Why Spray Coating on PCB is Crucial for Optimal Performance

What is Three-Proof Paint?

Three-proof paint is a type of specially formulated coating used to protect circuit boards and related equipment from environmental corrosion. It has excellent resistance to high and low temperatures. Once cured, it forms a transparent protective film that provides superior insulation, moisture resistance, leakage prevention, shock resistance, dustproof, corrosion resistance, anti-aging properties, and resistance to electrical arcing.

Under real-world conditions such as chemical exposure, vibrations, high dust levels, salt spray, humidity, and high temperatures, circuit boards may experience corrosion, softening, deformation, mold growth, and other issues, leading to circuit failures. Three-proof paint is applied to the surface of circuit boards, forming a protective film that guards against moisture, salt spray, and mold.

In situations where circuit boards are not coated with three-proof paint, they may be susceptible to corrosion, mold growth, short circuits, and other faults when exposed to chemical substances (such as fuel, coolant, etc.), vibrations, humidity, salt spray, and high temperatures. By using three-proof paint, circuits can be protected from damage, improving the reliability and safety factor of circuit boards, and ensuring their longevity.

Furthermore, since three-proof paint prevents leakage, it allows for higher power and closer spacing between printed circuit boards, thereby meeting the goal of component miniaturization.

The Specifications and Requirements for Three-Proof Paint Technology

Requirements for Spray Painting

1. Paint Thickness: The thickness of the paint film should be controlled between 0.05mm and 0.15mm. The dry film thickness should range from 25um to 40um.
2. Secondary Coating: To ensure the desired thickness for highly protected products, a secondary coating can be applied after the paint film has cured. The decision for secondary coating should be based on specific requirements.
3. Inspection and Repairs: Visual inspection should be conducted to check if the coated circuit board meets the quality requirements. Any issues identified should be addressed through appropriate repairs. For example, if there is three-proof paint on the pins or other protected areas, a pair of tweezers can be used to remove lint-free cotton swabs or clean cotton swabs dipped in a cleaning solution to wipe them clean. Care should be taken not to remove the normal paint film while wiping.
4. Component Replacement: After the paint film has cured if component replacement is required, the following steps can be followed: (1) Desolder the component directly using an electric iron, and then use a cotton cloth dipped in a cleaning solution to clean the surrounding area of the solder pad. (2) Weld alternative components. (3) Use a brush dipped in three-proof paint to coat the soldering area and allow the paint film to dry and cure.

These guidelines ensure proper application and maintenance of the three-proof paint for optimal protection.

Operational Requirements

1. Clean and Dust-Free Workspace: The working environment for three-proof paint should be clean and free from airborne dust. It is essential to maintain good ventilation and restrict access to unauthorized personnel.
2. Personal Protective Equipment (PPE): During operations, it is necessary to wear masks or respirators, rubber gloves, and chemical safety goggles to protect against any potential harm to the body.
3. Tool Cleaning and Container Handling: After completing the work, promptly clean the used tools and ensure the containers containing three-proof paint are tightly sealed.
4. Anti-Static Precautions for Circuit Boards: Adequate anti-static measures should be implemented when working with circuit boards. Avoid stacking the circuit boards and ensure they are placed horizontally during the coating process.

These precautions and measures help ensure a safe and effective working environment for applying three-proof paint.

Quality Requirements

1. No Overflow or Dripping: The surface of the circuit board should be free from any paint overflow or dripping. When using a brush for painting, ensure that no paint drips onto isolated areas.
2. Smooth and Glossy Coating: The three-proof paint layer should be smooth, glossy, and uniformly thin. It should effectively protect the surface of solder pads, surface-mounted components, or conductors.
3. Defect-Free Surface: The paint film and components should be free from bubbles, pinholes, wrinkles, shrinkage, dust, or any foreign particles. There should be no powdering or peeling. Note: Avoid touching the paint film until it has fully dried.
4. Avoid Coating Isolated Components: Do not apply three-proof paint to isolated components or areas.

These requirements ensure the high-quality application of three-proof paint, promoting the integrity and reliability of the coated circuit boards.

The Parts and Components that should not be coated with Three-Proof Paint

1. General Components Not to be Coated: High-power heat sinks, heat dissipation fins, power resistors, high-power diodes, cement resistors, DIP switches, potentiometers (variable resistors), buzzers, battery holders, fuse holders, IC sockets, tactile switches, relays, sockets, pin headers, wire terminals, DB9 connectors, surface-mount LEDs (not for indication purposes only), numeric displays, and grounding screw holes.
2. Components and parts specified as not to be coated according to the drawings.
3. Components specified in the “Non-Coatable Components (Areas) Catalog” as not suitable for three-proof paint. If the coating is required for the general components mentioned above, it can be performed based on specific requirements designated by the R&D department or indicated on the drawings.

These guidelines ensure that the appropriate components and parts are not coated with three-proof paint, preventing any adverse effects on their functionality and performance.

