive than the traditional wet chemistry coating methods used for acrylics, silicones and other substances.

Parylene's vapor polymerization process eliminates problems common to competitors' liquid application processes. With the effects of gravity and surface tension eliminated, parylene achieves superior coating of even the most complex structures, largely because of its vapor monomeric quality.

The Wet Chemistry Application of Non-parylene Coatings
Non-parylene conformal coatings like acrylic, epoxy, silicone or urethane rely on a liquid, wet chemistry coating technique. Typically, application of these conformal coatings involves either:

• Dipping the substrate into a liquid bath consisting of the coating substance
• Brushing the coating onto the substrate
• Spraying the wet coating material directly onto the substrate surface

While these procedures are quicker and less costly than parylene deposition, they are also subject to the pinholes, pudding, bridging, run-off, thin-out along substrate surfaces, and tin-whisker problems virtually eliminated by parylene. In addition, liquid chemistry coatings lack the precision-application of parylene, limiting their use for a wide range of specialized aerospace/military, consumer, medical, and associated MEMS/ nanotechnologocial products.

Moreover, acrylic conformal coatings lack parylene's resiliency when exposed to solvents, offering considerably less protection. The same can be said for temperature standards: acrylics' continuous operating temperature top-out at 125ºC; urethane conformal coatings have a maximum operating temperature of 200ºC. Parylenes can match this, and also remain operable at -200ºC.

Application processes affect utility. For instance, to be effective, silicone must be applied far more thickly than other coatings, reducing flexibility while limiting its MEMS/ nano-uses. Urethane generates lesser heat (125ºC) and vibration protection, making it largely unsuitable for ruggedization products and processes.

Conclusion
Parylene's vapor-deposition polymerization process produces a uniform thickness conforming completely to the substrate, generating assured pinhole-free coverage. The result is excellent chemical, dielectric barrier and moisture protection. Parylene's non-liquid application process derives a coating free from the edge-effect and meniscus problems common to competing conformal coatings. The vapor-phase deposition technique separates parylene from competitors, and is largely responsible for its superiority as a conformal coating for a wide range of products.

A specialized chemical vapor deposition (CVD) process attaches conformal coatings composed parylene (XY) to substrates. CVD uniformly encapsulates all exposed substrate surfaces as a gaseous monomer; completely eliminating wet coatings’ liquid phase and need for post-deposition curing. Synthesizing in-process, CVD polymerization requires careful monitoring of temperature levels throughout.

Beneficial thermal properties of XY protective coatings include reliable performance through an exceptional range of temperatures. Parylene is available in variety of material formats, prominently Types C, N, F, D and AH-4. Each has a particular range of properties that determine its optimal uses. Types C and N exhibit faster deposition rates than other parylenes, making them useful for a wider range of coating functions. However, operating temperature is a significant determinant of use: Much depends on chemical composition.

• Used more frequently than other XY varietals, Parylene C is a poly-monochoro para-xylene. It is a carbon-hydrogen combination material, with one chlorine group per repeat-unit on its main-chain phenyl ring. In oxygen-dominated atmospheres, C conformal films regularly provide reliable assembly security at temperatures of 100° C (212° F/water’s boiling point) for 100,000 hours (approximately 10 years). C is suggested for use in operating environments reflecting these temperature conditions. Chemical, corrosive gas, moisture, and vapor permeability remain consistently low. C generates exceptional vacuum stability, registering only 0.12% total weight-loss (TWL) at 49.4° C/10-6 torr (1 torr = 1/760 SAP (standard atmospheric pressure, 1 mm Hg). C can also be effective at temperatures below zero, to -165º C.
• With a completely linear chemical format, Parylene N is the most naturally-occurring of the parylene series. Used less regularly than Type C, N is highly crystalline; each molecule consists of a carbon-hydrogen combination. N’s melting point of 420° C is greater than most other XY types. Vacuum stability is high, registering TWL-levels of 0.30% at 49.4° C, and 10-6 torr. These properties encourage higher temperature applications. Compared to other XY varietals, N’s low dielectric constant/dissipation values also recommend uses with assemblies and parts subjected to higher levels of unit vibration during operation. N’s electrical/physical properties are not noticeably impacted by cycling from -270º C to room temperature, adding to its versatility.
• Parylene F has fluorine atoms on its aromatic ring. Possessing aliphatic -CH2- chemistry, F’s superior thermal stability is attributed to this aliphatic C-F bond, compared to Type C’s C-C bond. Better thermal stability, and reduced electrical charge/dielectric constant expand its use for ILD (inner layer dielectric) applications, such as those for ULSI (ultra large-scale integration), where a single chip can incorporate a million or more circuit elements. F is a good choice for many microelectromechanical systems (MEMS)/nanotech (NT) solutions.
• Originating from the same monomer as Type C, Parylene D’s chemical composition contains two atoms of chlorine in place of two hydrogen atoms. Like Type C, D conformal films can perform at 134° C (273° F), dependably securing assembly performance in oxygen-dominated environs for 10 years, at a constant 100° C. Parylene F resists higher operating temperatures and UV light better than C or N.
• Parylene AF-4’s melting point is greater than 500° C. It survives at higher temperatures/UV-exposure better than other parylenes for long durations because it possesses CF2 units, situated between its polymer-chain rings.

Because oxygen-free environments prohibit oxidative degeneration, XY’s operational temperature range increases significantly if used in inert atmospheres, characterized by absence of air. Depending on XY type, parylene can function at temperatures as low as -270º C, or as great as 450º C, representing total sustained operation within a thermal span of 720º C. CVD for higher temperature range XY types are costlier. Table I provides temperature data pertinent to using major parylene types.

