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Comparing Parylene to Liquid Conformal Coatings

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Comparing Parylene to Liquid Conformal Coatings Comparing Parylene to Liquid Conformal Coatings By: Sean Horn Diamond-MT, Inc. 213 Chestnut Street Johnstown, Pa 15906 +1 (814) 535 3505 phone +1 (814) 535 2080 fax www.paryleneconformalcoating.com [email protected] Conformal Coatings: A Quick Primer Consisting of various polymeric materials, conformal coatings preserve the operational integrity of electrical and mechanical assemblies, insulating the substrate, improving performance and extending product lifecycle. Conformal coatings provide printed circuit boards (PCBs) and similar electric devices with a protective, non-conductive dielectric layer, safeguarding these assemblies from: 1. Abrasion resulting from contact with acids, chemicals, solvents or, bodily fluids, in the case of medical equipment. 2. Conductor electro-migration, corrosion, dendritic growth, and electronic short-circuits. 3. Exposure to extreme changes in temperatures and humidity, working conditions requiring prolonged operation, or harsh physical environments. They also provide superior insulation and stress-relief to assure ongoing functionality. Major conformal coating types are composed of acrylic, epoxy, parylene, silicone and urethane. The type of coating material used depends upon such circumstances as: the precise character of the assembly/product being coated, the operational environment of the item's intended use, the coating substance's ability to provide appropriate protection, and the coating's process-requirements and cost relative to other coating materials. Four of the five types of conformal coating — acrylic, epoxy, polyurethane, and silicone — are applied by liquid methods, by either brushing, spraying, or dipping the coating on the substrate, then letting it dry on the surface of the designated assembly. Parylene is the major exception, employing a unique chemical vapor deposition (CVD) polymerization process, which generates superior performance for most uses, compared to liquid coatings. Whatever coating type is used, PCB manufacturers need devices that withstand heat, cold, rain, snow, vibration, fungus, oxidation, and corrosion through decades of operation, something normal circuit boards cannot do without protection. Compared to liquid coatings, parylene’s CVD application process penetrates into the substrate surfaces, generating the highest levels of protection available for many products. Comparisons of available conformal coating types usefully demonstrates the value of each type, and parylene’s comparative functional advantage in most cases. Comparing Parylene to Liquid Conformal Coatings Parylene Each conformal coating has its own unique properties which dictate its particular range of product uses and the required coating-thickness necessary to assure reliable performance. These conditions vary according to product and purpose. In comparison to liquid coatings, parylene surfaces are the most consistently pinhole-free, shielding substrates from potential environmental damage, at thickness-levels finer than other materials. In addition, Parylene generates chemical, dielectric, moisture, and thermal protection exceeding that of liquid conformal coatings. Additional beneficial qualities associated with parylene use are: Adaptability to creviced-surfaces, those with exposed internal surfaces, points or sharp edges, or other unusual coating conditions. Electrical insulation with low dielectric constancy and high tension strain. Non-conductive qualities that eliminate electrostatic, magnetic or radio frequency interference during operation. Reliable resistance to acids, bases and solvents. Thermal stability between -195 °C to +350°C. Parylene coating films are exceptionally thin and resilient, especially adaptable to the coating requirements of PCBs, microchips, their sensors and other electrical assemblies. Parylene coatings can be effective at considerably finer levels than liquid coating substances; that is, thinner layers of parylene provide equal or superior protection, compared to liquid coatings (measured in millimeters (inches)): Parylene -- 0.013 – 0.051 (0.0005 to 0.002). Silicone -- 0.051 – 0.203 (0.002 to 0.008). Acrylic, urethane, epoxy -- 0.025 – 0.127 (0.001 to 0.005) Liquid coatings measure between 25-75 micrometers (µm/microns) thick for most uses. Parylene coatings are effective at significantly thinner levels, between 0.1-70 µm, improving their functionality for microelectricalmechanical/nano-technology (MEMS/NT) purposes, characterized by smaller moving parts requiring more freedom to function appropriately. Its specialized CVD process ensures parylene penetrates far more completely into the substrate surface, generating superior protection of product mechanisms