The concept of no-clean materials is that the residues remaining on the surface are of no consequence to functionality of the assembly during its useful life.

Obviously, there are many variables such as assembly design, intended use (including environment) along with the design of the no-clean material itself that should be considered, and one presumes the needs of a child's electronic toy would be different from those of a pacemaker.

The job of judging where a particular assembly falls between a toy and a pacemaker falls to the manufacturer.

Yes, flux activators are often hydroscopic. Yes, those activators can turn white, or react with the mask and turn it white. Yes, Analytical Laboratories, such as Kyzen's or pure failure analysis labs can pin down those facts.

However, it strikes me that the humid environment is a fundamental part of this assemblies life cycle. Likewise, there may be nothing wrong with your process other than that it is incomplete. Perhaps you should consider a cleaning process to remove these residues.

Once gone, there is nothing left to be affected by the environment. Kyzen's Application Laboratory conducts such testing free of charge everyday for organizations facing similar challenges. Feel free to contact me directly if that is of interest.

Most No Clean or No Residue fluxes are not Hygroscopic and tested according to the various standards and do not require cleaning even in High Reliability applications. If organic acid from no Clean fluxes are left on a PCB in a condensing atmosphere then the worst that will happen is reduced SIR figures.

Without seeing the residue the problem sounds like the flux has been absorbed by a porous solder resist and has been released along with the solder resist filler which is Mineral Salts or Talcum Powders.

Easy way to test for this is if the residue cannot be cleaned in 'normal' cleaning solvents then it is likely to be Mineral Salts that can only be cleaned in Mineral Acids. If this is the case then speak to your bare board manufacturer to increase either the temperature or time of cure to overcome this issue.

Especially if the PCB is HASL finished as this will not be reliable to be left on the PCB and it does have the potential to ionize in the environment you describe.

It is true that most resins and rosins used in no-clean flux will turn cloudy and white when they get wet. High humidity can produce this effect.

If there is unspent activator left in the flux residue and that residue does not protect the unspent activator from water, then it is possible to have issues with corrosion and electromigration.

That said, human handling can leave more aggressive acids and salts on a board that partially spent flux. If you decide to assess the situation with testing, be sure of what you are observing with any analytical tests you perform.

First your description of the conditions does appear to be the reaction of partially heat activated flux that is able to absorb moisture will change from clear to white hazy in 3 weeks to 3 months. Depending on how much moisture is available.

Properly heat activated flux will stay clear and only turn white when a large amount of moisture is on the PCBA, such as standing water or running it through a cleaning system with water.

The reason the partially heat activated no clean flux is absorbing moisture is that it never completely released the carrier due to not enough heat for the required amount of time for the volume and will be both moisture absorbing and conductive residues on the board.

Dendrites may not grow but leakage pathways are available to cause intermittent and failing performance when these PCBA's are put into the field.

Check the cleanliness of the assembly using Ion Chromatography and localized C3 extractions. To identify the areas with the heaviest flux that need the optimization of heat during preheat and soldering and review the amount of flux for the soldering needs.

The board contractor may have to provide more information to us on cleaning and inspection of these boards. Do these boards see an aqueous or solvent based cleaning agent wash process before shipment to end customer?

Even after the wash process, does the inspection involve a visual inspection under microscope 40X-60X magnification as well as an ionic contamination / ionic chromatography test to confirm the cleanliness level of the boards? These are crucial points.

It is true that the activators in flux are hygroscopic in nature and would tend absorb moisture from the atmosphere. Such behavior would then result in failures such as electrochemical migration, contamination induced leakage current.

Even if the flux type is No-clean, it doesn't necessarily mean that it doesn't require a wash process. It does; especially to meet high reliability requirements.

On the other hand side, the issue could also be related to a solder mask defect, but then again that has to be tested and verified as well. This can be done at Zestron's Technical Center and provide a better overview of the situation to the board contractor.