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Design News, March 2013

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Source: Dow Corning ufacturing processes that are already in place, while also minimizing any cost increases. But they often end up being more expensive, and likely less effective, than if such protection had been factored into the design of a device right from the start. Environmental protection strategies often involve trade-offs among performance, perceived value, and cost. Added circuitry and component protection can enhance customer satisfaction and the value of a device, but they may not always justify a higher price to the purchaser. Although many different materials have been used in the past for A silicone conformal coating has been spray applied to automotive controllers, conformal coatings, three main types and fluoresces under UV light to aid visual inspection of the coating's coverage. dominate for use with electronics. These are acrylics, urethanes, and silicones. Each has its own advantages and limitations. Acrylics (sometimes abbreviated as "AR") are often inexpensive and easy to apply. They provide a hard, tough coating that is relatively easy to repair. However, they have a narrow useful temperature range and are typically supplied in the form of solvents. Some of these solvents can pose handling, safety, and environmental challenges. Acrylics generally do not have a cure mechanism; instead, solvent evaporation dries the film. Polyurethanes (sometimes abbreviated as "PU") overcome some of the limitations of acrylics by being usable over a much wider range of temperatures. Though typically not as hard, polyurethanes tend to be tough, and A controlled pattern silicone conformal coating has been spray applied to automotive they add relatively low stress to sensicontrollers, and fluoresces under UV light to aid visual inspection of the coating's coverage. tive solder joints and wire bonds on a circuit board. These advantages come with their own trade-offs. Polyurethanes are more diftypically a higher price per kilogram.Yet several factors are ficult to use, since circuit boards often must first be dried influencing engineers to choose silicone conformal coatings before they are applied. There are also toxicity concerns over other options to provide greater reliability and durabilwith some urethane formulations, which must be carefully ity to electronic devices. These factors are low stress, lower addressed before they can be used. production costs, tin whisker issues associated with lead-free Silicones (sometimes abbreviated as "SR") have the widsolder, high-temperature processing issues associated with est usable temperature range and are very easy to handle. lead-free solder, higher reliability, and faster processing. Most of them are solventless and add very little stress. HowElectronics are shrinking in size even as they expand in ever, these superior properties do come at a cost, which is functionality. This results in boards with higher component Design News | march 2013 | www.d esign n ews.com –37–

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