Green corrosion when using Rosin Flux?
In many applications where soldering is done with an excessive amount of rosin flux there appears on the copper surface a green residue similar to corrosion.
Chemical Reaction
Commercial water-white rosin consists of about 80% sylvic (or abietic) acid. The
balance is other isomeric forms of diterpene organic acids which do not enter into
the reaction of soldering. Abietic acid when heated combines directly with the
oxide on the copper surface, yielding a copper abietate. This is the green, soapy looking
material which resembles corrosion products of copper. The formation of
copper abietate is noticed readily on wire under clear Teflon for two reasons. First,
simply because the clear Teflon can be seen through. Second, because the
Teflon during soldering rapidly expands and contracts, thus trapping the rosin with
some ionizing solvent under the insulation. The abietic acid does not attack the
copper metal under any condition. This abietate formation is peculiar to copper
surfaces since the only common abietate salt is of copper.
Activated FluxesMost soldering applications require a more active flux in order to clean the surface
oxide. Activated rosin fluxes contain small quantities (0.2% to 5%) of organic
activating agents. The purpose of these activators is to catalyze the rosin copper
oxide reaction so that a better soldering job can be obtained. The catalytic agents
do not enter into the reaction and do not cause subsequent corrosion. However,
the presence of these activators does impel the rosin (abietic acid) to combine with
the copper oxide to form the green copper abietate compound. Copper abietate is
not conductive and forms a green insulating coating on copper. Usually the dark
rosin residue conceals this normal formation of green copper abietate.
Solution to the Problem
Concern develops over the green residue because copper abietate cannot readily
be distinguished from corrosion products of copper. The degree to which the flux
is activated has little bearing on the problem. The activators are still present in
sufficient quantity to trigger the copper abietate formation. The best way to
minimize the green residue is to use copper with a minimal amount of oxidation.
Reducing the amount of rosin flux used will also minimize the amount of residue