Fluxes contain a wide variety of components and especially in the case of solder paste specific chemicals may be present at low levels to deliver particular features in the behavious of the product.
Flux exists to clean metal surfaces of their natural surface oxide layer so that molten solder alloy can wet the underlying metal and form a permanent strong bond. Of course fluxes deliver other process-related features (rheology for printing solder paste, tackiness to hold components etc) but these are generally physical rather than chemical properties.
Given the purpose of flux, it follows any chemical that removes metal oxides from common electronics materials at soldering temperatures could be used but in practice, and particularly with No Clean fluxes, it is critical that the flux is not so aggressive that it dissolves a lot of metal as well as the oxides.
It is also important that the fllux has very little chemical activity below soldering temperatures. Formulators achieve the correct balance of activity by using mild carboxylic acids (one of the most important being rosin and its derivatives) and small amounts of halides and amines. Essentially (but very much simplified) the acids dissolve oxide films while the halides and amines disrupt them, making dissolution faster.
There is some relationship between acidity and flux activity but most flux systems are non-aqueous so acidity is not always a meaningful concept. Activity as seen by a user is a combination of inherent reaction speed with oxides (activators may be more effective on some metals and not active at all on others), sustainability (the capacity to dissolve large amounts of metal oxides) and durabability (activators are deliberately selected sometimes to decompose to provide neutral residues). Add to this the fact that formulators often "chemically bind" activators so they only present in the flux when it is heated to soldering temperature and I think you can see why there is no simple answer to the question.
Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.
Common fluxes are ammonium chloride or resin acids (contained in rosin) for soldering copper and tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing, braze-welding ferrous metals, and forge welding.
Ellyes Perry, Best Soldering Iron