Table I: Significant Temperatures for Selected Parylenes

Poor parylene adhesion negates many of the coating’s most-valued functional properties, including dielectric strength, and resistance to the effects of chemicals, corrosive agents, and moisture. Surface treatments that amplify the interface adhesion between the deposited parylene and the coated substrate are therefore highly desirable. These treatments entail depositing parylene on a clean hydrophobic surface before its chemical vapor deposition (CVD) process is enacted.

Parylene is applied to substrates at ambient temperatures within a specialized vacuum, conducted at pressures of around 0.1 torr. To assure complete impingement of the parylene monomer, uniformly encapsulating the substrate, provision of appropriate surface support prior to CVD limits subsequent factors of peeling force, soaking undercut rate, and vertical attack bubble density (VABD). that can lead to lack of coating adhesion and delamination.

A truly conformal coating, parylene provides superior, uniform barrier protection on almost any surface geometry or topography. However, any contaminants present on a substrate surface prior to CVD will inevitably have a negative impact on parylene adhesion. Chemicals, dust, oils, organic compounds, process residue, wax – contaminants of any kind – need to be thoroughly removed, leaving the substrate surface entirely devoid of their presence; if unattended, issues such as mechanical stress can develop. 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.

Cleanliness Inspection and Testing
Thorough surface inspection is the first step to delivering a substrate surface suitable to parylene adherence. Identifying contaminants significantly lowers the risk of incomplete surface cleansing, while informing selection of task-appropriate materials and methods.

Costly cleaning and rework issues can emerge if thorough surface-inspection is overlooked at any stage during the production/coating process. Poor inspection fails to detect and identify contaminants, leading to delamination, exposed surfaces and component dysfunction. In such cases, it is not uncommon for leakage of non-organic, electrically-conductive sediments beneath the parylene to interfere with and ultimately wreck the performance of electrical components.

Useful surface inspection techniques for organic contaminants include Gas Chromatography (GC) and Fourier Transform Infrared Spectroscopy (FTIR). Sometimes used in conjunction with mass spectroscopy, GC splits unidentified organic chemical mixtures into their distinct components, specifying their discrete properties. FTIR identifies specific organic contaminants by comparing evidence from spectrum analysis to those of known substances; contaminants such as silicon oils and mold-release agents are identified with FTIR. Valuable for determining the presence of inorganic contaminants like chloride, fluoride, potassium, or sodium, Ionic Exchange Chromatography (IOC) uses electrical-charges to separate the compounds’ ions and polar molecules.

In all cases, the aim is verifying not only the contaminant substance, but also the optimal solvent and cleaning system suitable to its eradication.

Parylene Surface Cleaning Agents
A variety of nonhazardous cleaning agents can be effectively applied to substrates, according to their precise identification. Regular detergent cleaning is suggested for soluble contaminants. Less soluble contaminants require use of biodegradable, multi-faceted, solvent-strength solutions like deionized water, isopropyl, and methyl ethyl.

Cleansing methods are also dependent on the composition of both the identified contaminants and surface materials, to achieve satisfactory levels of substrate neutralization. Solvent immersion, surface-spraying, substrate-tumbling, or vapor-degreasing are primary disinfectant procedures. However, the substrate surface may also require manual, hand-cleaning, or application of batch, inline, or ultrasonic methods.

Masking
Integral to surface preparation, the masking process is implemented to assure designated components of a PCB or similar electrical assembly are protected from the effects of the parylene itself, which can interfere with expected functionality. Some of parylene's key properties can be both desirable and detrimental to an assembly, if applied to the wrong areas. For instance, parylene’s excellent dielectric properties simultaneously disable a PCB's contacts, rendering it inoperable, even as they safeguard the substrate surface from electrical interference.

Masking the contacts resolves this issue, coating only those PCB-parts that won’t be negatively impacted by conformal protection. In this way masking preserves an assembly’s operational integrity and performance. This critical pre-phase of the parylene coating process can be exceptionally labor-intensive. Considerable operator attention to the task is necessary to ensure effective masking of each connector, sealing it from penetration by gaseous parylene molecules during deposition. All tape, or other covering materials, must thoroughly shelter the keep-out regions, without gaps, crevices or other openings, to ensure connector function is retained after coating.

A-174 Silane
A-174 silane adhesion promoter chemically bonds with the substrate surface to stimulate resolute parylene adhesion. Manual-spray, soaking, or vapor-phase processing methods are used to apply A-174 to the substrate after the masking-operation, forming a chemical bond with the surface. Substrates responding well to treatment with A-174 silane prior to implementation of parylene coating processes include those made of elastomer, glass, metal, paper and plastic.

A-174’s molecules form a unique chemical bond with the substrate surface, sufficient to improve parylene’s mechanical adhesion. However, not all substrate materials benefit from A-174. In its place:

• Plasma-surface treatment methods have limited parylene delamination for medical implantables.
• Silicon substrates roughened with xenon difluoride gas demonstrate enhanced parylene adhesion.

Researchers continue to seek additional cleansing/adherence agents to improve parylene's conformal utility for these purposes.

The diversity of adhesion promotion methods requires a similarly diverse list of raw materials and techniques. Surface treatments prior to CVD begin with cleanliness-testing and cleaning to remove surface contaminants, followed by masking of connectors and electrical components. Materials such as glass, metal, paper and plastic benefit from application of A-174 silane adhesion promoter for necessary, pre-CVD surface modification. Establishing best-adhesion practices and strict adherence-standards is critical to maintaining quality conformal coatings and minimizing delamination.