Conformal Coating Education Center

Parylene Inspection

Applied through chemical vapor deposition (CVD), parylene penetrates deep within substrate surfaces, generating a level of assembly security surpassing that offered by liquid coatings such as acrylic, epoxy, silicone and urethane. Yet, although XY is applied in a vacuum, it’s capacity to provide these extraordinary qualities does not exist in one.

In this Section:


Inspecting Parylene Coating

Methods and Procedures

Parylene conformal coating (XY) provides insulative protection for complex electronic circuit assemblies expected to function through rigorous operating conditions -- potential chemical, electrical, moisture and vapor incursion during performance. Applied through chemical vapor deposition (CVD), parylene penetrates deep within substrate surfaces, generating a level of assembly security surpassing that offered by liquid coatings such as acrylic, epoxy, silicone and urethane. Yet, although XY is applied in a vacuum, it’s capacity to provide these extraordinary qualities does not exist in one. Parylene’s durable protective value depends on film adhesion, a quality subject to persistent, thorough inspection throughout the production process.

Diamond MT’s parylene services include coating provision for clients’ printed circuit boards (PCBs), medical devices, and similar products, shipped to our production facility. The incoming inspection process begins immediately after items awaiting parylene coating have been unpacked:

  • Received assemblies are counted to verify quantity in comparison to the client’s provided packing slip/purchase order.
  • Damage-inspection verifies assemblies arrived at our facilities without breakage or defacement.

Individual processing follows these procedures, with additional inspection and cleanliness testing. This is imperative; substrate contaminants may have accumulated during manufacture, handling and transportation. Without question, the most significant factor affecting parylene (or any conformal coating) adhesion is surface cleanliness; contaminated surfaces lead to poor coating quality, limited adhesion, and delamination, defeating the purpose of XY application. To this degree. cleanliness inspection is a vital step in the coating process, assuring the substrate surface is ready to accept parylene conformal coating without incident.

Visual inspection alone is insufficient to confirm a PCB’s suitable cleanliness and other stage-readiness for XY coating. Throughout the production-run, every phase of the process must be consistently measured and monitored; this ongoing performance inspection averts costly cleaning issues. At Diamond, we maintain a sampling process throughout each production run, designed to:

  • confirm the readiness of a customer's assemblies for parylene coating, per IPC-J-STD-001 stipulations,
  • ensuring consistent quality levels throughout XY coating procedures.

According to IPC-J-STD-001 specifications, surface cleanliness levels should register 10µgm NaCl/in2 or less. Diminishing adherence to this standard is inadvisable; doing so jeopardizes coating and assembly performance. Substrate contamination undetected prior to film application requires process-cessation, and substrate recleaning, until acceptable non-contamination levels are achieved. These costly missteps are avoided by appropriately implemented cleanliness inspection before XY application is commenced.

XY inspection for quality assurance also details the degree of the coating thickness necessary to meet assignment specifications, PCB-area of coverage, visual, and adhesion-testing requirements. Subject to intensive inspection and evaluation, micron-thin XY-films are constructively measured:

  • using spectral reflectance directly on components or
  • by comparing project film-application with witness coupons previously XY-coated to ascertain similarity of result.

InspectionQuality attributes for parylene typically specify the use of various inspection procedures that verify appropriate surface purity. These include:

  • Ionic Exchange Chromatography (IEC), for identifying the presence of inorganic substances like chloride, fluoride, potassium and sodium. Specifying contaminants, IEC aids in selection of appropriate solvent/cleaning systems to resolve the issue.  
  • Fourier Transform Infrared Spectroscopy (FTIR) denotes the presence of specific organic contaminants, like mold agents or silicon oils.
  • Gas Chromatography can also detect/identify surface organic contaminants; sometimes used in conjunction with Mass Spectroscopy, when more complex contaminants are detected.

Once cleaning has been enacted, masking processes assure parylene coating doesn’t penetrate assembly keep-out areas, in accordance with client specifications. Subsequent masking inspection verifies compliance, leading to implementation of CVD procedures (the coating process). After XY deposition, masking materials are removed, and the batch is subject to further inspection, to assure even, pinhole free coating without tears along formerly masked regions. Thickness inspection verifies appropriate film thickness has been achieved.

