Here is what I have in mind: img51.imageshack.us/img51/7204/screenshotjv.jpg (again unfortunately this forum doesn't allow me to show it as image because it says that I don't have enough posts...)

How can that non conductive flux residue, along with oxide and impurities that remain inside it, be an advantage in the electric conductivity of that?

how can you remove that residue if it remains between the tin and the bar, incorporated within the soldering?