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Conformal coatings is a term that refers to the fact that these coatings conform to the shape of the circuit assembly to which they are applied. Conformal coatings, except parylene, are semi permeable membranes; they do not make a coated circuit assembly waterproof. However, they provide protection from environmental elements, and can contribute to a longer operating lifetime for a circuit exposed to water, humidity, and other environmental effects.
Circuits that are targeted for operating in a hazardous environment will usually be treated with some form of conformal coating. Among the benefits of a conformal coating is protection from ionized contaminants, such as salt. Additional benefits include resistance circuit damage from shock and vibration, due to the adhesion properties of the particular coating to the surrounding components and the printed circuit board (PCB).
A number of different conformal coatings are commonly in use for PCB protection, including acrylics, epoxy, silicone, urethane, and parylene coatings. Acrylics are solvent-based coatings. Once the coating is applied to a PCB, the solvents evaporate, leaving a solid material (formerly suspended in solution) to coat the circuit board. Such coatings can be applied by dipping, spraying, or brushing on a PCB. This type of coating has advantages of not shrinking during curing, a long pot life, and excellent humidity resistance. Disadvantages include low abrasion resistance, low chemical resistance, and sometimes a too-rapid curing process that can trap air bubbles within the coating.
Epoxy conformal coatings, which can be single compounds, are more typically two-part compounds that start curing immediately upon mixing. They can be applied by dipping, spraying, brushing, or pouring on a PCB. Epoxy conformal coatings offer advantages of good chemical and abrasion resistance, and fairly good humidity resistance. Since they are typically opaque once applied, they can provide some protection against reverse engineering. On the down side, shrinkage of epoxies during curing can damage some components. Epoxies are also difficult to remove when rework is needed for a PCB, and the exothermic reaction of epoxies can cause thermal stress at the component level.
Silicones are usually single-component compounds that begin curing upon exposure to moisture in the air, along with temperature. They can be applied by dipping, spraying, or brushing on a PCB, and they can handle high humidity with good thermal endurance and they exhibit a low dissipation factor which is useful for high-impedance circuitry. On the down side, using silicones and other material in the same environment may present issues with cross-contamination. In addition, on certain difficult, low surface energy surfaces, adhesion may be improved by priming or by special surface treatment such as chemical or plasma etching. Also, with RTV-cured coatings, the adhesion of silicones typically lags behind the curing time, and it can take as much as 72 hours to achieve proper adhesive with a coating.
Urethane conformal coatings, which can be single- or two-compound materials, are applied by dipping, spraying, or brushing on a PCB. These coatings offer good humidity and abrasion resistor, with high dielectric properties and good chemical resistance. But they can be difficult to rework, turning yellow when heated for reworking purposes. In addition, stripping with chemicals may leave an ionic residue on the PCB.
Parylene conformal coatings are applied by means of chemical vapor deposition (CVD). They feature consistent thickness with a bubble-free coating, and offer good dielectric properties, with low thermal expansion and good abrasion resistance, as well as outstanding chemical resistance. But Parylene conformal coatings are also difficult to remove for PCBs that need rework, and they can be expensive to add because of the labor-intensive process of using CVD, and requiring extensive masking of the PCB prior to the parylene coating application.
The thicknesses of these different coatings vary somewhat, from typically 12.5 to 50?m thick for parylene coatings, 25 to 75?m thick for acrylic, epoxy, and polyurethane coatings, and 50 to 200?m thick for silicone coatings. The table, which is from a conformal coating manufacturer's web site, provides a quick comparison of these conformal coating materials.
Factors to consider when choosing a conformal coating material include its electrical and thermal properties as well as its mechanical durability. In addition, the material should be judged in terms of its moisture permeability, chemical compatibility and resistance, method of application, and repairability/reworkability.
Choosing A Coating
Why use a conformal coating? Circuit designers choose to conformally coat a PCB for a number of reasons. With the shrinking size and increasing circuit density of PCBs, more designers are turning to conformal coatings for increased protection and reliability, for example, to prevent electrical shorts, to protect again interaction from foreign materials that might come in contact with the PCB, to increase the PCB's resistance to humidity, and to boost resistance to shock and vibration.
Conformal coating can help maintain a PCB's surface insulation properties and impede leakage currents in the process. A conformal coating allows the use of reduced conductor spacing while helping to reduce corrosion, reduce electro-migration, and reduce dendritic growth. Conformal coatings can also protect PCBs against the effects of tin whiskers. Apparently due to the RoHS initiative, tin whiskers have become more of a problem with PCBs. While it is still possible for tin whiskers to penetrate through a conformal coating, there is little evidence to indicate that those tin whiskers can migrate to another metallic surface and penetrate back into the conformal coating. Little is known about the growth mechanism of tin whiskers although it is known that the formation does not require metal dissolution or the presence of a magnetic field. The small scale of typical tin whiskers (see figure) can cause problems if migrating through a PCB.
Some things to watch for during application of a conformal coating can be learned by consulting IPC standards, such as IPC-A-610 for "Acceptability of Electronic Assemblies" and IPC-CC-830B with Amendment 1 for "Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies." The requirements summary of IPC-A-610A regarding 10.8.1 for a conformal coating explain that a coating must be transparent, must be uniform in color and consistency, and must provide uniform coverage on both the PCB and its components. The requirements summary of IPC-A-610E regarding 10.8.2 for conformal coating note that there should be no loss of adhesion in the coating, no voids or bubbles, no discoloration or loss of transparency, no dewetting, mealing, peeling, wrinkles, cracks, ripples, fisheyes, or orange peel, no entrapped foreign particles that violate a minimum electrical clearance, and the coating must be completely cured and uniform. The requirements of IPC-A-610E for encapsulation (10.9) explain that there can be no bubbles, blisters, or breaks that affect the PCB assembly operation or sealing properties of the encapsulate material. These issues can be best controlled by controlling the application process and environment.
As general requirements, circuit boards meant for coating should be clean and dry prior to the coating process. No foreign particles should be allowed to settle on the boards prior to coating. The coating environment should be free of silicone, which can prevent coating adhesion if allowed to contaminate the circuit board or coating process. Issues that can contribute to problems occurring during a coating process include contaminated materials, not enough coating material deposited on the PCB, having coating sprayed too far from a circuit board, resulting in the coating drying before it reaches the board, and use of an incorrect curing profile. Bubbles can form in a coating if the humidity is too high, if air is entrapped in the material prior to application, if the coating material is applied too thickly, and if the pot pressure is too high.
Gel materials represent a relatively recent option for PCB conformal coatings. These are higher-viscosity versions of materials already applied as conformal coatings which, by their viscosity, reduce or eliminate the need for masking of the PCB. These materials can also provide additional benefits compared to standard conformal coatings, by preventing capillary action in connectors.
Some general recommendations when executing a conformal coating process include the use of a vacuum chamber to draw all of the air out of the conformal material before putting it into the coating pot. If using a gel, a smaller-diameter dip tube should be used in the pot, since a dip tube with too large of a diameter will draw an air channel through a conformal coating fluid. For best coating results, stainless-steel fittings should be used, since brass seems to be incompatible with some coating materials.
The choice of conformal coating material will depend on the requirements of an application and circuit. In all cases, attention to process details and environments can help ensure optimum results when applying a conformal coating. Fingerprint oils on a PCB, for example, can greatly hinder the adhesion of a conformal coating to the PCB. Even slight variations in the humidity can change the viscosity of a conformal coating and the effectiveness of the final results, so environmental factors must be tightly monitored and controlled in a conformal coating operation. Humidity of 50 percent or greater has been shown to have an impact on the effectiveness of applied conformal coatings.
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