1. What is the three-proof paint?
Three anti-paint is a special formula of paint, used to protect 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 actual 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 actual 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).
2, the 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 products with high protection requirements, secondary coating can be performed after the paint film is cured (determine whether to perform secondary coating according to 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 protective 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. Replacement of components: 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 board water to clean the material around the pad
(2) Welding alternative components
(3) Use a brush to dip the three-proof paint to brush the welding part, and make the paint film surface dry and solidify.
Operation requirements:
1. The three-proof paint workplace must be dust-free and clean, and there must be no dust flying. Good ventilation must be provided 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 circuit board should not have paint flow or dripping. When the paint is painted, it should not drip to the partially isolated part.
2. The three-proof paint layer should be flat, bright, uniform in thickness, and 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.
3. Parts and devices that cannot be coated with conformal paint
(1) Conventional non-coatable devices: paint high-power radiator, heat sink, power resistor, high-power diode, cement resistor, code switch, potentiometer (adjustable resistor), buzzer, battery holder, fuse holder, IC sockets, light touch switches, relays and other types of sockets, pin headers, terminal blocks and DB9, plug-in or SMD light-emitting diodes (non-indicating function), digital tubes, ground screw holes.
(2) The parts and devices specified by the drawings that cannot be used with three-proof paint.
(3) According to the “Catalogue of Non-Three-proof Components (Area)”, 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 specified by the R&D department or the drawings.
Four, the precautions of the three anti-paint spraying process are as follows
1. The PCBA must be made with a crafted edge and the width should not be less than 5mm, so that it is convenient to walk 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.
V. Introduction of circuit board 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, ensure 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 uses high-speed particles ejected from a nozzle to “grind” 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.