Coating is the last process on the line and the easiest one to get wrong. It goes over finished solder joints, so a board that was already suspect looks perfect afterwards, and a board with flux residue trapped underneath fails later with no visible cause. The choice of family, the thickness and the masked areas are all decided long before the spray gun runs, usually on a drawing that says only the word coating.
What follows is what the drawing needs to say, and what each decision costs when it is left open.
The four families and what each one is for
| Family | Typical dry thickness | Service range | Rework | Fits |
|---|---|---|---|---|
| Acrylic (AR) | 30 to 130 micrometre | Around -55 to 125 C | Easiest, dissolves in solvent | General industrial, consumer, boards reworked often |
| Silicone (SR) | 50 to 210 micrometre | Wide, up to around 200 C | Hard, needs mechanical removal | High temperature, thermal cycling, flexible boards |
| Urethane (UR) | 30 to 130 micrometre | Around -55 to 125 C | Hard, solvent resistant | Chemical exposure, abrasion, fuel and solvent environments |
| Parylene (XY) | 10 to 50 micrometre | Very wide | Very hard, abrasive or plasma | Best barrier per thickness, tight spaces, high impedance nodes |
The family decides more than protection. Acrylic tolerates repair and is the safe default for a product that will see field rework. Silicone survives heat and movement but fights every repair. Urethane handles chemicals and abrasion and makes a coating defect expensive to fix. Parylene gives the best barrier at the lowest thickness and needs vacuum deposition, masking that survives the process, and a plan for removal before any rework.
Thickness is a process output, not a wish
A drawing that says 50 micrometre does not produce 50 micrometre. Thickness varies with spray pattern, number of passes, board topography, and how much coating pulls away from tall parts and connector bodies. What works is a range with a measurement method: for example 30 to 130 micrometre dry film on a flat coupon that runs through the line with the panel, measured at defined points, plus a visual criterion for coverage on the assembly itself.
The minimum on the flat area proves the barrier. Coverage on edges, around leads and under low components proves the process reaches the surfaces that fail first. A board can pass a thickness check on a coupon and still have an uncoated shadow behind a tall capacitor.
The keep-out list
Masking is where most coating defects are created. Every feature that has to stay conductive, mate with something, or be probed needs a decision before the first panel runs.
| Feature | Why it is masked |
|---|---|
| Connector contacts and mating faces | Coating insulates the contact and changes insertion force |
| Test points and probe pads | A coated pad blocks the fixture pin and gives a false pass or a false fail |
| Card edge fingers | Coating thickness changes the edge connector fit |
| Press-fit holes | Coating in the barrel raises insertion force and can crack the plating |
| Mounting holes and chassis ground pads | Grounding depends on bare metal contact |
| Thermal pads and heatsink interfaces | Coating adds a thermal resistance layer |
| Optics, sensors and windows | Any film changes the optical path |
| Labels, barcodes and serial marks | Covered print becomes unreadable at traceability |
| Battery contacts and fuse holders | Contact resistance and service access |
The list belongs on the drawing as a marked layer, not as a note saying connectors to be masked. Two suppliers reading a note mask two different feature sets, and the difference surfaces at incoming inspection.
The sequence that avoids rework
Clean, verify cleanliness, dry, mask, coat, cure, inspect, unmask, test. Each step exists because skipping it moves the failure somewhere worse.
Cleaning comes first because coating seals in whatever is on the board. Ionic contamination under a coating is worse than no coating: the residue sits against the conductor with a barrier holding moisture in, and the leakage shows up as drift on high impedance nodes. A cleanliness check on the lot before coating turns that into a process signal rather than a field return.
Drying matters for the same reason. Moisture in the laminate and under components expands during cure, and the result is bubbles and pinholes that look like application faults. Boards stored in a humid warehouse need a bake before coating, as do assemblies cleaned in water.
Cure has to match the chemistry. Acrylic and urethane cure by solvent release, silicone by moisture or heat, and UV cure only reaches what the light reaches. A board with tall components has shadowed areas that stay wet while the visible surface is dry, so the cure schedule needs a thermal step or a second pass, not a shorter belt.
Inspection then has to prove three things: coverage where it is required, no coating on the keep-outs, and no voids bridging adjacent conductors. UV tracer in the coating makes the first two visible in seconds. Thickness measurement on the coupon and a cross section on the first article settle the third when the product is medical, defense or automotive.
How SUPERB Assembly handles coating
SUPERB Assembly applies acrylic, silicone and urethane with a selective coating machine that sprays, brushes or dips, so the keep-outs are programmed rather than taped by hand on every panel. An automated UV and IR curing oven brings cure down from hours to minutes, which matters when a lot has to ship the same week it was coated. Coating inspection uses UV light plus thickness measurement against IPC-CC-830, and the cleanliness gate ahead of coating is an ionic contamination test.
The boards underneath are built to IPC-A-610 Class 2 or Class 3 against J-STD-001, in a floor that runs ESD S20.20 at 22 plus or minus 3 C and 45 to 65 percent relative humidity. Where the product also needs a housing or a harness, coating sits in the same build sequence.
- Coating selection guide: matching the family to the environment the product sees.
- Reliability testing: thermal cycling and damp heat that expose a weak coating.
- PCBA workshop: the coating, cure and inspection equipment in one view.
Frequently asked questions
Can a press-fit connector be coated?
The connector body can be coated if the barrel and the pin area are masked. Coating inside a press-fit hole changes the interference that holds the pin, so the hole is always a keep-out.
Does coating replace cleaning?
No. Coating is a barrier, not a cleaner. Residue under a coating is trapped against the board, and the flux activators stay available for corrosion in the presence of any moisture that gets through.
How is a coated board repaired?
Local removal, rework, reclean, recoat and cure, following the repair standard the contract names. The repair record should state the coating removed, the method and the recoat thickness, because a field failure at a repair site is often a coating defect rather than a solder defect.
Next step
Send the Gerber data, the BOM and the assembly drawing with the environment the product sees and the features that must stay bare. The reply proposes a coating family, a thickness range with the measurement method, the masking layer and the inspection scope, and quotes the lot with all three written in.
