Establish and define the process flow and requirements for applying UV adhesive conformal coating on PCBA.
Coating machine Recommedation：HC-P450S Click to view machine parameters
2. Scope
Applicable to the conformal coating process of PCBA for all products of the company.
3. Content
3.1 Coating Preparation
Conduct visual inspection on the PCBA to be coated. If there are obvious fingerprints, stains, or other appearance defects on the board, clean it with a cleaning agent and wait for the cleaning agent to evaporate completely before proceeding to the next process.
3.2 Protection for Non-sprayable Parts
3.2.1 Protected Parts
- Sockets, pins, DIP switches, buttons, knobs, both sides of pads for components to be reworked, both sides of metallized screw holes, upper end surface and inner surface of riveted nuts, terminal blocks, wire terminals, circuit breakers, IC sockets and ICs inserted in IC sockets, buzzers, LEDs (currently, partial paint on SMD LEDs basically does not affect brightness), LCDs, batteries, and other specially specified parts (subject to dedicated documents for each model).
Components in the blue frame cannot be coated with conformal coating
Components in the blue frame cannot be coated with conformal coating
3.2.3 Precautions
- Check if the tooling has leakage or contamination; if so, set it aside and report promptly.
- When applying adhesive tape (or anti-solder paste), do not cover the paper barcodes on the board or components.
- Wear an anti-static wrist strap when applying/removing adhesive tape (or anti-solder paste).
- Protect the board during operation to avoid tilting or dropping components.
- Separate and label boards with operation-induced defects.
- After operation, the operator must self-inspect the boards. When placing boards, leave a gap of about 1cm between them, and push the transfer cart with both hands—forbid violent pushing or letting the cart roll freely.
- After curing, use an ultraviolet lamp to visually inspect the PCBA when removing adhesive tape (or anti-solder paste) for any missed spraying or SMD components covered by tape/paste; if found, touch up the coating.
- After curing, use an ultraviolet lamp to visually inspect for defects such as bubbles, white spots, wrinkles, cracks, peeling, or contamination; report any issues to the supervisor promptly.
There are bubbles after brushing and the thickness is uneven
Double-row pins contaminated with conformal coating
Component leads and whitening spots on PCB surface
Conformal coating on PCB has ripples
Partial wetting of components on PCB surface
3.3 Coating Operation
3.3.1 Operator Protection
The operator must wear a mask and, if possible, protective goggles.
3.3.2 Air Path Drainage
Drain the water from the oil-water separator storage tank, once in the morning and once in the afternoon before use each day.
3.3.3 Coating Mixing
Before preparing new conformal coating material, first turn off the compressed air of the pressure tank, open the pressure relief valve to release the pressure inside the tank, then pull the trigger of the spray gun to let the conformal coating in the pipeline flow back to the bucket, and finally open the cover of the pressure tank and place it on the designated shelf.
2. For the fluorescent conformal coating , the mixing ratio of coating to thinner is 5:1 by volume, subject to the spraying equipment and dry film thickness test results Note: Conformal coatings with the same batch code must be used for the same batch of PCBA; mixing is prohibited.
3. Unscrew the thinner bucket lid, pour a certain amount of thinner into a measuring cup, then re-tighten the lid and pour the measured thinner into the stainless steel inner bucket of the spraying tank. Return the used thinner bucket to its original place.
4. Unscrew the conformal coating bucket lid and pour the coating into the stainless steel inner bucket of the spraying tank that already contains the specified amount of thinner. Measure the coating volume with a marking rod, ensuring it reaches just below the upper heat-shrinkable sleeve on the rod.
5. Cover the pressure spraying tank, close the pressure relief valve, slowly shake the mixing handle 10 times clockwise and 10 times counterclockwise to evenly mix the thinner and coating. Then turn on the compressed air, confirm the tank pressure is 0.4±0.1kgf/cm², and pull the trigger until the paint is discharged evenly to release the pressure air in the pipeline.
3.3.4 Coating
1.Confirm the pressure in the spraying tank is 0.4±0.1kgf/cm².
2.Open the pressure regulating valve on the spray gun to maximum, then adjust the pressure regulating valve with a pressure gauge connected to the gun to 3.0–4.0kgf/cm² (conversion: 3.0–4.0kgf/cm² = 2.94–3.92 bar = 42.68–56.90 psi).
3.After spraying the entire board, apply an additional coat to key areas, such as the lead regions of power transistors on heat sinks.