Prior to packaging coated assemblies, final inspection is necessary. Encompassing every aspect of the product, this process ensures

  • successful implementation/completion of all phases required by the specific XY coating assignment, and
  • absolute compliance with the client’s drawings and specifications.

After passing final inspection, assemblies are ready to ship. At Diamond, return-to-client typically takes about ten business days, but faster turn-around can be negotiated at your request.


Does Parylene Get Everywhere?

Parylene Applications

After pertinent research you’ve determined parylene (XY) is the best conformal film for your coating assignment. Especially relevant were XY’s uniform protective and insulative properties, which are useful for numerous applications, ranging from printed-circuit boards (PCBs) to medical implants to military-grade purposes. Among parylene’s other advantages are:

  • adherence to an exceptional quantity of substrate geometries/materials,
  • biological/chemical inertness, 
  • bubble- and pinhole-free conformability/flexibility at film thicknesses greater than 0.5 microns,
  • excellent dielectric/moisture barrier properties,
  • high optical clarity,
  • penetration of extremely small crevices/spaces,
  • tin whisker mitigation, and
  • withstanding autoclave-level heat

Applied through a chemical vapor deposition (CVD) process, gaseous XY s can be deployed and adhere on any surface that touches air. Thus, it has the capacity to coat under components, inside minute substrate fissures, and inside semi-sealed areas. Unlike liquid film materials, the micron-level thinness of parylene films generate coatings without forming bridges in tight areas.

These properties have been verified repeatedly through parylene’s use and have been extended as application technologies improve. But, for your own coating assignments, you need to know if the parylene film reaches and adheres everywhere required by the coating assignment’s specifications. You’ll want the most reliable proof available of the coating’s absolute conformality.

Verifying Parylene Conformality
Verifying XY conformality – the property of uniform parylene application throughout the project-specified surface for each coated assembly -- may require specialized inspection methods. With greater resolution power than a light microscope, electron microscopy (EM) offers much higher magnification than most alternatives; it permits finely-detailed views of much smaller objects, like XY’s many microelectricalmechanical systems (MEMS)/nano-tech applications. EM uses a beam of electrons –stable, negatively-charged subatomic particles found in all atoms, the primary carrier of electricity in solids -- to create an image of the specimen. This technique can provide images suitable for confirming XY conformality.

Parylene Applications

However, additional methods may be required. Physically-cleaving a coated specimen where XY film thickness exceeds 200 nanometers (nm) allows more precise imaging; cross-sectional scanning electron microscopy (SEM) will generate images suitable for conformality determination. SEM images show how well the parylene has coated the specimen, and

  • if the coating thickness is pinhole-free/uniform, or
  • where any gaps in the coating exist.  

Sequential cross-sectioning helps determine conformality (or its lack) along the surface of a single specimen or through an entire sample.

Physical cross sectioning may not work for all substrate topographies. Use of ion/electron beam ablation (I/EBA) can successfully image the XY film/substrate interface, to determine if the parylene has indeed adhered everywhere intended by the coating assignment’s specifications.

Verifying XY conformality becomes proportionally more difficult as coating layers decrease in size, as with MEMS/nano-tech applications, where layers frequently are less than 100 nm. Analysis of SEM cross-sectioning becomes more difficult under these circumstances, suffering from Z-contrast/charging effect inconsistency. These conditions can be rectified using a focus ion beam (FIB) system in conjunction with transmission electron microscopy (TEM). TEM is used to view thin specimens – like tissue sections, molecules, in addition to conformal layers -- through which electrons can pass generating a projection image.

Electron microprobe analysis (EMPA) can enrich TEM imagery. Working similarly to an SEM, EMPA is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. While this technique is adept at verifying the conformality of thinner, more complex layers of parylene, its accuracy can be challenged by ion damage, as film thicknesses diminish (>20 nm).

In such cases, using SEM images prior to- and following CVD can generate a reasonable view of coating covering and conformality. This technique is valuable in cases characterized by property changes to the substrate surface initiated by CVD. Such applications – where preservation of the precursor functionality down the depth of feature is necessary – benefit from combined (before/after) SEM imaging. Comparing prior-with-final assembly properties verify applied XY conformality in these cases.

A Simple Solution
These methods will be very useful in determining the exact placement of Parylene at different levels of the object that was coated. For most applications, simply looking at the areas that have been de-masked, as we discussed in the how to inspect Parylene article. If you can see the de-mask lines, take solace in the fact that coating HAS to be on the rest of the board. It simply cannot NOT be there.


