PCBA Conformal Coating Application Process and Precautions

Traditionally, three-proof protection for electronic equipment refers to resistance to moisture, mold, and salt spray. It encompasses environmental-condition assessment, environmental-resistant design, process protection, testing technology, and engineering implementation for electronic equipment.

1. Preparation Before Coating

Before applying conformal coating to a PCBA, inspect the board's external appearance first.

Materials and tools include conformal coating, a coating container, rubber gloves, a mask or respirator, brushes, masking tape, tweezers, ventilation equipment, drying racks, and an oven.

2. Protecting Components That Must Not Be Coated

The following components and areas must not be coated with conformal coating:

• High-power components with heat-dissipation surfaces or heatsinks, power resistors, power diodes, and cement resistors.
• DIP switches, trimmer resistors, buzzers, battery holders, fuse holders, IC sockets, and tactile switches.
• All types of sockets, pin headers, terminal blocks, and DB connectors.
• Through-hole or surface-mount LEDs and numeric displays.
• Areas and components explicitly identified in the drawings as unsuitable for insulating varnish.
• Screw holes on the PCBA.

Protection may be provided with dedicated fixtures. When fixtures are unavailable, masking tape or spot-applied solder-mask protection material may be used. Dedicated protective sleeves or fixture covers are preferred whenever possible.

3. Technical Requirements and Precautions

1. Conformal coating by brush must be performed only after the PCBA has passed assembly-related testing and inspection and has been thoroughly cleaned.
2. The brush must be kept clean and must not be reused for other operations. During brushing, prevent coating from dripping onto areas that do not require protection. After use, clean the brush thoroughly with thinner or an equivalent cleaning agent.
3. The coating must be transparent and must cover the PCB and components evenly. Its color and viscosity should remain consistent.
4. The process sequence is: coat side A, allow the surface to dry, coat side B, and then cure at room temperature.
5. The specified coating thickness is 0.03–0.15 mm.
6. All coating operations should be performed at a temperature of at least 16 °C and a relative humidity below 75%. Because a PCB is a composite material, it can absorb moisture. If moisture is not removed, the conformal coating may not provide its intended level of protection. Pre-drying or vacuum drying can remove most of the absorbed moisture.
7. Check the fixture for leakage and contamination.
8. Wear an antistatic wrist strap when applying or removing masking tape or spot-applied protective coating. Do not cover paper barcodes, and do not knock components out of position or detach them.
9. Maintain a gap of 1 cm between individual boards when placing them for processing.
10. After curing, inspect the PCBA under a UV lamp while removing the masking tape or protective coating. If coating defects are found, brush on additional coating where necessary. The finished coating must be free of bubbles, white spots, wrinkles, cracks, delamination, and contamination.

4. Preparation for Spray Coating

1. Operators must wear protective masks. Safety glasses are also recommended.
2. Drain the water collection tank of the oil-water separator before the start of both the morning and afternoon shifts.
3. Prepare the coating as follows:

1. After the coating in the pressure tank has been used up, prepare a new batch. Before mixing, shut off the compressed air, open the pressure-relief valve to release the tank pressure, and trigger the spray gun so that residual coating in the line flows back into the tank. Open the tank cover only after these steps, and place it on the designated rack.
2. Determine the coating-to-thinner ratio according to the spray equipment and the measured dry-film thickness. PCBA panels in the same batch must use coating with the same identification number. Do not mix different coating batches.
3. Open the thinner container, pour the required quantity of thinner into a measuring cup, and reseal the container. Pour the measured thinner into the stainless-steel inner tank of the spray vessel. Return the thinner container to its designated location after use.
4. Open the conformal-coating container and pour the coating into the inner tank containing the thinner. The liquid level should reach the lower edge of the heat-shrink sleeve on the level-indicator rod.
5. Tighten the pressure-tank cover, close the pressure-relief valve, and slowly stir the contents through 10 forward and 10 reverse rotations to obtain a uniform mixture. Open the compressed-air supply and adjust the tank pressure to 0.4 ± 0.1 kgf/cm². Finally, trigger the spray gun until the coating flow is uniform and all air has been purged from the line.