4.Adjust the paint flow rate using the needle valve at the rear of the gun. Before starting spraying each day, first spray on a metal measurement plate (as per 4.3.4.6), then test the wet film thickness with a wet film thickness gauge to ensure it is ≥120 microns, and record it in the Wet Film Thickness Test Record. For manual coating, spot-check at least once every 2 hours; for automatic coating, once every 4 hours. After use, clean the metal plate with a cloth dipped in thinner and the wet film tester with thinner. Relevant managers must verify the test records. Note: If the spray gun operator is changed, re-test the paint thickness. Preferentially use a snap gauge for measurement due to its smaller measurement area, which is more suitable for direct measurement on the board, as shown in the figure. Read the measurement values under an ultraviolet lamp.
Note: Wet film thickness testing has large errors and serves only as a reference for process control; spray quality is based on dry film thickness testing. Wet film thickness testing is not required for dip coating or brush coating.
5.Push the transfer cart with painted PCBA to the air-drying position.
6.Start spraying from the upper-left corner of the PCBA. Move the spray gun horizontally from left to right at a uniform speed with the adjusted parameters (used for thickness testing).
7.For automatic spraying, use the overlapping coverage method as shown in the figures. Set the spray pattern size and overlap according to specific equipment. After spraying the entire board, apply an additional coat to key areas, such as the lead regions of power transistors on heat sinks.
8.Inspect the board for uniform spraying under an ultraviolet lamp; touch up any uneven areas.
9.After manual spraying, air-dry, naturally dry, or bake; automatic spraying is generally connected to a baking line for baking. The film surface-dries at room temperature in 3–5 minutes.
10.For double-sided spraying of PCBA, the principle is: spray the side with taller components first, air-dry for ≥20 minutes or bake, confirm surface-drying, then flip the PCBA to spray the side with shorter components. If both sides have SMD components, prop up the downward-facing side during manual spraying to prevent components from contacting the tray.
11.After spraying, place the spray gun on a dedicated rack with the nozzle immersed in thinner. Add thinner promptly if insufficient to ensure full immersion.
12.Secondary Coating
Due to non-standard manual operations causing multiple thin coating quality issues, secondary coating is mandatory for manual spraying/brush coating of products with high protection requirements. The coating method is the same as the first coating.
For machine spraying of high-protection products, adjust the spraying speed or increase the nozzle flow to meet thickness requirements instead of performing secondary coating.
13.Secondary Wet Film Thickness Testing
Same method as the first spraying wet film thickness test.
14.Secondary Film Surface-Drying
After manual spraying, air-dry, naturally dry, or bake; automatic spraying is generally baked on a line. After surface-drying, the product can be transferred without collision or friction to the film.
3.3.5 Brush Coating
1.Prepare a suitable container (e.g., a disposable paper cup) to hold the conformal coating (no thinner needed for hand-brushing). Do not pour too much (about 2 hours’ usage). Replace the liquid exposed to air for >2 hours. Disposable cups cannot be reused (other containers can be cleaned and air-dried for reuse).
2.Prepare a soft brush for brush coating (choose an appropriate size based on PCB size, and ensure a small brush with a width of ~1cm or smaller for edge areas or hard-to-reach spots). The brush must be replaced (per shift) or immediately if bristles are split or dispersed (affecting coating quality or risking coating non-target areas). Dip the soft brush in an appropriate amount of conformal coating (saturated but not dripping), and brush at a 45° angle to the PCB surface with overlapping strokes. Avoid repeated brushing in the same area to prevent bubbles. Brush slowly to ensure the liquid fully wets the PCB and components.
3.After ~2 hours of brushing the coating in the paper cup, replace the cup, pour a new 2-hour supply, and continue operation.
4.For double-sided brush coating, place the board horizontally to brush one side, wait for surface-drying, then brush the other side (board horizontally placed).
5.Lamp boards or small boards generally use brush coating, while large boards preferably use a conformal coating machine, subject to dedicated documents for each model.
3.3.6 Curing
1.The coating line uses a conveyor curing oven. After spraying and inspection, the PCBA carrier enters the oven via a chain for curing. Set the oven temperature and curing time as per dedicated documents
2.Adjust the oven track (to align with the previous track) before cuing, and set the oven temperature and chain speed (faster speed = shorter curing time, vice versa).
3.When the PCBA exits the oven, hold the carrier edges with both hands, place them on a transfer cart, and push to a ventilated area for air-cooling or room-temperature cooling.