In-Line Parylene Processing?

Parylene Process

The phrase “in-line parylene processing" is deceptive because it does not accurately describe the method in which parylene (XY) is applied as a conformal coating. It is true that some aspects of the traditional production line are relevant, but primarily in a fractional way. without the traditional station-to-station regimentation of standard in-line manufacturing processes.

A conventional factory production line incorporates a structure wherein the object being manufactured – automobile, bookshelf, dishwasher, textiles, etc. -- is passed through a set linear sequence of mechanical or manual operations, from start-to-finish, until the product is complete. In-line manufacture began in the 18th century and, as production technology developed, spurred the Industrial Revolution and the growth of Western capitalism. Its impact remains strong, influencing how goods are made and sold in our consumer society.

Despite remarkable advances in production technology, the in-line structure continues to prevail today for many industries. Many of the computer-related items parylene is called on to coat are themselves manufactured through in-line processing methods. For instance, printed circuit boards (PCBs), a prime recipient of XY protection, are typically manufactured, stamped and soldered in a sequential manner, from process-to-process, in an ongoing procession of successive activities, without pause or intervention until the board is complete.

And, if the PCB manufacturer has its own conformal coating mechanism, applying the film can become part of the in-line process. However, this applies more for liquid materials, whose dip or spray coating methodologies are more readily incorporated into a sequential series of work processes, with perhaps a spray and curing function at the end of the line.

In-line Processing

Yet, conformal coating processes require in-line methods of their own, apart from product design, construction and manufacture. For wet coatings like acrylic, epoxy, silicone and urethane, typical line processing includes:

  • Cleaning, to remove contaminants from the PCB.
  • Applying coating by dipping or spraying the liquid film material onto the substrate.
  • Drying the wet component allows it to be handled for further use.
  • Curing permits attainment of the product’s optimal electrical/mechanical performance properties and conformal protection.
  • Inspection ensures the coating is conformally appropriate to the assembly’s function.  

There is a sequential order to these production events that can be achieved through in-line processing; however, all occur after the PCB or other item-to-be-coated has been produced, and thus may be separate from the original in-line production sequence.

Parylene conformal coating requires specialized processes for application, which makes it even less adaptable to in-line production. XY's unique vapor-phase polymerization differs considerably from the application processes of other coating materials, eliminating the intermediate liquid deposition procedure necessary to wet coatings.

Called chemical vapor deposition (CVD), it is more complex than liquid methods, which apply pre-synthesized coating onto substrates. CVD synthesizes the conformal film in-process, depositing XY directly onto the substrate, while penetrating into its surface. Opportunities to enact in-line production methods are severely limited because, to be successful, CVD needs to be implemented in a specialized vacuum chamber. In addition to providing superior conformal film adaptable to a multiplicity of purposes, CVD offers these process advantages:

  • elimination of specialized surface treatment prior to film deposition, causing
  • a chemically stronger consistency than conventionally assembled monolayers.

The relative complexity of XY processing makes it more difficult to modify for in-line manufacture; once implemented, the process is self-governing, from beginning-to-end, more suitable to batch processing.

Parylene does require some extra steps to be used that are not for liquid conformal application. Cleaning substrates prior to application is recommended, and masking/de-masking almost always is required. However, XY only needs priming in the case of noble metal substrates, when a pre-application of silane can improve adhesion. CVD, a longer process more suitable to batch production, replaces liquid application methods, with generally better results. Parylene requires no drying/curing.

Thus, three of the in-line steps necessary for wet coating are eliminated, and the basic process – XY application – is another method entirely; CVD’s coating product offers greater performance reliability, but is also more costly and time-consuming. There seems to be no way to incorporate in-line methods to parylene coating processes; the requirements of successful CVD simply contradict the sequential, piecemeal approach of the production line. Once CVD is initiated, it proceeds autonomously; the series of processes for completion take place within the deposition chamber, without participation from human or automated workers. Perhaps future, unforeseen technical innovations will alter these circumstances. Until they do, CVD transformation to in-line processes will remain infeasible. The need for batch-processing negates reliance on conventional production line/in-line manufacture for parylene conformal film application.


Does Parylene De-Wet?

