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Design007-May2021

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90 DESIGN007 MAGAZINE I MAY 2021 case. Most of these coatings are single-part sys- tems that are easy to apply and quick to cure with little temperature rise. On the downside, single-part coatings are solvent based to mod- ify their viscosity for application purposes, which can have environmental repercussions. Conformal coatings can be applied manually using a paint brush, a spray gun, or simply by dipping the circuit board into a bath contain- ing the coating material. Where large numbers of circuits must be treated in a fast-moving production line, coatings are more likely to be applied by closely controlled robotic selective- coating systems for maximum consistency. Conformal coatings are applied in their liquid state as thin films which cure to provide a dry film thickness of between 25 and 100 microns, adding minimal weight increase to the assem- bly. ese coatings are usually clear and allow rework, so coated components are visible and easy to replace. e level of chemical resistance and thermal protection that conformal coatings provide is generally good for short exposures. Encapsulation resins and potting compounds can be applied in thicknesses from 0.5 mm, but are generally applied much thicker than this, which can lead to a significant weight gain for the assembly. Weight gain aside, this increased thickness does mean that the PCB is far better protected against chemical attack, particularly in cases of prolonged immersion. Depending upon the formulation, a resin can also provide superior protection against physical shock since its bulk will help to dissipate the forces across the PCB, rather than allowing them to be concentrated. A bonus for protecting your intellectual property and design advantages, a layer of dark-coloured resin can obscure the circuit layout and components from prying eyes. One thing to remember, though, is that attempting to remove the resin will damage the PCB, severely limiting opportunities for component replacements. Compounds and resins are generally two- part systems, in which a resin is mixed with a hardener in a precise ratio to form a cross- linked polymer when cured. It's also possible to add mineral substances (fillers) to resins to improve their performance under certain operating conditions. Like conformal coatings, most resins will cure at room temperature, and while this can be a relatively slow process in the case of potting resins, cure time can be reduced by applying heat. It might interest you to learn that, in certain applications, where a two-part resin formula- tion may have been the first choice for circuit protection, a two-part conformal coating may turn out to be the better approach, thanks to its superior mechanical properties, as com- pared with one-part coatings. For example, Electrolube developed the 2K range of sol- vent-free coating materials, based on similar two-part chemistry to resins, but designed to be applied by selective coating equipment in the 200–400-micron range, combining many of the advantages of both technologies and minimising many of the drawbacks of each. Moreover, switching from a resin to a confor- mal coating will eliminate the weight penalties of the former, which may be critical to some applications. Two-part conformal coatings can be applied relatively thickly without risk of cracking, giving a sharp edge coverage that performs somewhere between where a con- ventional conformal coating fails, and potting is required. A two-part conformal coating's environmental protection capability is also rather impressive. The level of chemical resistance and thermal protection that conformal coatings provide is generally good for short exposures.

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