Coating over no-clean flux residues is a standard practice, but as with any engineering question, the answer is it depends...

On the type of coating chemistry you are using and the specific nature of the flux residue. Some coatings are more tolerant of flux residues, especially those with very high solvent contents.

Others such as water-based materials are very sensitive to flux residues, with specific defects including dewetting (the coating won't completely coat the residue -- it will pull back away from the residue), poor adhesion, CTE mismatch between the coating and flux residue causing cracking during thermal cycles/shock, or more insidious cases of electrochemical corrosion...

This last defect/failure mode is generally believed to be caused by the coating interacting with the residue to liberate the excess activators from their resin matrix which during high humidity conditions can lead to the formation of corrosion products.

First point of call, I'd check in with your coating and flux suppliers to see if they have any experience or data regarding the 'compatibility' of the materials. This data whilst useful to you, is unlikely to replicate your exact process (thermal profiles etc) so you would want to make an internal evaluation (flux/coating suppliers should be able to help you verify your process) to ensure you are not solving one problem and inadvertently causing another.

Aside from the flux residues, general cleanliness of the assembly (residues from handling, bare board fab etc) and the cure conditions of the solder mask will have a big impact on the overall level of reliability you achieve.

This is why cleaning would generally be considered the defacto standard for high reliability applications, but even cleaning has its own pitfalls beyond the scope of your question.

I hope this helps for now, please don’t hesitate to contact us if we can assist you in any way.

It should not have any issues.

If the coating of selection is not adhering most likely it will be a harmful excess of no clean residue that is not supposed to be there in any case.

A standard average no clean flux residue is normally acceptable.

Many people do this the feasibility of doing so depend on both the conformal coating and the flux. There are two main areas of concern, one is inhibition of cure of the coating which is typically limited to thermal cure silicones.

The other is adhesion to the flux reside, poor adhesion to the flux residue can allow pooling of moisture under the coating, this is difficult to predict and will need to be tested.

I would not recommend that any coating be applied over any surface, without that surface being cleaned to remove possible contamination. The adhesion of the conformal coating is directly proportional to the adhesive forces between the coated layer and the coated surface.

ANY particulate or chemical contamination, even at microscopic or trace levels, will weaken these forces, which, in turn, will weaken the continuity and effectiveness of the conformal coating. It is possible that delamination can occur in these weakened areas, depending on the environmental stresses the board will experience.

As an experienced coatings chemist, my recommendation is to CLEAN the board before conformal coating. "No-clean" flux is more of a marketing catch-all phrase than a practical reality, for high performance requirements

The short answer is yes.

The majority of circuit boards that are coated are no clean.

However it is a good idea to get the combination of no clean flux and coating compatibility tested. Most coating suppliers offer this service.