Bulletin Category: Adhesion & Surface Problems | Typical Environments: Manual spray, selective spray, dip coating and cleaning processes.

De-wetting is one of the most common coating defects seen after cleaning, especially where assemblies appear visually clean but still contain contamination or unstable surface conditions. Assemblies may appear clean before coating, yet the coating still pulls away, separates or refuses to form a continuous film.

The issue is often incorrectly blamed on the coating material itself. In reality, de-wetting is normally linked to surface condition, contamination, cleaning residues or changes in surface energy.

A cleaned assembly is not automatically a coating-ready assembly. This bulletin explains why apparently clean boards can still reject conformal coating during production.

What Engineers Usually See

De-wetting rarely affects the entire assembly uniformly. Instead, it normally appears as isolated local coating defects.

• Coating pulling back from local areas of the PCB.
• Circular “fish-eye” style defects.
• Patchy or uneven coating spread.
• Edge pull-back around pads, leads or solder joints.
• Random localised non-wetting areas.
• Defects appearing intermittently between batches.

Common Hidden Causes

De-wetting is usually linked to contamination or surface-energy disruption rather than coating chemistry failure.

• Cleaning residues: incomplete rinsing or chemistry carry-over can leave invisible surface films.
• Handling contamination: fingerprints, gloves, oils or packaging contact can affect local wetting.
• Low surface energy: certain plastics, residues or process chemicals reduce coating spread.
• Silicone contamination: even very small amounts can severely disrupt coating behaviour.
• Flux residues: no-clean fluxes may still affect coating adhesion and wetting.
• Surface ageing: cleaned assemblies left exposed before coating may change condition over time.

Practical Engineering Checks

When de-wetting appears, the first response should be systematic process review rather than immediate coating replacement.

• Review cleaning chemistry and rinse effectiveness.
• Check whether handling practices changed before coating.
• Compare failures against specific PCB areas or materials.
• Review time delay between cleaning and coating.
• Check for silicone-containing materials or nearby contamination sources.
• Confirm whether the defect is batch-specific or process-wide.

When This Becomes an Engineering Issue

If de-wetting is recurring, batch-dependent, inconsistent or linked to field reliability concerns, the issue usually extends beyond simple coating application.

SCH can support cleaning assessment, coating trials, plasma cleaning review, contamination troubleshooting, adhesion investigation and process-control improvement.

Related Technical Resources

• De-Wetting in Conformal Coating
• Surface Preparation & Cleanliness
• Plasma Cleaning for Conformal Coating & Parylene Adhesion
• Why Does Cleaning Improve the Adhesion of the Conformal Coating?
• Engineering Consultancy

Why Choose SCH Services?

SCH Services combines practical coating production experience with technical consultancy, training, process engineering and specialist coating support.

Our work covers conformal coating, Parylene coating, masking, coating removal, inspection, process troubleshooting and advanced functional coating applications.

This allows us to support customers not only with coating knowledge, but with the practical controls needed to make coating processes repeatable, inspectable and production-ready.

Technical Guidance Disclaimer

This bulletin provides general technical guidance only. Coating behaviour, cleaning effectiveness, contamination control, surface condition and process suitability must always be validated against the specific assembly, material set, operating environment and production requirements.

SCH Services can provide further engineering support where application-specific assessment, testing, validation or process development is required.