The Production Process for Conformal Coating

The production process for conformal coating goes through eight steps. Each step is related to how effective, reliable, and efficient the coating will be on a printed circuit board (PCB).

1. Cleaning

Cleaning is the initial stage of the procedure that gets rid of residues from operator handling, soldering, machine strain, and environmental contamination. Among the cleaning methods are:

Aqueous Chemistries: This method cleans the board using water-based solutions.

Solvent Cleaning: This method uses solvent-based chemicals for contaminant removal.

Plasma Cleaning: Effective cleaning of the board surface is done using ionized gas.

Validation tests for cleanliness are equally important, and these include:

Ionic Contamination Testing

Ion Chromatography

Surface Insulation Resistance

2. Priming

The primer application involves the application of a preparatory coating over the board, enhancing adhesion and forming a binding layer for the applied conformal coating. The need for priming is variable and dependent upon the type of resin used:

Acrylic and Polyurethane: These coatings are wet and generally adhere satisfactorily to most substrates without the use of a primer.

Silicone Conformal Coating: This coating usually needs primers to promote a successful coating operation.

Parylene Coatings: Priming involves either the gas phase or liquid processes.

3. Masking

Masking is used to protect particular board components and areas from being coated. Of course, this is required since the coating functions as an insulator. Some typical masking materials are:

Masking Tapes, Dots, and Shapes

Liquid Latex

Masking Boots

The type of masking material one chooses depends on the application and must be selected to ensure masking does not fail.

4. Application

Conformal coating can be applied by brushing, spraying, dipping, and vapor deposition. All these methods are chosen based on the requirements of the coating and the board.

5. De-masking and Finishing

After application, the masking materials are removed. The finishing processes ensure the following:

Quality of Coating

Prevention of Masking Leakage

Verification of Standards

Prevention of Coating Defects

The identified defects are repaired before proceeding to the next step.

6. Drying and Curing

It is important to understand that drying is not the same as curing. The difference is as follows:

Drying: The time it takes for the PCB to become safe for operator handling, which might range from a few minutes to many days.

Curing: This step ensures the coating attains its preferred electrical and mechanical properties to protect the board during operation.

These are the cure mechanisms developed to effect the cure, such as:

Heat Cure

Moisture Cure

UV Cure

Catalytic Cure

7. Inspection

The most critical step in inspection is to ensure coating quality against the standards. This consists of:

Manual Inspection

UV Lamp Visibility

Automated Optical Inspection (AOI)

8. Conformal Coating Chemistries

Available chemistries are based on requirements, which include:

Acrylic (Organic Liquid):

 Easy rework

 Easy drying

 Good moisture resistance

 Easy viscosity adjustment

Polyurethane (Organic Liquid):

 Dielectric Properties

 Abrasion resistance

 Solvent resistance

 Good moisture resistance

Epoxy (Organic Liquid):

 Good dielectric properties

 Higher Tg, glass transition temperature

 Effective at 150℃

 Good abrasion resistance

Silicone (Inorganic Liquid):

 Stable over a wide temperature range

 High Tg

 Resistance to solvents

 Good dielectric strength