Hi, I´m trying to braze 0.7mm thick aluminum square tube to a 1mm thick aluminum sheet. I thought of using soldering tin as a filler metal because of the low melting point so as to avoid going through the 0.7mm tube and to minimize bending of the sheet.
Does anyone know the chemical composition of the flux I should use and how to apply it?

I was thinking on Caustic Soda; but I´m not sure.

Thanks,
Diego.