Conformal coating is a material that is applied to electronic products or assemblies to protect them from solvents, moisture, dust or other contaminants that may cause harm. Coating also prevents dendrite growth, which may result in product failure. This paper will discuss the variables that affect the application of conformal coatings, and review in detail those variables that impact the process of selective coating of printed circuit boards.
Methods of Applying Conformal Coating
As with most processes in the electronics industry, there are several ways to apply conformal coatings to the product. Some of the methods are typically performed manually while others are automated.
Dip Coating
One of the oldest and best-known methods of coating is the dip process. The dip process can be done manually or automatically. In the manual mode, operators immerse the PCB in a tank of coating. The PCB is either immersed totally or to a predetermined level on the board. Some manual dip systems will automatically move the board down into the tank and remove the board from the tank. This allows for more control. Automatic dip systems consist of a tank of coating and a conveyor to move the PCB’s. The PCB’s are put on hangers, conveyed to the tank, moved through the coating and then removed. The speed of the conveyor determines the amount of material applied. With either manual or automatic systems, components that can not be exposed to the coating but are below the dip line must be masked.
Brush Application
Brushing the material on the PCB is another option. This is a manual process where an operator dips a brush into a container of coating material and brushes the material onto the PCB. There is no equipment investment, no tooling or masking required and the process is simple. Disadvantages include operator exposure to material, inconsistencies in coverage, material contamination and viscosity variances. Although brushing may be adequate for low volume prototype runs, this process is not viable for mass production.
Atomized Air Spray
Air spraying (painting) the board manually is another common method used to conformally coat PCB’s. Since air spraying involves a lot of overspray, components on the PCB that can not be exposed to coating must be masked. Masking is done manually with either tape or boots. After the masking is done, the boards are laid out or hung to allow for exposure to the spray. An operator will then spray the PCB’s with a hand held spray gun similar to those used to spray paint. The boards are allowed to cure and then the masking material is removed.
