After seeing a question about soldering flux and in particular flux paste which I'm interested in trying, I was reminded that there is a lot of confusion and scare about solder flux. I'm one of the confused.
Plumbing flux is bad
There are two basic things people ask about: No-clean and Plumbing flux. My question now is only about plumbing flux, which we all know not to use on electronics. If you accidentally use plumbing flux it must be cleaned. A few random quotes from high-rep users:
Fluxes designed for metalwork, such as soldering pipes, is generally an acidic flux and must never be used on PCBs or it will corrode things horribly.
In general, though, solder pastes are acid-based, and much more aggressive than rosin-based. You can use such fluxes, but intense care in washing off the residue is required to avoid corrosion later.
Just buy the good stuff!
With the influx (get it?) of large international marketplaces it can be difficult to know for sure what you are buying. Translation errors, different brands in different regions, different regulations on what has to be printed on the label, the possibility of the sender to stock or ship the wrong thing, counterfeit products, etc. Since the internet still is pretty U.S.-centric (for natural reasons), a lot of recommendations are for U.S.-based products not readily available in other parts of the world, or are much more expensive.
But how do you know?
Sometimes they tell you straight up on the package: "Not applicable to PC boards", but you're not always that lucky. The user in the question that prompted me to ask here thought that his flux was corrosive because it had warning labels, but here is a quality U.S. made flux paste (SRA #135) that is obviously rosin-based and designed for electronics repair, and still warns against being "irritating to eyes, respiratory system, and skin".
How can I verify or test at home that a solder flux I have bought is suitable for electronics?
Can I do it in a way that does not take 3 months? In a way that does not require a chemistry lab? What am I actually looking for? From what I understand there are degrees of corrosiveness so maybe it's not even possible to say for sure?