Precautions for the Spray Coating Process of Three-Proof Paint

1. PCBA Design: The PCBA (Printed Circuit Board Assembly) should have process edges with a minimum width of 5mm to facilitate the handling and tracking process.
2. PCBA Size: The maximum dimensions of the PCBA should be 410x410mm, while the minimum dimensions should be 10x10mm.
3. Component Height: The maximum allowable height for components mounted on the PCBA should be 80mm.
4. Minimum Distance: The minimum distance between the spray-coated area and the non-coated area of the PCBA should be 3mm.
5. Thorough Cleaning: Proper cleaning of the PCBA ensures the removal of corrosive residues and promotes strong adhesion of the three-proof paint to the board surface. The recommended paint thickness is between 0.1mm and 0.3mm. Bake conditions: 60°C for 10-20 minutes.
6. Components Not to be Coated: During the spraying process, certain components should not be coated with three-proof paint. These include high-power components with heat dissipation surfaces or heat sinks, power resistors, power diodes, cement resistors, DIP switches, potentiometers, buzzers, battery holders, fuse holders, IC sockets, and tactile switches.

Adhering to these precautions ensures the effective application of three-proof paint and maintains the integrity of the PCBA during the coating process.

Introduction to the rework process for Three-Proof Paint on Circuit Boards

When reworking a circuit board, there are two common methods that can be used. The first method involves removing only the expensive components from the board and discarding the rest. However, the more common method is to remove the protective film from the entire board or specific locations and replace the damaged components one by one.

When removing the protective film of the three-proof paint, it is crucial to ensure that the underlying substrate, other electronic components, and nearby structures in the rework area are not damaged. There are several methods for removing the protective film, including using chemical solvents, micro abrasion, mechanical methods, and desoldering through the film.

Careful attention should be given to the rework process to prevent any further damage to the circuit board or its components. Following proper rework procedures ensures the successful replacement of damaged components and the restoration of functionality to the circuit board.

• The most commonly used method for removing the protective film of three-proof paint is through the use of chemical solvents. The key factor lies in understanding the chemical properties of the protective film to be removed and the specific chemical properties of the solvent used.
• Microabrasion involves using a nozzle to spray high-speed particles that ‘grind’ away the protective film of the three-proof paint on the circuit board.
• Mechanical methods are the easiest way to remove the protective film of three-proof paint. Desoldering through the film involves creating a discharge hole in the protective film, allowing molten solder to be expelled.

These methods serve as effective techniques for removing the protective film, depending on the specific requirements and circumstances of the rework process. It is important to exercise caution and choose the appropriate method to ensure the successful removal of the three-proof paint protective film while minimizing any potential damage to the circuit board or its components.

Correct Methods and Precautions for the Spray Coating Process of Three-Proof Paint

1. Personnel Safety: Operators must wear appropriate protective equipment during the coating process.
2. Cleaning after Use: If there is any remaining three-proof paint in the spray container after application, invert the container and press the nozzle until only gas is expelled to clean the spray valve.
3. Multiple Coating Layers: If a thicker coating layer is desired, it is recommended to apply two thin layers. Ensure that the first layer is completely dried before applying the second layer.
4. Areas to Avoid Coating: Connectors, sockets, switches, heat sinks, plug-in areas, and other specified areas should not be coated with the paint. It is advised to use removable solder masks for covering these areas.
5. The thickness of the Coating: The thickness of the coating depends on the application method. Adding more diluent and having lower viscosity results in a thinner coating, while higher viscosity results in a thicker coating.
6. Ambient Conditions: All coating operations should be conducted at a temperature not lower than 16°C and relative humidity below 75%. PCBs, as composite materials, absorb moisture, which can hinder the protective function of the three-proof paint. Pre-drying and vacuum drying can remove most of the moisture.
7. Selective Coating: Selective coating is suitable for accurate and efficient application, mainly used in medium to large-scale equipment. The thickness of the coating is influenced by coating properties, temperature, pressure, and equipment design. Selective coating can be used for medium to large-scale applications where only specific areas require coating. It typically does not require masking as the coating is only applied to selected regions. The equipment used for selective coating tends to be more expensive and requires higher maintenance compared to immersion or spray coating. Viscosity, temperature, pressure, and the design of the dispensing head all affect the thickness of the coating.
8. Automatic Whole Board Coating: Spray can-type products are convenient for repair and small-scale production, while spray guns are suitable for large-scale production. Automatic whole-board coating is a cost-effective method for medium-sized equipment, making it easier to form a protective film. However, it can be challenging to coat the bottom of components, requiring additional treatment. The thickness of the coating is influenced by the linear relative movement speed, material temperature, and spray equipment pressure. Spray coating is often used for medium-volume production with high production rates and low costs. It is difficult to coat underneath components and requires more maintenance. The type of feed system, line speed, material temperature, and atomization pressure used will affect the thickness of the coating.

These methods and precautions ensure the proper application of three-proof paint during the spray coating process, resulting in reliable and effective protection for the circuit board.

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