Types of Conformal Coating
There are five main types of conformal coating available on the market today. They each possess a unique set characteristics and properties to offer protection from a wide range of conditions. These coatings can be divided into the following categories:
Parylene (Type XY)
Parylene is a common generic name for a unique series of polymers based on paraxylene. The Parylenes are formed by the pyrolysis of a di-p-xylene (dimer) in a vacuum environment which is then deposited on the cooler, room temperature substrate within the vacuum chamber. Parylene is applied in a vacuum as a gas that disperses evenly throughout the chamber to target surfaces the material forms as a clear polymer (plastic) film.
Acrylic (Type AR)
Acrylic conformal coatings are perhaps the most popular of all conformal coating materials due to their ease of application, removal and forgiving nature. Acrylics dry rapidly, reaching optimum physical properties in minutes, are fungus resistant, provide long pot life, and moderate dielectric constant. Additionally, acrylic coating gives off little or no heat during cure, eliminating potential damage to heat-sensitive components. They do not shrink during cure and have good humidity resistance and exhibit low glass transition temperatures. Mil-I-46058C and IPC-CC-830 approved.
Silicone (Type SR)
Silicone Conformal Coating is a transparent, soft, flexible coating which offers excellent moisture, corrosion and humidity resistance, good dielectric properties, and has exceptional light transmission capabilities for solar applications. It provides good chemical resistance, salt spray resistance and are very flexible. It functions well in high temperate applications and is resistant to UV light and dirt pick-up. This coating can be “solder-through” for easy repair. However, silicones are not solvent soluble and require chemical strippers for removal or repairs. MIL-I-46058C and IPC-CC-830 approved.
Polyurethane (Type UR)
Polyurethane coatings are available as either single or two-component formulations. Both formulations provide excellent humidity resistance and far greater chemical resistance than Acrylic coatings. Single component Polyurethanes, while easy to apply, enjoy long pot life but sometimes require very lengthy cure cycles to achieve full or optimum cure. In some instances, the cure cycle can be accelerated by the judicious application of heat.
Epoxy (Type ER)
Epoxy resins (ER) are usually available as two-part compounds that start curing upon mixing, but single part coatings that can be cured thermally or with UV exposure, are also available. Epoxy resins exhibit good abrasive and chemical resistance, as well as reasonable humidity resistance. The coating is virtually impossible to remove and rework requires burning through with a soldering iron. A buffer is recommended around delicate components, since film shrinkage occurs during polymerization. The shrinkage can be minimized by curing at a low temperature. Epoxy resins can be applied by brush, spray, or dip-coating.