Bubbles and foam are two of the leading causes of failure during conformal coating inspections. Because of this, it’s worth looking at these defects more closely.
Bubbles cause voids in the coating, leaving areas of the substrate exposed to potential contaminants. Knowing this, a question we often hear is:
- How do I prevent bubbles from forming in my conformal coating?
The answer is: Understand how and why bubbles form in the first place. Then you can take steps to prevent them.
5 Common Causes of Bubbles in Conformal Coating (and Their Remedies)
Here are the five most common causes of bubbles in conformal coating, along with their remedies:
Cause #1: Wet Surface Skins Over
When applying a wet conformal coating, the surface of the coating can skin over, trapping solvents under the surface, which can bubble or burst out.
The remedy: When applying the conformal coating, ensure the wet film surface is not too thick. This is particularly important when spray coating. It is better to apply multiple thin layers than one thick layer.
Cause #2: Coating too Thick or too Viscous
If the coating is applied too thick or too viscous, any bubbles that are created can become trapped in the coating before they settle out.
The remedy: Apply thinner individual layers, and allow a flash-off time between coats to let any excess coating evaporate.
Cause #3: Air Trapped Under Components
During the conformal coating process, air sometimes gets caught under components of a circuit board. When this happens, air can seep out during the drying and curing process and cause bubbles.
The remedy: To avoid air entrapment under circuit board components:
- Dip the circuit board or device slower into the coating dip tank.
- Add a dwell time to allow penetration of the coating around and beneath components.
- Use a lower viscosity version of the conformal coating to underfill components more effectively.
Cause #4: Pressure Pots
Pressure pots with conformal coating inside can absorb air, which can cause champagne bubbles.
The remedy: When using pressure pots with conformal coating inside, try to avoid leaving the pots pressurized for long periods of time. You should avoid running the pressure pots more than 50% full and letting them stand. This will minimize the champagne effect in the coating.
Cause #5: Coating is too Viscous
Brush coating with too viscous a material or working the coating can cause bubbles.
The remedy: When brush coating try to blend the coating so it flows easily and “flow” the material onto the PCB. Do not “work” the coating into the PCB as it creates bubbles.
To learn more about common conformal coating defects, download our whitepaper: