Maintaining proper thickness for silicone conformal coating is critical because it is applied very thick compared to other conformal coatings. If a coating is applied too thick, it may create excessive stresses on solder joints and components (particularly glass-bodied components). If a coating is applied too thin, it may not reach the optimum properties described on the Technical Data Sheet. For this reason, the IPC created the J-STD-001 to regulate and standardize the thickness that coatings are applied at. For silicones, the J-STD-001 calls out 0.00197 to 0.00827 in. Our operators strive to hit between .002” and .008” for silicone applications.
In order to get the proper thickness, silicone conformal coating can be applied three ways:
Spray application can be done via hand-spray, aerosol can, or by robotics. The material is usually diluted with solvents to get to a predetermined viscosity and is sprayed from all four quadrants at a 45 degree angle, when done manually or with aerosols.
Dip coating is typically done by an automated machine, although it can be done manually as well. The assemblies are typically hung by an arm and then lowered in a dip tank containing the coating with an immersion rate determined by the population density of the PCB to be dipped and the desired thickness of the coating.
Brush coating is done manually by an operator with a brush. Brush coating is mainly employed during coating touchup operations or for conformal coating rework.