PCB on the three-proof paint specifications and precautions

What is conformal coating? Conformal coating is a specially formulated coating used to protect circuit boards and related equipment from environmental damage. Conformal coating has good resistance to high and low temperatures; After curing, a transparent protective film will be formed, which has excellent insulation, moisture-proof, anti-leakage, shockproof, dust-proof, anti-corrosion, anti-aging, corona resistance and other properties. Under realistic conditions such as chemistry, vibration, high dust, salt spray, humidity and high temperature, the circuit board may be corroded, softened, deformed, mildewed, etc., which can lead to the failure of the circuit board. The conformal coating is applied to the surface of the circuit board to form a protective film with the functions of 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 temperatures, uncoated circuit boards may be corroded, mold and short circuits can occur, resulting in circuit failure, and 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, because the conformal coating prevents leakage, it allows for higher power and smaller PCB spacing, which can meet the needs of component miniaturization. In the actual production process, the correct use of conformal coating is the key to ensure the quality and performance of the circuit board. From the perspective of the characteristics of the conformal coating itself, it provides multi-faceted protection for the circuit board, which plays an indispensable role in meeting the function of the circuit board itself under harsh environmental conditions and meeting the function of the circuit board itself. For example, the presence of conformal coating greatly reduces the risk of failure of the circuit board in terms of protecting it from chemical attack and environmental factors such as vibration, moisture, salt spray, etc., thereby improving its reliability and extending its service life.

Specifications and requirements of the conformal coating process Painting requirements:

<!--[if !supportLists]-->1.   Paint thickness: Paint film thickness should be controlled at 0.05mm - 0.15mm, dry film thickness is 25um - 40um.。

<!--[if !supportLists]-->2.   <!--[endif]-->Secondary coating: In order to ensure the thickness of products with high protection requirements, secondary coating can be carried out after the paint film has cured (whether to carry out secondary coating according to the requirements).

<!--[if !supportLists]-->3.   <!--[endif]-->Inspection and repair: Visually inspect the coated circuit board to meet quality requirements and fix the problem. For example, if the pins and other protected areas are stained with conformal coating, you can use tweezers to clip absorbent cotton balls or clean cotton balls dipped in washing water to scrub them clean, and be careful not to wipe off the normal paint film when scrubbing.

<!--[if !supportLists]-->4.   <!--[endif]-->Component replacement: After the paint film is cured, if you want to replace the components, you can do the following: (1) directly solder the components with an electric soldering iron, and then use a cotton cloth dipped in board water to clean the materials around the pad; (2) welding of alternative components; (3) Use a brush dipped in conformal paint to brush the welded parts, and make the surface of the paint film dry and solidify.

Operational Requirements:

<!--[if !supportLists]-->1.   <!--[endif]-->The conformal coating workplace is required to be dust-free and clean, dust-free, must have good ventilation measures, and prohibit unrelated personnel from entering.

<!--[if !supportLists]-->2.   <!--[endif]-->Wear protective equipment such as masks or gas masks, rubber gloves, and chemical protective glasses when operating to avoid physical injury.

<!--[if !supportLists]-->3.   <!--[endif]-->After the work is completed, the used tools should be cleaned in time, and the container with conformal coating should be sealed and covered tightly.

<!--[if !supportLists]-->4.   <!--[endif]-->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:

<!--[if !supportLists]-->1.   <!--[endif]-->There can be no paint flow and dripping on the surface of the circuit board, and attention should be paid to the part that does not drip into the local isolation when the brush is painted.

<!--[if !supportLists]-->2.   <!--[endif]-->The conformal coating layer should be smooth, bright, and uniform in thickness, and the surface of the pad, SMD component or conductor should be protected.

<!--[if !supportLists]-->3.   <!--[endif]-->The surface of the paint layer and the components can not have bubbles, pinholes, corrugation, shrinkage holes, dust and other defects and foreign substances, no chalking, no peeling, note: the paint film is not dry before the surface, do not touch the paint film at will.