5. Spray-Coating Operation

1. Maintain the pressure in the pressure tank at 0.4 ± 0.1 kgf/cm².
2. Open the spray-gun pressure adjustment valve fully, then use the pressure regulator with gauge to set the spray pressure to 3.0–4.0 kgf/cm².
3. Adjust the spray-pattern control valve on the side of the gun. At a gun-to-board distance of 10–15 cm, the spray pattern should be elliptical, with a maximum width of approximately 100–120 mm. After coating the entire board, apply one additional pass to critical areas such as the leads of power devices mounted on heatsinks.
4. Adjust the needle valve at the rear of the spray gun to control the coating flow rate. Before starting work each day, spray a test panel and use a wet-film thickness gauge to confirm that the wet-film thickness is at least 120 μm. Record the result in the wet-film thickness test log. For manual coating, perform a sampling inspection every two hours; for automated coating, inspect every four hours. Clean the test panel and thickness gauge with thinner after measurement, and have the responsible supervisor verify the record. Recalibrate the spray thickness whenever the operator changes. Whenever possible, use a coating-thickness comb to measure directly on a single board and read the value under a UV lamp.

Wet-film thickness measurement has a relatively large error and is intended only for process monitoring. Final acceptance should be based on the dry-film thickness. Wet-film thickness measurement is not required for dip-coated or brush-coated products.

1. Move the coated PCBA trolley to the air-drying area.
2. Start the spray gun at the upper-left corner of the PCBA. Move it horizontally to the right at a constant speed using the established parameters. At the board edge, move the gun downward approximately 150 mm, then return using the same parameters. Repeat this motion until the entire tray of PCBAs has been coated. Rotate the tray 90° and repeat the same process.
3. Automated spray coating must use an overlapping-pass method. The spray width and overlap distance depend on the equipment settings. After the overall coating pass, apply an additional pass to critical areas.
4. Inspect coating uniformity under a UV lamp. Apply additional coating to any nonconforming areas.
5. After manual spray coating, the PCB assembly may be air-dried, rack-dried, or oven-dried. Automated spray coating normally proceeds directly to the curing line. The surface-dry time is approximately 3–5 minutes.
6. For double-sided coating, coat the side with taller components first. Allow it to air-dry for at least 20 minutes or bake it until the surface is dry before coating the side with shorter components. If both sides contain surface-mount components, raise the downward-facing side during manual coating to prevent the components from contacting the tray.
7. After spraying, place the spray gun on its dedicated rack. The nozzle must remain immersed in thinner; add thinner promptly if necessary to ensure complete immersion.
8. For high-protection products, apply a second coating because manual operation can produce nonuniform thickness. The second coating uses the same method as the first.
9. For machine spraying, the required thickness can generally be achieved by adjusting the spray speed or flow rate, so a second coating is normally unnecessary.
10. Use the same wet-film thickness measurement method for the second coating.
11. The second coating may be air-dried, rack-dried, or oven-dried. After the surface has dried, avoid scratching the coating with hard objects during handling.

6. Brush-Coating Operation

1. Use a disposable paper cup to hold the conformal coating. Thinner may be omitted for brush application. Limit the quantity to what can be used within two hours. Replace the cup after this period and do not reuse it.
2. Use a soft-bristle brush. Replace it once per shift, and replace it immediately if the bristles split or fray. Pick up an appropriate amount of coating and apply it at an angle of 45°. Overlap adjacent brush strokes while avoiding repeated brushing, which can create bubbles. Use slow movements so the coating fully wets the PCB and components.
3. Replace the paper cup every two hours and continue operation with fresh coating.
4. For double-sided brush coating, apply the first side while the board is placed horizontally. After the surface has dried, coat the opposite side.
5. LED panels and small boards are generally brush-coated, while large boards are better suited to a spray-coating machine. Follow the dedicated process documentation for the specific equipment model.

