Assembly Planbook - April 2008 - (Page 52) ASSEMBLY Dispensing Put Your Coat On Conformal coatings protect electronic assemblies from the elements. A Parylene is biocompatible and biostable, so it’s suitable for coating medical devices. By Bill Boyd Equipment Program Manager Specialty Coating Systems Inc. Indianapolis pril might officially be spring, but here in the Midwest at least, you’d be wise to have a raincoat and a stout umbrella if you don’t want to get soaked. Many electronic assemblies need protection from the elements, too. Conformal coatings shield electronic assemblies from thermal shock, moisture, humidity, corrosion, fungus, dust, dendrite growth, oxidation and even handling during final assembly. Conformal coatings do more than just keep electronic products dry. These polymer coatings strengthen delicate components and leads and support small devices that can’t be mechanically secured. And, because conformal coatings are electrically insulative, they prevent current leakage and inhibit arcing. This is a great benefit to electronics designers who face the challenge of packing more power and capability into an evershrinking footprint. A variety of polymers can be used as conformal coatings. The most common chemistries are acrylics, epoxies, Parylenes, silicones and urethanes. Less common polymers include amorphous fluoropolymers, fluorocarbons and perfluoroethers. Many coatings are hybrids of two or more of these chemistries. Most are produced as traditional, solvent-based materials, but some are available in solvent-free formulations. Acrylics are relatively hard with a smooth, glossy finish. They are one of the easiest coatings to handle: They are easy to apply and dry quickly. They do not require a curing process, and can easily be removed with solvents. Acrylics resist moisture as well as urethanes and silicones. They have moderate abrasion resistance, but are not so good at withstanding heat, alcohol or petroleum Conformal coatings do more than just keep electronics dry. solvents. They form a thin, tough shield, but are best used where environmental exposure is minimal. Because they lack resistance to solvents, acrylics are easily reworked. Adhesion problems can occur if the substrate is not 100 percent clean and dry. Acrylics provide good dielectric protection and have a temperature range of –59 to 132 C. Epoxies are typically two-part thermoset materials. They offer good humidity resistance and high abrasion resistance. They resist all solvents and adhere strongly to most surfaces. Epoxies form a hard, smooth surface that resists chipping, peeling or cracking. A drawback to epoxies is that they shrink 52 ASSEMBLY / April 2008 www.assemblymag.com http://www.assemblymag.com
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