No Clean Flux

No clean flux is flux, whose residue, if left on board assembly after soldering, does not necessarily have to be removed. Under normal operating conditions of the board assembly in the field, corrosion on account of any residues is not a concern. Fluxes not classified as no-clean frequently contain acids or chlorides. These compounds do not decompose during the soldering process, and they can be found, still active, on the surface of the assembly after soldering. To eliminate the chance of corrosive action, these residues must be washed off from the board assemblies immediately after the soldering process.