Specification and precautions for three proofing paint on PCB
Time:2023-10-16
Three proofing paint is a specially formulated coating used to protect circuit boards and related equipment from environmental erosion. Three proofing paint has good resistance to high and low temperatures; After curing, it forms a transparent protective film, which has superior insulation, moisture resistance, leakage resistance, shock resistance, dust prevention, corrosion resistance, aging resistance, corona resistance, and other properties.
Under real-world conditions, such as chemistry, vibration, high dust, salt spray, humidity, and high temperature, circuit boards may experience problems such as corrosion, softening, deformation, and mold, leading to circuit board failures.
The three proofing paint is applied to the surface of the circuit board to form a protective film (three proofing refers to moisture resistance, salt spray resistance, and mold resistance).
Under conditions such as chemical substances (such as fuel, coolant, etc.), vibration, moisture, salt spray, humidity, and high temperature, circuit boards that do not use the three proof paint may be corroded, mold grows, and short circuited, leading to circuit failures. The use of the 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, due to the ability of the three proof paint to prevent leakage, higher power and closer spacing between printed boards are allowed. This can meet the purpose of miniaturizing components.
Specifications and requirements for the three proofing paint process
Painting requirements:
1. Paint thickness: The paint film thickness should be controlled between 0.05mm-0.15mm. The dry film thickness is 25um-40um.
2. Secondary coating: To ensure the thickness of products with high protection requirements, secondary coating can be carried out after the paint film has solidified (whether to carry out secondary coating according to demand).
3. Inspection and repair: Visually inspect whether the coated circuit board meets the quality requirements and repair the problem accordingly. For example, if the contact pin and other protected areas are stained with three proofing paint, they can be wiped clean with tweezers using degreased cotton balls or clean cotton balls dipped in board water. When wiping, be careful not to remove the normal paint film.
4. Component replacement: After the paint film has solidified, if you want to replace the component, you can proceed as follows:
(1) Weld the components directly with electrochromic iron, and then use a cotton cloth dipped in board washing water to clean the surrounding material of the solder pad
(2) Soldering alternative components
(3) Dip a brush with a three proof paint brush to apply to the welding area and dry and solidify the paint film surface
Operating requirements:
1. The workplace of the three prevention paint requires dust-free and clean, without dust flying, and must have good ventilation measures, and unrelated personnel are prohibited from entering.
2. Wear protective equipment such as masks or gas masks, rubber gloves, chemical protective glasses, etc. during operation to avoid injury to the body.
3. After completing the work, it is necessary to clean the used tools in a timely manner, and close and tightly cover the containers containing the three proofing paint.
4. Antistatic measures should be taken for circuit boards, and circuit boards should not be overlapped. During the coating process, circuit boards should be placed horizontally.
Quality requirements:
1. The surface of the circuit board should not have flowing paint or dripping. When brushing the paint, be careful not to drip onto the partially isolated parts.
2. The three prevention paint layer should be flat, bright, and of uniform thickness, protecting the surface of solder pads, SMD components, or conductors.
3. The surface and components of the paint layer should be free of defects such as bubbles, pinholes, ripples, shrinkage, dust, and foreign objects. There should be no powdering or peeling. Attention: Before the paint film is dried, it should not be touched arbitrarily.
4. Components or areas that are partially isolated cannot be coated with three proofing paint.
Parts and devices that cannot be coated with three proofing paint
1. Conventional non coating devices: painted high-power heat sinks, heat sinks, power resistors, high-power diodes, cement resistors, pull-out switches, potentiometers (adjustable resistors), buzzers, battery bases, fuse bases, IC bases, touch switches, relays, and other types of sockets, pins, wiring terminals, DB9, plug-in or SMD light emitting diodes (non indicating), digital tubes, grounding screw holes.
2. The parts and components specified in the drawings that cannot use three proofing paint.
3. According to the detailed regulations in the "Catalogue of Non Three Prevention Components (Regions)", components with three prevention paint cannot be used.
For conventional non coating devices that require coating operations as specified in the regulations, the R&D department can specify requirements or indicate on the drawings for three proofing coating before coating.
Precautions for the spraying process of three proofing paint
1. PCBA must have a process edge and a width of no less than 5mm for easy access to the machine and track.
2. The maximum length and width limit of PCBA board is 410 * 410mm, and the minimum limit is 10 * 10mm.
3. The maximum height limit for PCBA mounted components is 80mm.
4. The minimum distance between the spray area and the non spray area of the PCBA component is 3mm.
5. Thorough cleaning can ensure that corrosive residues are completely removed and that the three proofing paint adheres well to the surface of the circuit board. The paint thickness should be between 0.1 and 0.3mm. Drying condition: 60 ° C, 10-20 minutes.
6. During the spraying process, some components cannot be sprayed, such as high-power components with heat dissipation surfaces or radiators, power resistors, power diodes, cement resistors, dial switches, adjustable resistors, buzzers, battery holders, fuse holders (tubes), IC holders, light touch switches, etc.
Introduction to the repair of circuit board's three proofing paint
When a circuit board needs to be repaired, expensive components on the board can be removed separately and the remaining parts discarded. But a more common method is to remove all or partial protective film on the circuit board and replace damaged components one by one.
When removing the protective film of the three proofing paint, it is necessary to ensure that it does not damage the substrate below the component, other electronic components, or structures near the repair location. The removal methods of the protective film mainly include: using chemical solvents, micro grinding, mechanical methods, and welding through the protective film.
The use of chemical solvents is the most commonly used method for removing the protective film of the three proofing paint, and its key lies in the chemical properties of the protective film to be removed and the specific chemical properties of the solvent.
Micro grinding is the use of high-speed particles sprayed from a nozzle to "grind" off the protective film of the three proofing paint on the circuit board.
Mechanical methods are the easiest way to remove the protective film of the three proofing paint. Welding through a protective film involves opening a discharge hole on the protective film to allow molten solder to be discharged.
