Your conformal coating film is supposed to be clear and transparent, but it’s cloudy-white and dull … it’s blushing. This condition is an indication that your coating film is trying to tell you something, and it isn’t that it’s embarrassed. This blog will help you better understand why your conformal coating film is blushing from acquired moisture contamination and help you eliminate this defect.
Coatings appear cloudy because they have inadvertently acquired moisture, either due to their hygroscopic nature or by artificial means (a cooling mechanism created by solvent evaporation), which then reacts negatively with the coating resin – exhibiting itself as a milky, colloidal-type substance. Many different types of conformal coating chemistries can be sensitive to blushing under certain circumstances. These circumstances are usually related to specific ambient conditions, application, cure, condition of the assembly/substrate/associated components, storage environment and/or equipment under use.
These conditions include:
- High humidity, ~70% or higher
- Low (or cooling) temperatures, ~16oC or below
- Assemblies and associated components exposed to ambient moisture during storage prior to coating, can significantly increases the chance of blushing. To overcome this, baking is often recommended. Not baking your boards after cleaning is a prime source of moisture uptake into assemblies
- Storage conditions (opened containers, inside pressure pot, RH resident in the charging gas of the application equipment) and interaction with moisture already on the assembly.
In conjunction with this variety of circumstances, certain solvent-borne chemistries can aggravate this condition due to the fast evaporation of internal solvents. This creates a cooling mechanism as they volatize from the film. This cooling mechanism will condense any moisture vapor that may be present in the immediate atmospheric area.
High solids, low volatile room temperature vulcanized, or UV conformal coatings (with secondary moisture cure functionality) may also show some degree of “blush.” They are more sensitive, possibly hygroscopic, to the presence of ambient moisture vapor as this is utilized to activate their primary or secondary curing mechanisms.
Problems that can be created due to moisture vapor intrusion/blushing:
- Surface tackiness/incomplete cure
- Below standard aesthetic quality
- Coating discoloration
- Poor gloss retention
- Poor adhesion
- Wrinkling during second coat application/over-coating.
Here are some simple steps to start with … DIY:
- Maintain nominal ambient humidity in the production environment between 45% and 65%.
- Maintain nominal ambient temperature in the production environment between 18oC and 27o
- Increasing the flash time between coating application and cure may help, as moisture vapor in the wet film may have the potential to re-evaporate
- Use different thinner to change evaporation rate.
- Slowing the thinner compound average evaporate rate will minimize or potentially eliminate condensation of moisture vapor due to fast mechanism of solvent evaporation
- Please refer to Rafael Hernandez’s blog posting for different solvent evaporation rates…Bubbling.
- Use desiccated air/nitrogen to push coating through applicator
- Post-bake assemblies after aqueous wash (if applicable) to eliminate any entrapped (under or between components) moisture.
- Ensure that all partial containers are properly sealed when stored. Nitrogen purge in partially used container helps to eliminate moisture.
We hope that this helped you understand what blushing is, how it occurs, and how to prevent/alleviate it from occurring. Remember that moisture is the root cause of cloudy or blushing conformal coating. With many potential sources of moisture contamination, following our simple recipe will lead you back to a crisp, clear, and transparent film, and zero defects.