Conformal Coating Specifications and Precautions on PCB

 1. Conformal coating

Conformal coating is a specially formulated coating, to protect PCB and related devices from corrosion. Conformal coating has anti high temperature and anti low temperature functions. After conformal coating is set, it becomes a transparent protective film, which has superior insulation, moisture-proof, leakage-proof, shock-proof, dust-proof, anti-corrosion, anti-aging, corona resistance and other properties.

                                                                         

Under realistic conditions, such as chemical materials, vibration, high dust, salt spray, humidity and high temperature, the circuit board may have corrosion, softening, deformation, mildew and other problems, resulting in circuit board failure.

Conformal coating is applied to the surface of the circuit board to form a protective film of conformal coating (conformal refers to moisture-proof, anti-salt spray, and mildew-proof).

                                                               

Circuit boards that do not use conformal coatings under conditions such as chemicals (e.g., fuel, coolant, etc.), vibration, moisture, salt spray, humidity and high temperatures, can have corrosion, mildew and short circuits. And this can cause circuit failures. The usage of conformal coatings can protect circuits, increase the reliability and safety of circuit boards and ensure its service life.

In addition, because the conformal coating prevents leakage, it allows higher power and closer PCB spacing. This  achieves the purpose of miniaturizing components.

2. Specifications and requirements for conformal coating process

Painting requirements:

1) Painting thickness: The thickness of conformal coating is controlled at 0.05mm-0.15mm, and the thickness of the dry conformal coating is 25um-40um.

1) Coating for second times: to ensure the thickness of products of high protection, painting should be done for second times after conformal coating is set. (It is necessary to decide to adopt coating for second times or not according to requirements)

2) Check: Visually inspect whether the coated circuit board meets the quality requirements or not, and do some maintenance. For example, the pins and other protection areas are glued with conformal paint, you can use tweezers to clip absorbent cotton balls or clean cotton balls dipped in water to scrub circuit board clean, and pay attention not to wash off the normal conformal paint when scrubbing.

3) Replacement of components:

Remove the components directly with electrochrome iron, and then use a cotton cloth dipped in the water to clean the material around the pad; solder the replacement component; Solid brush dipped in conformal paint to brush the soldering part, and let the surface of the conformal coating dry and solidify.

Process requirements:

The workplace of conformal coating should be dust-free, and there must be good ventilation measures, and irrelevant personnel are prohibited from entering.

Wear masks or gas masks, rubber gloves, chemical protective glasses and other protective equipment during operation to avoid harming the body.

After the work is completed, the used tools should be cleaned in time, and the container with conformal paint should be closed and covered tightly.

Anti-static measures should be taken for the circuit board, and the circuit board should not be placed on top of each other, and the circuit board should be placed horizontally during the coating process.

                                                                             

 

Quality requirements

There can be no flow coating and dripping on the surface of the circuit board, and attention should be paid that conformal coating does not drip into the local isolation when the brush is used to paint.

The conformal coating layer should be flat, bright, and uniform in thickness to protect the surface of pads, SMT components or conductors.

The surface of the conformal coating and the components can not have bubbles, pinholes, corrugations, shrinkage holes, dust and other defects, no chalking, no peeling, note: before conformal coating is dry, do not touch the conformal coating film at will.

Locally isolated components or areas should not be coated with conformal coating.

 

3. Parts and devices that cannot be coated with conformal coating

1) Conventional non-painted devices: high-power radiators, heat sinks, power resistors, high-power diodes, cement resistors, DIP switches, potentiometers (adjustable resistance), buzzers, battery holders, fuse holders, IC holders, touch switches, relays and other types of sockets, pin headers, terminal blocks and DB9, plug-in or SMT light-emitting diodes (non-indicative), digital tubes, grounding screw holes.

       

                                                                   

1) The drawings show that the parts and components that cannot use conformal coating.

2) Devices that cannot be coated by conformal coating are specified in the details of the "Catalogue of Non-Conformal Components (Regions)".

If usual non-painted devices in the regulations need to be coated, they can be coated as per the R&D department  requirements or the drawings with the conformal coating mark.

 

4. Precautions for conformal coating spraying process

1) The PCB must have a process edge and the width cannot be less than 5mm, so that it is convenient to walk on the track;

2) Maximum length and width of PCB is 410*410 mm, and the minimum length and width of PCB is 10*10mm;

3) The maximum height of SMT components is 80 mm;

4) The minimum distance between the spraying area and the non-spraying area of the components on the PCB is 3mm.

5) Thorough cleaning ensures that corrosive residues are completely removed and that the conformal coating adheres well to the surface of the board. The thickness of the conformal coating is between 0.1~0.3mm. Baking conditions: 60°C, 10~20 minutes.

6) In the process of spraying, some components can not be sprayed, such as: high-power with heat dissipation surface or radiator components, power resistors, power diodes, cement resistors, DIP switches, adjustable resistors, buzzers, battery holders, fuse seats (tubes), IC holders, touch switches, etc.

 

5. Maintenance

The protection film on PCB should be removed, and broken components should be replaced.

When the conformal coating film is removed, make sure that the substrate underneath the components, other electronic components, and structures near the maintenance location will not be damaged. The removal methods mainly include: the use of chemical solvents, micro-grinding, mechanical methods and dismantlement through the protective film.

The use of chemical solvents is the most common method of removing conformal coating protective film, and the key lies in the chemical properties of the protective film to be removed and the chemical properties of the specific solvent.

Micro-grinding is the use of high-speed particles ejected from the nozzle to "grind" off the conformal coating protective film on the circuit board.

The mechanical method is the easiest way to remove the conformal coating protective film. To remove soldering through the protective film is that a discharge hole is made in the protective film so that the molten solder can be discharged.