Blistering

Definition

Author

• Dr Lee Hitchens, Nexus

Blistering is the formation of dome- or circular-shaped bubbles of the coating film held away from the substrate.

Blistering can be the start of the conformal coating delaminating.

However, it can also happen within the conformal coating itself and not just the interface of the conformal coating and the laminate surface.

Download our PDF guide Solving conformal coating problems in the application process now.

Examples of conformal coating blistering

Examples are shown in the images where the conformal coating where blistering has occurred.

Causes of blistering in conformal coating

Factors that influence blistering include:

• Cleanliness: The cleanliness of the surface for good adhesion. If the surface is contaminated across a wide area then blistering can occur due to the poor adhesion of the coating to the laminate.
• Compatibility: the conformal coating and the surface to be adhered to need to bond teogether. If they are incompatible then they will not bond. This incompatibility may by due to surface energies mismatch. If the conformal coating is not matched to the laminate then blistering can occur.
• Permeability to moisture: If the coating allows the moisture to permeate into the region between the coating and the laminate, then a difference in vapour pressure occurs. This could lead to osmotic pressures building due to a change in temperature and pressure, which lift the coating away from the laminate and cause a blister.
• Degree of coating cure: If the conformal coating is not cured correctly then this can lead to adhesion problems.

How to prevent blistering in conformal coating

It is possible to minimise blistering effects by using techniques such as:

• Cleanliness: Clean the surface of the board to improve the adhesion of the coating. Remove contamination such as mould release agents on components before application.
• Compatibility: Match the conformal coating to the solder resist and laminate so that good adhesion is achieved. Change the coating and / or surface finish so the surface energies are more favourable.
• Permeability to moisture: Choose a less permeable conformal coating.
• Reduce the coating thickness: This reduces the problems associated with CTE mismatch with the components and laminate that lead to flexing of the coating.
• Lower the Cure Temperature: Reduce force drying (thermal profile) during curing to avoid excess stress and strains on the coating.
• Prime the surface: Apply a primer to treat the board surface. This will increase the bonding of the conformal coating to the substrate.

Download our PDF guide Solving conformal coating problems in the application process now.

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