Wet Application Processes

Liquid conformal polymers – resins of acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – use wet application processes to attach to substrates. Most prominent of these are brushing the wet coating onto an assembly, dipping (immersing) the assembly in a bath of liquid coating, or spraying the conformal film onto the designated surface. The coating materials are wet when they are applied. If

  • application processes are inadequate or
  • targeted substrates are inadequately cleaned
  • conformal adhesion diminishes and can lead to delamination.

One of the failure mechanisms that can emerge under these circumstances is de-wetting.

De-wetting

De-Wetting of Liquid Coatings
De-wetting is the tendency of the coating material to refuse to wet the surface of assemblies to which it has been applied. De-wetting deteriorates the conformal coating. Thin polymer films can fracture into small, non-conformal droplets through de-wetting, which has several distinct phases:

  • Hole-formation occurs either spontaneously (spinodal de-wetting), or because of film contact with surface contaminants.  
  • Reduced surface area of the polymer/air interface stimulates further hole growth.
  • Other holes may emerge in consequence, further decreasing the diameter of the coating’s polymer fibers.
  • Hole collision creates thinning polymeric lines throughout the film, which continues to diminish in thickness as the film material drains to the apexes of its polymeric rings.
  • Rayleigh instability – increased fluctuations on the film surface -- develops.
  • Holes eventually dissolve into droplets, disrupting the uniformity of the liquid coating material, and jeopardizing conformal protection.

De-wetting and hole-growth in wet polymer conformal films is a major failure mechanism, diminishing their protective qualities. Liquid films polymeric nature adds to the non-linear viscoelastic effect of their shear thinning.

Surface contamination prevents coating solutions from evenly sticking to and ‘wetting’ the substrate. Lack of proper adhesions leaves assembly areas uncoated, exposing the substrate to additional contamination and subsequent coating failure. Cleanliness is the key to preventing de-wetting; causes of surface contamination include:

  • flux-residue when no-clean flux is used,
  • soldering processes,
  • hot air solder leveling (HASL) rinse-operations stimulating corrosion,
  • component mold release agents,
  • silicone oil left from production adhesives,
  • cleaning bath contamination and
  • operator handling.

When de-wetting occurs, solder fails to adhere to components. In addition to contamination and corrosion, extremely high temperatures above a film material’s glass transition temperature can stimulate de-wetting; by increasing the mobility of the polymer-chain molecules, a tendency toward separation from each other and the substrate surface develops, stimulating de-wetting.

The only viable solution is stripping the damaged coating from the affected area, re-coating it with a rigorous, manual re-work process.

Parylene and De-wetting
Providing an entirely conformal, durable, pinhole-free coating for PCBs and similar electronics, parylene (XY) offers a protective, insulative coating for a wide range of products and materials. Applied by chemical vapor deposition (CVD) rather than the liquid methods used by AR. ER, SR and UR, XY is converted from a solid to a gas, with no wet stage. Thus, unlike liquid coatings, parylene is not pre-synthesized and dispensed during application in a wet format.

Parylene’s CVD free radical polymerization technique creates XY coating, synthesizing the coating during application, using a reaction mechanism that forms resonance-stabilized XY diradicals, which eventually adsorb on and into a substrate near room temperature. The result is generation of a much better. conformally-thin polymer film on virtually all substrate surfaces than those supplied by conventional wet-solution methods.

However, XY’s specific material conditions and the CVD application method also quash chemically-based film adhesion for parylene; only mechanical adhesion is possible. Penetrating substrate surfaces gives a parylene a more dependable conformal coat than those provided by liquid polymers, as mechanical adhesion enters and fills pores/voids along covered surfaces, holding together by interlocking film elements.

No wet processes/liquid materials are used, The absence of solvent in XY CVD avoids de-wetting and pinhole-related defects, by enabling growth of high-purity, ultrathin layers of conformal coating. Precisely-controlled parylene CVD enables direct chemical synthesis of thin-film conformal coating formation in one-step processing. Unlike liquid materials, monomeric reactants in the CVD sequencing process require no solubility, bypassing de-wetting potential and other detrimental impacts accompanying solvent-use.


Are Parylene Noodles a Defect?

Parylene Inspection

Unlike liquid conformal coatings joined to substrate surfaces by wet application methods, polymeric parylene (XY) uses a unique chemical vapor deposition (CVD) process to assure adherence. There is no intermediate liquid phase. Rather, cross-link polymerization of powdered raw XY-dimer converts the solid to a vapor at the molecular level, polymerizing XY directly as a transparent film on assembly surfaces.