and operation. Liquid Conformal Coatings In comparison to parylene, properties of liquid conformal coatings include: Acrylic: Rapid-drying acrylic coatings do not contract during cure and display good fungus and humidity resistance. However, they have limited abrasive and stress-relieving capabilities, causing them to break-down more readily at higher temperatures. In addition, the recent decline in their cost advantage compared to other coatings reduces a major advantage of their use. Epoxy: Like acrylics, film-shrinkage during polymerization is common for epoxy resins. In addition, their stress resistance diminishes substantially when subjected to temperature extremes. These conditions somewhat nullify their extreme surface durability, and good abrasive/chemical resistance. Silicone: Easily repairable, with low toxicity, silicone conformal coatings have a low dissipation factor and good PCB-adhesion, with superior resistance to heat, humidity, moisture and ultraviolet light. Silicone has an operating temperature range of -55°C - +200°C, thus withstanding extreme temperature variations. Very versatile, silicone can be adapted precisely to a product's coating requirements. Urethane: Possessing excellent dielectric properties, good chemical resistance, low moisture-permeability, and reliable low-temperature flexibility, urethane coatings have limited overall bond-strength. They commonly flake and peel when applied to larger surface areas and have poor high-temperature resistance; repairing urethane surfaces is difficult. In all, liquid coatings lack parylene's reliable combination of adhesion, electrical conductivity, durability, and flexibility. Parylene consistently performs under conditions of duress that engender diminished performance or product failure when liquid conformal coatings are applied. Parylene Excellence Parylene displays better coating-thickness and temperature advantages, providing the thinnest effective coating application compared to liquid coating materials. Its CVD process penetrates deep into the substrate surface, generating exceptionally resilient pinhole-free surfaces, capable of withstanding extreme physical stress, while generating such high-value substrate-treatment properties as: chemical/dielectric/moisture barrier protection, dry-film lubricity, exceptional functional durability, reliable application and thermal stability. Each of the major types of conformal coatings offers particular advantages for a range of uses. Liquid coatings are less costly and easier to apply than parylene, but none displays parylene's functional versatility. Its exceptional dielectric properties make parylene the coating film-of-choice for a considerable range of electrical assemblies. While reliance on CVD application-processes can increase manufacturing costs, compared to liquid conformal coatings, parylene withstands specialized and often harsh environments with optimal functionality to the most reliable degree, making it more cost-effective long-term. Despite parylene’s functional superiority for many conformal coating purposes, liquid coating types have their uses. Realizing their capabilities is recommended to assure selection of the coating type most appropriate to your product objectives Comparing Acrylic and Polyurethane Conformal Coatings Acrylic (AR) and polyurethane (UR) conformal coatings are among the best known and most commonly used conformal coating materials. As liquid coatings, both can be applied to substrates through a variety of methods: Brushing the substance onto the substrate surface. Dipping properly masked components into vats of coating material. Atomized/non-atomized spraying procedures, using either human or robotic labor. However, sharing basic application techniques and some product end uses does not mean they can be applied interchangeably. Significant differences need to be addressed prior to determining how appropriate either conformal coating is for use with any particular project. Basic Properties of AR and UR Understanding the fundamental differences between AR and UR is essential to their effective use. Acrylic: Protecting electronics from corrosion, dirt, dust, fungus, moisture, and thermal shocks, AR conformal films also possess dielectric properties capable of withstanding most static/voltage discharge. With a dielectric strength of V 300/Mil., a dielectric constant of 2.5, and a dissipation factor of 0.01, ARs’ effective operating temperatures range from -65'C through +125'C. Following application – usually via liquid brush or spray methods – they dry quickly to a clear, salt-resistant conformal finish. Fluorescent levels are consistently high. While their abrasive/chemical resistance is commensurately low, AR coatings are rather easily applied, cleaned and removed; they are also useful for many component miniaturization
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