Three-proof paint spraying process

Conformal coatings are primarily classified into acrylic, silicone, and polyurethane types based on their chemical composition. From the perspective of curing methods, they can be further divided into solvent-based curing, room temperature curing, thermal curing, and UV curing, among others. The selection of a specific process typically depends on the characteristics of the product itself. Manufacturers generally provide specialized thinners to adjust the concentration of the product, meeting the requirements of various processes such as spraying, brushing, and dipping.

Solvent-based acrylic conformal coatings are the most widely   used products in the  market, offering several advantages: fast surface drying and curing, excellent protective properties (against moisture, salt spray, and mold), cost-effectiveness, transparent color, flexible texture, and ease of repair.

The conformal coating used in this study is of the solvent-based acrylic type. The transparent acrylic protective film it forms effectively resists erosion from weak acids, alkalis, alcohol, and moisture. Additionally, this coating exhibits high-temperature resistance, softening at 100°C and becoming viscous at 120°C without dripping. Containers for the conformal coating should be made of metal or plastic, avoiding wood or PVC materials to prevent chemical reactions that could damage the container and compromise the coating's protective performance. Furthermore, the conformal coating should be stored in a cool, dry area, with the storage environment temperature controlled not to exceed 54°C.

In practical usage environments, circuit boards often face various harsh conditions, such as chemical exposure (e.g., fuels, coolants), vibration, high dust, salt spray, humidity, and high temperatures. These conditions can lead to issues like corrosion, softening, deformation, and mold growth on the circuit boards, ultimately causing circuit failures. After applying the conformal coating, a thin and flexible film, approximately 25–50 microns thick, forms on the surface of the circuit board. This flexible film provides effective protection for the circuit under the aforementioned harsh conditions, preventing damage.

Content and Scope: This operating procedure specifies in detail the operating standards, process requirements, and quality requirements for brushing conformal coating (against damp heat, salt spray, and mold) on all types of PCBA boards within the company.

Materials and Tools: Conformal coating, coating container, rubber gloves, mask or respirator, brush, masking tape, tweezers, ventilation equipment, drying rack, oven.

Technical Requirements

3.1 Conformal coating protection for PCBA boards must be performed after testing, inspection, and confirmation of qualification, and the surface must be thoroughly cleaned before assembly.

3.2 The brushes used should be kept clean and not used for other tasks; during brushing, prevent the coating from dripping onto non-coated areas; after use, clean thoroughly with thinner immediately.

3.3 The coating should be transparent and uniform, completely covering the PCB board and components, with consistent color and viscosity.

3.4 The process steps are: first coat side A, after surface drying coat side B, and finally perform room temperature curing.

3.5 The spraying thickness should be controlled between 0.1mm–0.3mm.

3.6 All coating operations should be carried out in an environment with a temperature not lower than 16°C and relative humidity below 75%. Since PCB is a composite material prone to moisture absorption, if moisture is not removed, the conformal coating may not provide optimal protection. Moisture can be removed through pre-drying or vacuum drying.

Brushing Process Requirements for Conformal Coating:

4.1 First, clean and bake the PCBA board to remove surface moisture and water. Before brushing, thoroughly remove dust, moisture, flux residues, and other impurities to ensure good adhesion of the conformal coating to the circuit board surface. The baking conditions are: temperature controlled at 60°C, baking time for 30–40 minutes. Coating immediately after removal from the oven while still warm can yield better results.

4.2 Based on the product size and the layout of the board and components, select an appropriate brush. Pour the conformal coating into a dedicated container, then dip the brush into an appropriate amount of the coating and apply it evenly to the surface of the circuit board.

4.3 The brushed area should be slightly larger than the area occupied by the components to ensure complete coverage of the components and pads.

4.4 During brushing, the PCBA board should be kept as horizontal as possible. After brushing, there should be no dripping, the coating should be smooth without any exposed areas, and the brushing thickness should be 0.1–0.3mm.

4.5 After brushing is completed, place the side with more inserted components facing up, horizontally on a drying rack for surface drying and curing (heating can accelerate the curing process).

4.6 When applying conformal coating to the PCB, all connectors, sockets, switches, heat sinks (or fins), and heat dissipation areas (refer to the notes for components that should not be coated) must not be coated with conformal coating. During the brushing process, use masking tape to cover and protect these areas.