Applied in a gaseous state, XY penetrates deep within substrate surfaces, providing an authentically conformal protective covering. In many ways, parylene coatings are superior to those provided by wet materials like acrylic, epoxy, silicone or urethane, for a wider variety of products and purposes. Micro-thin film performance makes parylene especially useful for coating printed circuit boards (PCBs) and in microelectromechanical systems (MEMs)/nano technologies.

The XY deposition process assures neither heat nor cooling is needed for coating adherence. The circuit board neither expands nor shrinks, reducing coating-stress. Depositing XY as a dry vapor helps the coated items endure minimal changes during the application process, eliminating another major risk factor of coating defect.

De-wetting

Polymeric Noodles
As a polymer, parylene begins as a monomer-based linear chain fused covalently. The ongoing chain entanglements characterizing parylene morphology stimulate a degree of viscoelastic behavior. Viewed microscopically under normal conditions, they resemble noodles, sometimes elongated but neither precisely straight nor clustered together; often compared to a bundle of spaghetti noodles, they are held together by a few chemical cross-links. More precisely,

  • the shape of a Gaussian coil develops,
  • collected as parylene (or other polymer) molecules join,
  • ranging from several nanometers to several tens-of-nanometers in length,
  • measured by the root-mean-square end-to-end distance Ree,
  • scaled as the square root of the coil‘s total number of monomers (N) or molecular weight (Mw).

In this basic form, which encompasses parylene morphology, noodles are NOT a defect, but a normal and characteristic part of the polymer’s physical structure. Increasing their density across the strands provides shape and strength, limiting their ability to pull away from each other, while increasing their functional and load-bearing uses. These capacities help XY polymers achieve architectural/performance networks necessary for conformal coating purposes.

Defective Noodling
Despite its general superiority as a conformal coating, parylene application and use can suffer defects. While common XY defects can often be identified, planned for and mitigated through proper procedures, they still occur. Inadequate application or deposition onto a surface unprepared for adhesion can compromise XY function.

Defective noodling is the result of deformation mechanisms developing on the surfaces and interfaces of parylene coated systems. These factors can cause loss of the parylene film’s surface pattern-effectiveness, disrupting the structural integrity of the coating’s typically reliable noodle-like entanglement. The surface is then characterized by highly disordered structural configuration, resembling a plate of noodles winding chaotically around each other, interfering with parylene’s usual uniform, pinhole-free surfaces. Disruption of the coating’s performance integrity can lead to both current leakage and voltage breakdown.

If inappropriately cleaned before application, or inadequately deposited, liquids or other substances can penetrate both at the parylene-substrate interface and through the polymer layer, stimulating an environment of disrupted noodle development. Film instabilities can also occur on parylene surfaces when temperatures exceed the polymer’s standard glass transition temperatures (Tg). Basic outcomes include sequential disruption of hierarchical coating formation, a condition that can be generated despite the protection usually afforded by XY’s reliable CVD application method. Poor adhesion and residual stress can also lead to bending, cracking, peeling and noodling of parylene conformal films.

Parylene conformal coating defects can be caused by a range of factors. However,

  • cleanliness of the product surface,
  • carefully matching the parylene type to the coating assignment/purpose, and
  • expert performance of the CVD process
  • mitigate the potential for these problems to arise.

Undetected trace contaminants disrupt the bond between parylene film and underlying surfaces, leading to disruption of noodle configuration.

Thus, noodles, a basic element of parylene morphology, can themselves be transformed from a tasty dish of conformal coating to one that may need to be scrapped or redone.

  • Conventional, non-defective parylene noodles resemble a properly cooked and stirred pot of spaghetti, with a bit of olive oil stirred in, to keep them from only adhering to each other.
  • Rather, they adhere to the substrate. This recipe can be delicious!!
  • When defects occur, the noodles resemble a tangled mat of unstirred, cooked spaghetti, assuming a random shape, of little use to the conformal coating project.

Not always immediately apparent, disordered adhesion will eventually compromise the coating and, ultimately, the end-product, thus neutralizing parylene’s protective benefits. As an integral structural XY-component, you can’t avoid the presence of parylene noodles, but you can control them.


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