PCB Tech - Three anti-paint specifications and precautions on PCB

Three anti-paint is a kind of specially formulated paint, used to protect PCB circuit boards and related equipment from environmental erosion. The three-proof paint has good resistance to high and low temperature; it forms a transparent protective film after curing, which has excellent insulation, moisture resistance, leakage resistance, shock resistance, dust resistance, corrosion resistance, aging resistance, corona resistance and other properties.

Under realistic conditions, such as chemical, vibration, high dust, salt spray, humidity and high temperature, the circuit board may have corrosion, softening, deformation, mildew and other problems, which may cause the circuit board to malfunction.

The three-proof paint is coated on the surface of the circuit board to form a layer of three-proof protective film (three-proof refers to anti-moisture, anti-salt spray, and anti-mildew).

Under conditions such as chemical substances (for example: fuel, coolant, etc.), vibration, humidity, salt spray, humidity and high temperature, the circuit board that is not used with the three-proof paint may be corroded, mold growth and short circuit, etc., leading to the circuit In the event of a failure, the use of three-proof paint can protect the circuit from damage, thereby improving the reliability of the circuit board, increasing its safety factor, and ensuring its service life.

In addition, because the three-proof paint can prevent leakage, it allows higher power and closer printed board spacing. Thereby, the purpose of miniaturization of components can be met.

Specifications and requirements of the three anti-paint process

Painting requirements:

1. Spray paint thickness: The paint film thickness is controlled within 0.05mm-0.15mm. The dry film thickness is 25um-40um.

2. Secondary coating: In order to ensure the thickness of the product with high protection requirements, the secondary coating can be carried out after the paint film is cured (determine whether to carry out the secondary coating according to the requirements).

3. Inspection and repair: visually check whether the coated circuit board meets the quality requirements, and repair the problem. For example, if the pins and other protected areas are stained with three-proof paint, use tweezers to hold a cotton ball or clean cotton ball dipped in the washing board water to clean it. When scrubbing, be careful not to wash off the normal paint film.

4. PCB component replacement: After the paint film is cured, if you want to replace the components, you can do as follows:

(1) Solder the components directly with electric chromium iron, and then use a cotton cloth dipped in the board water to clean the material around the pad

(2) Welding alternative components

(3) Use a brush to dip the three anti-paint to brush the welding part, and make the paint film surface dry and solidify

Operation requirements:

1. Three anti-paint workplaces are required to be clean and dust-free, and there must be good ventilation measures, and irrelevant personnel are prohibited from entering.

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

3. After the work is finished, clean the used tools in time, and close and tightly cover the container with the three-proof paint.

4. Anti-static measures should be taken for the circuit boards, and the circuit boards should not be overlapped. During the coating process, the circuit boards should be placed horizontally.

Quality requirements:

1. The surface of the PCB must not have flow paint or dripping. When the paint is painted, be careful not to drip to the partially isolated part.

2. The three-proof paint layer should be flat, bright, and uniform in thickness to protect the surface of the pad, patch component or conductor.

3. The surface of the paint layer and components must not have defects such as bubbles, pinholes, ripples, shrinkage holes, dust, etc. and foreign objects, no chalking, no peeling phenomenon, note: before the paint film is dry, do not touch the paint at will membrane.

4. Partially isolated components or areas cannot be coated with three-proof paint.

Parts and devices that cannot be coated with conformal paint

Conventional non-coating devices: paint high-power radiators, heat sinks, power resistors, high-power diodes, cement resistors, code switches, potentiometers (adjustable resistors), buzzers, battery holders, fuse holders, IC holders, Touch switches, relays and other types of sockets, pin headers, terminal blocks and DB9, plug-in or patch-type light-emitting diodes (non-indicating function), digital tubes, grounding screw holes.

2. The parts and components specified by the drawings that cannot be used with three anti-paints.

3. According to the "Non-Three-proof Components (Area) Catalogue", it is stipulated that devices with three-proof paint cannot be used.

If the conventional non-coatable devices in the regulations need to be coated, they can be coated by the three-proof coating according to the requirements specified by the R&D department or the drawings.

Precautions for three anti-paint spraying process

1. The PCBA must be made with a crafted edge and the width should not be less than 5mm, so that it can be easily tracked on the machine.

2. The maximum length and width of PCBA board is 410*410mm, and the minimum is 10*10mm.

3. The maximum height of PCBA mounted components is 80mm.

4. The minimum distance between the sprayed area and the non-sprayed area of the components on the PCBA is 3mm.

5. Thorough cleaning can ensure that the corrosive residues are completely removed, and make the three-proof paint adhere to the surface of the circuit board well. The paint thickness is preferably between 0.1-0.3mm. Baking conditions: 60°C, 10-20 minutes.

6. During the spraying process, some components cannot be sprayed, such as high-power radiating surface or radiator components, power resistors, power diodes, cement resistors, dial switches, adjustable resistors, buzzers, Battery holder, insurance holder (tube), IC holder, touch switch, etc.

Introduction to PCB Tri-proof Paint Rework

When the circuit board needs to be repaired, the expensive components on the circuit board can be taken out separately, and the rest can be discarded. But the more common method is to remove the protective film on all or part of the circuit board, and replace the damaged components one by one.

When removing the protective film of the three-proof paint, make sure that the substrate under the component, other electronic components, and the structure near the repair location will not be damaged. The protective film removal methods mainly include: using chemical solvents, micro-grinding, mechanical methods and desoldering through the protective film.

The use of chemical solvents is the most commonly used method to remove the protective film of the three-proof paint. 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 a nozzle to "grind" off the protective film of the three-proof paint on the circuit board.

The mechanical method is the easiest way to remove the protective film of the three-proof paint. Desoldering through the protective film is to first open a drain hole in the protective film to allow the molten solder to be discharged.