How to Avoid Bubbles in Conformal Coating

Bubbles and foam are two of the leading causes of failure during conformal coating inspections. Bubbles cause voids in the coating, leaving areas of the substrate exposed to potential contaminants. Therefore, it is important to understand what causes bubbles to form and the steps to take 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.

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.

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 the components of a circuit board. When this happens, air can seep out during the drying and curing process and cause bubbles.

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 under-fill components more effectively

Cause #4: Pressure Pots
Pressure pots with conformal coating inside can absorb air, which can cause the appearance of champagne bubbles.

When using pressure pots with conformal coating inside, try to avoid leaving the pots pressurized for long periods of time. Additionally, do not run the pressure pots more than 50% full and lett 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.

When brush coating, try to blend the coating so it flows easily and ease the material onto the PCB. Do not “work” the coating into the PCB as it creates bubbles.