4.7 If a thicker coating is desired, it can be achieved by applying two thinner layers (but the second layer must only be applied after the first layer has completely dried).

Operational Requirements

5.1 Brushing work with conformal coating should be conducted in a separate enclosed room, and the operating room must be equipped with adequate ventilation facilities.

5.2 Smoking, eating, and drinking are strictly prohibited in the operating room. Do not consume alcoholic beverages before work.

5.3 Operators should wear masks or respirators, rubber gloves, and chemical protective goggles to ensure personal protection.

5.4 After work, clean the utensils promptly, organize and wipe down tools and equipment, and tightly seal the conformal coating container.

5.5 The workplace should be kept clean and dust-free, and unauthorized personnel are prohibited from entering.

5.6 All tools and equipment used should be properly grounded, and electrostatic protection measures should be implemented.

5.7 During the operation, PCBA boards should not be stacked and should be kept level.

5.8 Before using each batch of raw materials, conduct a small-sample curing test (test quantity 3–5 PCS).

Quality Requirements

6.1 The PCB board surface must not show running, dripping, or partial wetting of the coating.

6.2 The conformal coating layer should be smooth, glossy, uniform in thickness, and effectively protect pads, SMD components, and conductor surfaces.

6.3 The coating surface must not exhibit orange peel, bubbles, pinholes, or ripples.

6.4 Components or parts that do not require protection should not be coated with conformal coating.

6.5 The printed board after conformal coating treatment should be clean and tidy, with no component damage.

6.6 The board surface and components must not have contaminants, fingerprints, ripples, pinholes, dust, or other defects and foreign matter, and there should be no chalking or peeling.

Storage Requirements

7.1 Raw materials should be stored in a cool, dry warehouse (room temperature should not exceed 30°C). The warehouse should be equipped with explosion-proof ventilation systems and equipment, and the raw materials should be kept away from heat sources.

7.2 They should be stored separately from other types of chemicals, with clear material identification. Mixed storage is strictly prohibited.

7.3 Fire prevention measures should be taken, and corresponding fire-fighting equipment should be available.

7.4 The storage warehouse should be equipped with leak emergency equipment and appropriate containment materials (such as sand, etc.).

7.5 The warehouse should maintain good ventilation to prevent the accumulation of leaked harmful gases and avoid accidents.

7.6 The storage container must indicate the valid use-by date.

7.7 The storage shelves or equipment should be well grounded, and equipment and tools that are prone to generating sparks are prohibited.

Precautions:

8.1 Conformal coating that has exceeded its shelf life is prohibited from continued use.

8.2 After opening the storage container, it should be sealed for storage.

8.3 Do not pour unused conformal coating back into the original storage container; it should be sealed and stored separately.

8.4 Before entering a workroom or storage room that has been closed for a long time (over 12 hours), ventilate it for 15 minutes first.

8.5 If conformal coating splashes into the eyes, immediately lift the eyelid and rinse with flowing clean water or saline, and seek medical attention as soon as possible.

8.6 If discomfort is felt during the brushing process, quickly leave the work area to a well-ventilated place; if breathing difficulties occur, administer oxygen immediately and seek medical attention.

Components that should not be coated with conformal coating:

9.1 High-power components with heat dissipation surfaces or heat sinks, power resistors, power diodes, cement resistors.

9.2 DIP switches, adjustable resistors, buzzers, battery holders, fuse holders (tubes), IC sockets, tactile switches.

9.3 All types of connectors, pin headers, terminal blocks, and DB connectors.

9.4 Through-hole or surface-mount LEDs, digital tubes.

9.5 Parts and components specified in the drawings as not to be insulated with coating.

9.6 Screw holes on the PCBA board.

Selecting the Correct Conformal Coating Fixture: For each circuit board requiring double-sided spraying, corresponding front and back side conformal coating fixtures should be provided. During spraying operations, the correct fixture must be used to ensure even coverage of the areas to be sprayed and prevent coating of non-spray areas (such as connector interfaces, DIP switches, etc.).

Quality Inspection Process

11.1 Visual Inspection: After brushing, first conduct a visual inspection. Inspectors should carefully check the PCB board surface for running, dripping, or partial wetting of the coating; ensure the coating layer is smooth, glossy, uniform in thickness, and properly protects pads, SMD components, and conductor surfaces; the coating surface must not have orange peel, bubbles, pinholes, or ripples; the board surface and components must not have contaminants, fingerprints, ripples, pinholes, dust, or other defects, and there should be no chalking or peeling; confirm that non-protected components are not coated with conformal coating, and the printed board is clean and undamaged.

