Assembly Planbook - April 2008 - (Page 53) while curing. This may stress components, so engineers should take that into account when designing their boards. Due to their extreme solvent resistance, epoxies are virtually impossible to strip. If rework must be done, it typically requires a soldering iron. Parylenes are a unique family of coatings. Applied through vacuum deposition, these polymers form a strong, ultrathin, pinhole-free coating. The vacuum deposition process applies a uniform coating of Parylene to every area on the assembly—even hidden surfaces that other coatings can’t reach. As such, contact points such as battery contacts or connectors must be carefully masked. Parylenes do not contain solvents or catalysts, and require no curing. Parylenes have excellent dielectric strength, high thermal stability (up to 450 C), and outstanding chemical resistance. They protect circuits against high humidity, intermittent immersion, fog, pollution and aggressive solvents. They’re also biocompatible and biostable, making them suitable for use with medical devices. Rework of Parylenes is possible. Removal methods include microabrasive blasting, incising, excimer laser etching, heat softening and plasma etching. Areas from which Parylene has been removed can be recoated to their Urethane coatings are available in one- or two-part chemistries. Some can be difficult to apply, but all are durable and provide a smooth, nonporous surface. original specifications. Silicones have good electrical insulation properties, making them suitable for high power and highly populated boards. Once cured, silicones are pliable. Their adhesive strength ranges from easily detachable to tightly bonded. They have good resistance to moisture, humidity, heat and ultraviolet light. Because they’re so flexible, silicones can be applied much thicker than other materials. They can withstand expansion Automatic coating equipment can be equipped with an atomizing spray head to provide general coverage or a needle dispenser to apply coatings into deep recesses and around tall components. and contraction without transmitting this stress to underlying components. Silicones are available in solvent-resistant and solvent-removable forms. Customized solvents are typically required to remove the former. The latter are removed more easily. Urethanes are available in one- or two-part chemistries. Some are more difficult to apply, but all are very durable and provide a smooth, nonporous surface. They have good humidity and chemical resistance, and high dielectric properties. Urethanes provide similar moisture protection as acrylics and silicones. Hardness can range from extremely hard to relatively soft. One-part urethanes are easier to apply, but may require a long cure time. Some two-component urethanes cure quickly. However, significant shrinkage can occur while curing, and the extremely hard film may stress components. Heat from a soldering iron will make urethanes pliable enough to remove, but workers should take precautions to avoid exposure to isocyanides, which can irritate the skin, nose and eyes. Choosing the best coating for a particular application can be confusing. It’s a good idea to consult a few vendors to learn the benefits and limitations of the www.assemblymag.com April 2008 / ASSE M B LY 53 http://www.assemblymag.com
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