The functions of soldering flux are to protect against re-oxidation of the joint during the soldering procedure, promote wetting that allows capillary action to begin, and to assist in residual oxide removal. The flux should be applied to surfaces that have been mechanically cleaned, and only enough should be used to lightly coat the areas on the tube and fitting that are to be joined.
An oxide film may re-form quickly on copper after it has been cleaned. Therefore, the flux should be applied as soon as possible after cleaning.
The fluxes best suited for soldering copper and copper alloy tube should meet the requirements of ASTM B 813, Standard Specification for Liquid and Paste Fluxes for Soldering Applications of Copper and Copper Alloy Tube.
Some fluxes identified by their manufacturers as "self-cleaning" present a risk in their use. There is no doubt that a strong, corrosive flux can remove some oxides and dirt films. However, when highly corrosive fluxes are used this way, there is always uncertainty whether uniform cleaning has been achieved and whether corrosive action from flux residue continues after the soldering has been completed.