11.2 Thickness Inspection: Use professional measuring tools to inspect the thickness of the conformal coating spray, ensuring it is within the 0.1mm–0.3mm range. If a thicker coating is achieved by applying two thinner layers, inspect the thickness of each layer and the total thickness to ensure they meet requirements, and confirm that the first layer is completely dry before applying the second layer.

11.3 Adhesion Test: Use specific methods to test the adhesion of the conformal coating to the PCB board and component surfaces. Insufficient adhesion may cause the coating to peel off, affecting circuit protection.

Employee Training and Assessment

12.1 Training Content: To ensure the quality and safety of conformal coating brushing work, comprehensive training should be provided to relevant operators. The content includes the chemical composition and characteristics of conformal coatings (such as the fast surface drying and curing, good protective properties, cost-effectiveness, transparent color, flexible texture, ease of repair, etc., of solvent-based acrylic conformal coatings), the applicable scope of various conformal coatings, correct usage methods of tools (brushes, coating containers, tweezers, etc.), brushing process steps (cleaning and baking boards, brush selection, brushing operation, surface drying and curing, etc.), quality requirements (appearance, thickness, adhesion), safety precautions (wearing protective equipment, environmental requirements, etc.), and knowledge of components that should not be coated with conformal coating.

12.2 Assessment System: Establish a strict assessment system to evaluate trained employees on both theory and practical operation. Theoretical assessment covers key knowledge points related toconformal coatings; practical operation assessment requires employees to complete the brushing of a PCBA board according to standard procedures within a specified time and meet the standards. Only those who pass the assessment are qualified to work.

Equipment Maintenance and Management

13.1 Regular Inspection: Equipment used for conformal coating brushing (such as ovens, ventilation equipment, etc.) should be inspected regularly. Confirm that the oven temperature control is accurate and heating is normal, ensuring that board baking and curing meet requirements; check if the ventilation equipment's air volume meets needs and if the ducts are clear to prevent the accumulation of harmful gases.

13.2 Cleaning and Maintenance: Clean equipment promptly after use. For example, brushes and coating containers must be cleaned with thinner to prevent residual conformal coating from drying and hardening; the interior of the oven should be cleared of dust and debris to maintain good heat dissipation of heating elements; ventilation equipment filters should be cleaned or replaced regularly to ensure ventilation effectiveness.

13.3 Equipment Update: With technological development and equipment aging, the need for equipment updates should be assessed in a timely manner. If equipment performance cannot meet increasing production demands, or if equipment failures are frequent and repair costs are too high, consider replacing with new equipment to improve production efficiency and product quality.

Environmental Protection

14.1 Waste Disposal: During the use of conformal coating, some waste is generated (such as used brushes, wipes contaminated with conformal coating, expired conformal coating, etc.). This waste should be classified and handled according to environmental protection requirements. Waste containing conformal coating must not be discarded arbitrarily; it should be properly collected and handed over to professional environmental agencies for treatment to prevent environmental pollution.

14.2 Exhaust Gas Emission: The exhaust gas discharged by the ventilation equipment in the workroom may contain volatile organic solvent components from the conformal coating. Ensure that the ventilation equipment is equipped with effective exhaust gas treatment devices to purify the discharged exhaust gas. It must meet national environmental emission standards before being released into the atmosphere to reduce air pollution.

14.2 Exhaust gas emissions: The exhaust gas emitted by the ventilation equipment in the workshop may contain volatile mechanical solvent components of conformal paint. It should be ensured that the ventilation equipment is equipped with effective exhaust gas treatment devices to purify the exhaust gas to avoid causing to the environment and only be discharged after meeting the standards, so as to reduce air pollution.

14.2 Exhaust gas emissions: The exhaust gas emitted by the ventilation equipment in the workshop may contain organic solvents and other components volatilized by conformal paint. It should be ensured that the ventilation equipment is equipped with effective exhaust gas treatment devices to purify the exhaust gas and discharge it into the atmosphere after meeting the environmental protection emission standards stipulated by the state, so as to reduce pollution to the atmospheric environment.