7. Curing

1. Spray-coating lines generally use a tunnel oven. After passing inspection, the PCBA enters the oven on a carrier through a conveyor chain. The baking temperature and time must be set according to the dedicated process documentation for each product model. Typical settings are 80 °C, with a maximum temperature not exceeding 95 °C, and a curing time of 10–30 minutes. The actual endpoint is determined by whether the coating film is no longer tacky to the touch.
2. Before baking, properly align the rail connections and set the oven temperature and conveyor speed.
3. After the PCBA exits the oven, wear protective gloves and hold the carrier by its edge. Place it on the transfer trolley and move it to a ventilated area for cooling or allow it to stand at room temperature to prevent burns.

8. Appearance Inspection Requirements

1. The PCB surface must be free of runs, drips, and partially wetted areas.
2. The coating must be smooth and glossy, with uniform thickness and complete coverage of the specified pads, surface-mount components, and conductors.
3. There must be no orange peel, bubbles, pinholes, or ripples.
4. No coating residue may remain on components that are not intended to be protected.
5. The finished board must be clean and free of component damage.
6. The board must be free of contamination, fingerprints, ripples, craters, dust, powdering, peeling, and other defects.

9. Dry-Film Thickness Measurement and Repair

1. After curing and cooling, preferably select a 10 × 10 mm copper area on a single board. First measure and record the green solder-mask thickness with a dry-film thickness gauge, since the solder-mask thickness is consistent within the same PCB batch. Then measure the total thickness of the insulating materials. The difference between the two measurements is the dry-film thickness of the conformal coating. If the result exceeds the process requirement, promptly notify the manufacturing engineer to adjust the spray-gun parameters.

For spray coating, if thickness cannot be measured directly on the board, spray an aluminum test coupon together with the PCBA and measure its thickness after curing:

• Products requiring a high level of protection: typically greater than 50 μm.
• General products: greater than 30 μm.
• For manual coating, measure once every four hours; for automated coating, measure once every eight hours.

1. Dip coating: immerse an aluminum coupon in the conformal coating for 5 seconds, remove it, cure it, and then measure its thickness. The thickness requirements are the same as those listed above.
2. Brush coating: brush an aluminum coupon according to the normal process, cure it, and then measure its thickness. The thickness requirements are the same as those listed above.
3. Wet-film thickness data is for reference only. Spray-coating quality must be evaluated according to dry-film thickness. When direct measurement on the board is possible, use the board measurement as the basis.

10. Component Replacement and Coating Repair

If a component must be replaced after the coating film has cured, follow this procedure:

The component can generally be removed directly with a soldering iron. Clean the area around the pads with a cloth moistened with Thinner 239. If removal is difficult, first use a cotton swab moistened with Thinner 239 to wipe the conformal coating from the pads, allowing the coating to dissolve or become thinner before soldering. Clean the area afterward, apply replacement coating, and allow it to cure.

11. Precautions When Using Conformal Coating

1. Strictly follow the operating procedures for the spray machine, oven, and other equipment. Perform required maintenance and inspections, and wear antistatic gloves.
2. Use the specified mixing ratio for conformal coating and thinner. Reseal both materials promptly after use. Store them in a cool, ventilated area below 30 °C, away from flames, power sources, and electromagnetic interference. Storage racks should be properly grounded.
3. Confirm that the product is within its shelf life before use. Do not use expired coating.
4. If coating contacts the skin, wash it promptly with soapy water or a cleaning agent. If swallowed, seek medical attention immediately. If it enters the eyes, rinse with clean water without rubbing and seek medical attention immediately.
5. After every 24 hours of operation, remove the coating inlet tube from the spray gun and flush it with thinner at least 10 times to prevent curing-related blockage. Disassemble and clean the spray-gun head weekly, including the nozzle and adjustment screws. A spray gun that will remain unused for more than 12 hours must also be cleaned. Clean brush-coating tools with thinner every day. Collect and seal waste liquid and have it handled by a qualified waste-treatment organization.
6. Seal empty containers securely and store them at the designated location. Residues and waste liquid must be collected and disposed of by a qualified waste-treatment company.

12. Conclusion

Conformal coating is a highly detailed process. Both spray and brush application require careful control, particularly with respect to film thickness and protection of connectors and other components that must remain uncoated. Many factors affect coating quality, so the process requires continued investigation and optimization to improve the consistency and protective performance of the finished PCBA.