Ok , everybody is saying it , but not out right and direct ..
FLUX , when heated , melts into a liquid form and spreads ALL over . Which , if it WAS conductive , it WOULD short out any nearby solder joints on a board with other electrical components if it were conductive . Flux does NOT disappear .. its that slimy greasy felling stuff left on solder joints . True , you CAN burn it all off , but , if you get your solder onnections that hot ... well , not sure your gonna have a run left there to solder to anyway .