Conformal coating cracks or lifts when a rigid film sits on a board that expands, contracts and vibrates, or runs hotter than the coating's range. A flexible silicone coating copes better. Micron 3708 SIL-26 Silicone Conformal Coating is a moisture-curable silicone coating whose page says it accommodates thermal expansion and vibration without cracking and lists LED drivers, power supplies and outdoor electronics. Its page gives no numeric temperature limit, so confirm your temperature with Micron.
Why it happens
- A rigid coating on an assembly that moves. Boards and components expand at different rates as they heat and cool. A stiff film cannot follow, so it cracks.
- Operating temperature beyond the coating's range. Micron's acrylic coatings (3100, 3110) are rated -70°C to 120°C. Hot spots near power components can exceed that.
You will see cracks in the coating near hot components and coating lifting after repeated thermal cycles, often on LED drivers, power supplies and outdoor equipment.
How to fix it
- Heat, vibration, thermal cycling or outdoor service: Micron 3708 SIL-26 Silicone Conformal Coating. Its page describes a flexible, highly dielectric film resistant to thermal cycling and vibration, IPC-CC-830B compliant, for PCBs, LED drivers and power supplies, automotive and outdoor electronics.
- Service stays within -70°C to 120°C and easy rework with thinner matters: Micron 3100 Protektor Conformal Coating, an acrylic that can be removed with Protektor Conformal Coating Thinner.
Directions from the 3708 page, in short:
- Make sure the PCB is clean, dry and free from oil, flux residue, dust and contaminants.
- Apply by brushing or dipping. The page gives a dip time; follow it and the TDS.
- Let excess coating drain.
- Let it dry under ambient conditions, or as your process requires, for the time stated on the page and TDS.
- Use only in well-ventilated areas.
Remove the old cracked coating and clean the board before applying a new one. Coating over a cracked film traps contamination.
What to avoid
- Use 3708 only in well-ventilated areas, as its page states, and avoid contact with eyes and mouth.
- Do not assume 3308 thinner removes the silicone coating. Micron's pages state removal only for the Protektor acrylic coatings.
- Do not use acrylic 3100 or 3110 on boards that run above 120°C.
- Do not coat over flux residue or dust.
When to ask an expert
Ask Micron technical if:
- you have a specific temperature requirement, because 3708 has no published numeric limit;
- you need to strip an existing silicone coating;
- cracking persists with a flexible coating, which may point to mechanical stress or a design issue.
FAQ
Why does my acrylic coating crack near power components?
Acrylic is relatively rigid and Micron rates its acrylic coatings to 120°C. Hot spots and repeated thermal cycling can crack it. A flexible silicone coating such as 3708 is designed for that kind of movement.
What temperature can 3708 handle?
The page describes high-temperature stability but gives no number. Tell Micron your operating temperature before choosing it.
Can I rework a board coated with 3708?
Micron's pages do not say which product removes 3708. Ask Micron before you plan rework on silicone-coated boards.
Is 3708 sprayable?
The page describes brushing, dipping or selective coating methods. It is supplied in bulk, not as an aerosol.
Should I switch every board to silicone coating?
Not necessarily. If boards stay within -70°C to 120°C and you rework them often, acrylic 3100 is easier to remove. Use 3708 where heat, vibration or thermal cycling is cracking a rigid coating.
See the product: Micron 3708 SIL-26 Silicone Conformal Coating. Related: 3100 Protektor Conformal Coating. More electronics fixes: electronics hub.