flux pcb can
I assume you are all talking about rosin flux?
All fluxes do the same thing, clean the surface as they boil away, allowing the solder to bond to unoxidized copper.
Unfortunately the effects of residual flux is hard to quantify. The type of flux, the impedance of the circuit, and the amount of moisture it has been exposed to all will have an effect. Both the aircraft part industry and military suppliers are required to test for residual flux to very low levels in a lab before shipping boards. This should indicate that there is a problem with flux conducting.
When I started, activated rosin flux was about all that was used (basically pine sap with some chemicals added). That flux tended to become corrosive if left on circuits and if exposed to high humidity it could become conductive for a long time even if moved to a low humidity environment. Couple that with extreamly high impedance tube (valve) circuits and it was a real problem. Lucky for us we had vapor degreasers and gallons of FREON TF to clean it with! I still think this is the best flux. Only problem is without FREON TF it's very difficult to clean.
Next (from what I know) was water soluble flux. It works fairly well (nowhere near as good as rosin) and the idea was to use a machine that was basically a dish washer to clean the boards after soldering. It gave off very irritating fumes and the we were never able to get it all off the boards!
In manufacturing, we are now using "No Clean" flux. I don't know a whole lot about it besides it is supposed to be non-corrosive, but I've still fixed operating problems by cleaning the flux off!
NOT ALL FLUX IS COMPATABLE WITH ELECTRONICS! Plumbers use acid flux (ever wondered why those throwaway lab brushes are called "acid brushes")? If that is used on a circuit board...give up now and throw out the circuit board. It will actually get under the circuit traces and eat the trace eventually. It is also VERY conductive when mixed with water (humidity) and I've actually seen it eat the fiberglass of the board. You can buy flux at the hardware store that will allow solder to work on both brass and stainless steel!
Oh BTW, I've tried all three types of flux on lead-free solder with no problems (higher heat, but that's the solder).