4.Wear protective gloves when opening the oven or handling the carrier to prevent burns from high temperatures.
3.4 Coating Inspection, Acceptance, and Issue Feedback
1.After PCBA coating, curing, and removal of protective materials, the operator must inspect protected areas for contamination. If defects are found, set them aside, label them, and report promptly.
2.Load the completed boards onto a transfer cart and move them to the assembly line or packaging line.
3.5 Dry Film Thickness Testing and Repair
1.Preferably test dry film thickness on the cured and cooled coated board using a dry film thickness tester as follows:
A. Before coating, select a 10×10mm copper foil area on the board to test the green oil thickness and record the value. Since solder mask is applied by screen printing, the thickness is consistent for the same batch of PCBs. Maintain thickness test records for each batch of PCB solder mask, as shown in the figure.
B. On the cured and cooled coated board, select a 10×10mm copper foil area to test the thickness of the insulating material (solder mask+ conformal coating) and record the value. The conformal coating thickness = this measured value – the solder mask thickness of the same batch of PCB. Note: This measurement is the thickness of the insulating layer on the copper foil, which equals green oil + conformal coating thickness. If the measured dry film thickness exceeds the SOP requirement, there is a risk of wet conformal coating contaminating non-coated components. Promptly report to the ME engineer for adjusting the spray gun flow. If the flow-exceeding machine has contaminated non-coated components, repair or replace them; if not, the machine can produce normal products.
For coating, if thickness cannot be measured on the board, place an aluminum sample plate (the above-mentioned metal measurement plate) in the tray to be sprayed with the PCBA. After curing and cooling, test the dry film thickness on the aluminum plate with a thickness tester.
Requirements:
A. For high-protection products, spray as per dedicated documents (generally thickness >50 microns).
B. For general products, the universal requirement is thickness >30 microns. For manual coating, test at least once every 4 hours; for automatic coating, once every 8 hours.
2.Dip Coating: Immerse the aluminum sample plate in conformal coating for 5 seconds, then remove and cure. Test the dry film thickness on the aluminum plate, with the same requirements as spraying.
3.Brush Coating: Brush the aluminum sample plate with conformal coating as per normal operation, cure, and test the dry film thickness, with the same requirements as spraying.
4.Wet film thickness testing has large errors and serves only as a process control reference; spray quality is based on dry film thickness.
5.For boards where dry film thickness can be directly tested, use board test data as the basis.
4.6 Component Replacement and Film Repair
After film coating and curing, if components need to be replaced, follow these steps:
1.Use a soldering iron to desolder the component normally, then clean the pad surroundings of yellowing substances with a cloth dipped in Thinner.
2.If desoldering with a soldering iron is difficult, first scrub the conformal coating on the pad with a cotton swab dipped in Thinner to partially dissolve or thin it, then desolder the component with a soldering iron and clean the pad surroundings with a cloth dipped in Thinner.
3.Solder the replacement component.
4.Brush the soldered area with coating and cure the film surface.
4.7 Precautions for Conformal Coating Use
1.When operating in the coating room (line), strictly follow the operation guidelines for the spraying machine, oven, and other equipment. Perform equipment maintenance and checks as required, and wear anti-static gloves.
2.Note the mixing ratio of conformal coating to thinner.
3.After using conformal coating and thinner, tighten the bucket lids and store them in a ventilated, shaded area below 30℃, away from open flames, power sources, and electromagnetic interference. The storage shelf must be well-grounded.
4.Check the production date before use and do not use expired products.
5.If conformal coating contacts the skin, wash with soap or detergent immediately. If ingested, seek medical attention immediately. If it enters the eyes, rinse with water immediately, do not rub, and seek medical help promptly.
6.After spraying each 24 hours, disconnect the paint inlet pipe of the spray gun, spray a can of thinner for ≥10 times to prevent curing blockage. Disassemble and clean the gun head weekly, thoroughly cleaning the nozzle and adjusting screws. Do not clean the needle valve with other components to avoid damage, and do not use the needle valve to poke objects. Clean the spray gun thoroughly if unused for >12 hours. Clean brush coating tools with thinner daily, collect the waste liquid in a sealed plastic bucket, and do not dispose of it casually for environmental protection.
7.Tighten all empty painting buckets and place them in designated areas. Empty buckets, residues, and waste liquid must be disposed of by a professional waste treatment company.