Obviously, if your customer instructs you to use a certain flux type, using THAT flux type is probably a good idea.
Responding to your question: Chloride is one of the more detrimental materials found on printed circuit assemblies. Chlorides can come from a variety of sources, but is most often attributable to flux residues. Chlorides will generally initiate and propagate electrochemical failure mechanisms, such as metal migration and electrolytic corrosion, when combined with water vapor and an electrical potential.
The amount of chloride that can be tolerated on an assembly depends on the flux chemistry being used. Assemblies processed with high-solids rosin fluxes (RA, RMA) can tolerate higher levels of chloride due to the encapsulating nature of the rosin. Organic [water soluble] fluxes and no-clean fluxes are generally based on resins or very low levels of rosin, and so do not have this encapsulating protection, therefore, they require lower levels of flux on final assemblies.
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