<!--[if !supportLists]-->4.   <!--[endif]-->Locally isolated components or areas should not be coated with conformal coating.

In terms of the process of conformal coating, the specifications and requirements involve painting, operation, quality and other aspects. There is a clear control range of thickness when painting, and secondary coating can be carried out according to the protection requirements, and there are also operational requirements such as subsequent inspection and repair and component replacement. Workplaces, personnel protection, tool cleaning, and anti-static and placement requirements for circuit boards are all required to ensure safe and effective operations. The quality requirements specify in detail the appearance state of the circuit board after the surface is coated with conformal coating, from the absence of flow paint, dripping to the smoothness, brightness, and no defects of the paint layer. There are also clear classification regulations for parts and devices that cannot be coated with conformal coating, and for special cases, they can be specified by the R&D department or coated according to the drawings. In the spraying process, there are detailed precautions for the process edge width, size range, component height, and distance between spraying and non-spraying areas of PCBA, and the cleaning and drying conditions and some non-sprayable components are emphasized.

Parts and devices that cannot be coated with conformal coating

<!--[if !supportLists]-->1.   <!--[endif]-->Conventional non-coatable 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, tact switches, relays and other types of sockets, pin headers, terminal blocks and DB9, plug-in or SMD light-emitting diodes (non-indicative), digital tubes, grounding screw holes.

<!--[if !supportLists]-->2.   <!--[endif]-->The parts and devices that are not allowed to use conformal coating as specified in the drawings.

<!--[if !supportLists]-->3.   <!--[endif]-->Devices that cannot be conformal by conformal coating are specified in the details of the "Catalogue of Non-Conformal Components (Regions)". If the conventional non-coatingable devices in the regulations need to be coated, they can be coated by the R&D department to specify the requirements or the drawings to mark the three-proof coating.

Precautions for the conformal coating process

<!--[if !supportLists]-->1.   <!--[endif]-->The PCBA must be set with a process edge and the width cannot be less than 5mm, so that it is easy to walk the track on the machine.

<!--[if !supportLists]-->2.   <!--[endif]-->The maximum length and width of the PCBA board is 410*410mm, and the minimum is 10*10mm.

<!--[if !supportLists]-->3.   <!--[endif]-->The height of the components placed in the PCBA is up to 80mm.

<!--[if !supportLists]-->4.   <!--[endif]-->The minimum distance between the spraying area and the non-spraying area of the components on the PCBA is 3mm.

<!--[if !supportLists]-->5.   <!--[endif]-->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 paint should be between 0.1 - 0.3mm. Baking conditions: 60 °C, 10-20 min.

<!--[if !supportLists]-->6.   <!--[endif]-->In the process of spraying, some components are not sprayable, 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 sockets (tubes), IC holders, tact switches, etc.

Introduction to Conformal Coating Rework of Circuit Board 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. However, the more common approach is to remove all or part of the protective film on the board and replace the damaged components one by one. When removing the conformal coating film, make sure that it does not damage the substrate underneath the component, other electronic components, and structures near the rework location. The removal methods of the protective film mainly include: the use of chemical solvents, micro-grinding, mechanical methods and desoldering through the protective film. The use of chemical solvents is the most commonly used method of removing conformal coating protective film, the key to which 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, a discharge hole is made in the protective film so that the molten solder can be discharged.

When a circuit board needs to be reworked, there are several ways to deal with it. Expensive components can be removed and the rest can be discarded, or damaged components can be replaced after some or all of the protective film is removed. There are various methods to remove the protective film, including chemical solvent method, micro-grinding method, mechanical method and desoldering method through the protective film, each of which has its own characteristics and application scenarios. In short, conformal coating plays an extremely important role in the production, use and maintenance of the entire circuit board, and the strict control of each link helps the circuit board to better play its function and ensure the stability of its performance.