Specifications and precautions for PCB conformal coating
1. What is conformal coating?
Conformal coating is a specially formulated coating used to protect circuit boards and related equipment from environmental erosion. Conformal coating has good high and low temperature resistance; after curing, it forms a transparent protective film with excellent insulation, moisture-proof, leakage-proof, shock-proof, dust-proof, corrosion-proof, aging-proof, and corona-resistant properties.
Under real-world conditions, such as chemical, vibration, high dust, salt spray, humidity and high temperature, the circuit board may corrode, soften, deform, mildew and other problems, causing the circuit board circuit to malfunction.
The three-proof paint is applied to the surface of the circuit board to form a three-proof protective film (the three-proof refers to moisture-proof, salt spray-proof and mildew-proof).
In the presence of chemicals (e.g. fuel, coolant, etc.), vibration, moisture, salt spray, humidity and high temperature, circuit boards without conformal coating may be corroded, mold grows and short circuits may occur, leading to circuit failures. The use of conformal coating 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, since conformal coating can prevent leakage, it allows higher power and closer printed board spacing. Thus, the purpose of miniaturization of components can be met.
2. Specifications and requirements of conformal coating process
Painting requirements:
1. Paint thickness: The paint film thickness is controlled at 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 performed after the paint film is cured (determine whether to perform secondary coating according to demand).
3. Inspection and repair: Visually inspect whether the coated circuit board meets the quality requirements and repair the problems. For example, if the pins and other protection areas are sticky with conformal coating, you can use tweezers to hold a cotton ball or a clean cotton ball dipped in board washing water to clean it. Be careful not to wash off the normal paint film when scrubbing.
4. Component replacement: After the paint film is cured, if you want to replace the component, you can do the following:
(1) Use an electric iron to directly solder the component, and then use a cotton cloth dipped in board washing water to clean the material around the pad;
(2) Solder the replacement component;
(3) Use a brush dipped in conformal coating to apply to the welding area, and let the paint film dry and solidify.
Operation requirements:
1. The work place of conformal coating must be clean and dust-free, with no dust flying. There must be good ventilation measures, and no unauthorized personnel are allowed to enter.
2. Wear protective equipment such as masks or gas masks, rubber gloves, chemical protective glasses, etc. during operation to avoid harming the body.
3. After work, clean the used tools in time, and close and cover the container containing conformal coating.
4. Anti-static measures should be taken for circuit boards. Circuit boards should not be placed overlappingly. During the coating process, the circuit boards should be placed horizontally.
Quality requirements:
1. There should be no paint flow or dripping on the surface of the circuit board. When applying paint with a brush, be careful not to drip on the partially isolated parts.
2. The three-proof paint layer should be flat, bright, and uniform in thickness, and protect the pads, SMD components or conductor surfaces.
3. There should be no defects and foreign objects such as bubbles, pinholes, ripples, shrinkage holes, dust, etc. on the surface of the paint layer and components. There should be no powdering or peeling. Note: Do not touch the paint film at will before the paint film is dry.
4. The components or areas that are partially isolated cannot be coated with three-proof paint.
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3. Parts and devices that cannot be coated with three-proof paint
1. Conventional devices that cannot be coated: high-power radiators, heat sinks, power resistors, high-power diodes, cement resistors, pull-out switches, potentiometers (adjustable resistors), buzzers, battery holders, fuse holders, IC holders, touch switches, relays and other types of sockets, pin headers, wiring terminals and DB9, plug-in or SMD light-emitting diodes (non-indicating functions), digital tubes, and grounding screw holes.
2. Parts and components that cannot be coated with conformal coating as specified in the drawings.
3. Components that cannot be coated with conformal coating as specified in the "Catalog of Components (Areas) Not Conformal Coating".
If the conventional non-coating components specified in the regulations need to be coated, the R&D department can specify the requirements or mark the drawings for conformal coating.
4. Precautions for the conformal coating process
1. The PCBA must have a process edge and the width must not be less than 5mm to facilitate the machine track.
2. The maximum length and width of the PCBA board is 410*410mm, and the minimum is 10*10mm.
3. The maximum height of components mounted on the PCBA is 80mm.
4. The minimum distance between the spraying area and the non-spraying area of the components on the PCBA is 3mm.
5. Thorough cleaning ensures that the corrosive residues are completely removed and the conformal coating is well adhered to the surface of the circuit board. The thickness of the paint should be 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 heat dissipation surface or radiator components, power resistors, power diodes, cement resistors, dip switches, adjustable resistors, buzzers, battery holders, fuse holders (tubes), IC holders, touch switches, etc.
5. Introduction to the rework of circuit board three-proof paint
When the circuit board needs to be reworked, 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 three-proof paint protective film, make sure that the substrate under the component, other electronic components, and the structure near the rework location will not be damaged. The methods for removing the protective film mainly include: using chemical solvents, micro-grinding, mechanical methods, and desoldering through the protective film.
The use of chemical solvents is the most common method for removing conformal coatings. The key lies in the chemical properties of the coating to be removed and the chemical properties of the specific solvent.
Micro-grinding uses high-speed particles ejected from a nozzle to "grind" away the conformal coating on the circuit board.
Mechanical methods are the easiest way to remove conformal coatings. To remove solder through the protective film, a discharge hole is first opened on the protective film to allow the molten solder to be discharged.
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