Silicone Coatings

Silicone conformal coatings are most widely used in harsh environment applications, including under hood automotive applications, due to their innate ability to withstand prolonged exposure to higher temperatures. Silicones are also capable of being applied in thicker films, making the coatings useful as a vibration dampening/isolation tool if the assembly is to be placed in a high vibration environment.

One of the keys for providing protection to underlying circuitry and other metallic surfaces is attaining good wetting and adhesion of the cured silicone to all surfaces of the electronic device or module. Water, in liquid form, is easily excluded at the cured surface of silicone coatings, but water vapor, present as humidity in the air, can permeate down to the substrate surface. Provided there is good adhesion of the coating, there are no gaps in which the moisture can penetrate, thus no damage is caused to the underlying circuitry.

Silicone Conformal Coating Process

Silicone coatings can be applied using one of these three common methods:

• Dip: Dip coating can be accomplished through use of an automated or manually-controlled arm that holds the device and lowers it into a dip tank filled with coating. Finished coating thicknesses can be highly repeatable when using automated systems due to programmable dip, dwell and removal times.
• Spray: The spray method uses an aerosol can or spray gun to apply conformal coatings. Quick to the quick nature of application, spraying can be useful with high volumes. For large production volumes, robotic spray processes can be used to increase efficiency.
• Brush: With the brushing method, the operator uses a brush to let the silicone coating flow over the device. Brushing is common for low volume production runs, rework and repairs.

Rework of Silicones

Silicone conformal coatings can be more difficult to rework than acrylics or polyurethanes due to their high heat tolerance, chemical resistance properties and increased coating thickness.