Conformal coating de-wets, bubbles or lifts when there is oil, flux or dust on the board underneath. Clean the board with a PCB cleaner and let it dry fully before coating. Micron 3400 Protektor PCB Cleaner is a solvent-based PCB cleaner and degreaser whose page says it provides a clean surface for the re-application of conformal coating and evaporates to a clean, dry, residue-free surface. If old Protektor acrylic coating has to come off first, use Micron 3308 thinner.
Why it happens
- Oil, flux or dust on the board before coating. Coatings need a clean surface to wet out and bond. Fingerprints, flux residue and dust all get in the way.
You will see coating de-wetting (pulling back into beads), poor adhesion, bubbles, or coating lifting after a short time in service.
How to fix it
- Cleaning boards before coating or recoating: Micron 3400 Protektor PCB Cleaner, 550 ml aerosol or 1 L bulk liquid. Its page says it removes thin oils, thermal paste, light flux residue, ink and grime, and is specifically formulated for delicate electronics.
- Old Protektor acrylic coating has to be stripped before recoating: Micron 3308 Protektor Conformal Coating Remover and Thinner. The Protektor coating pages say cured film can be removed using Protektor Conformal Coating Thinner. Clean with 3400 afterwards.
The 3400 page has no step-by-step directions. Follow the instructions on the can and the TDS. A general prep routine:
- Power off and disconnect the board.
- Work in a ventilated area away from ignition sources.
- Remove old coating where needed (3308), then clean the whole area to be coated (3400).
- Handle the board by its edges with clean gloves from here on.
- Let it dry fully.
- Mask connectors and anything that must stay uncoated.
- Coat, then inspect.
What to avoid
- Let the board dry fully before coating. Trapped solvent can cause bubbles.
- Boards must be unpowered. 3400 is a solvent: treat it as flammable until the SDS says otherwise, keep it away from sparks, flames and hot surfaces, and use ventilation.
- Do not touch cleaned areas with bare hands.
- Do not use 3308 on silicone or UV-cured coatings; that is not stated on its page.
When to ask an expert
Ask Micron technical if:
- adhesion failures persist after cleaning;
- you are recoating over a coating you cannot identify;
- your customer has a cleanliness or adhesion test you need to pass.
FAQ
Why does my coating pull back into droplets?
That is de-wetting, and it usually means there is oil, flux or another contaminant on the board. Clean with a PCB cleaner and let it dry before coating again.
Can I coat straight after cleaning?
Wait until the board is fully dry. The 3400 page says it evaporates quickly, but check under components where solvent can linger.
Do I need to remove the old coating before recoating?
If the old coating is damaged, contaminated or lifting, yes. For Protektor acrylic coatings, the product pages say the cured film can be removed with Protektor thinner (3308).
Does 3400 leave a film that affects coating?
Its page says it leaves a clean, dry, residue-free surface and provides a clean surface for re-application of conformal coating.
How do I know the board is clean enough?
A quick check is to look for even wetting when you coat: the coating should flow out smoothly with no beads or holes. If your customer specifies a cleanliness test, use that and ask Micron technical for help meeting it.
Should I clean the whole board or only where I coat?
Clean everything that will be coated, plus a margin around it. Contamination at the edge of a coated area is where lifting often starts.
See the product: Micron 3400 Protektor PCB Cleaner. Related: 3308 Conformal Coating Remover and Thinner. More electronics fixes: electronics hub.