Sine Cu foil is (reasonably) isotropic and etching is normally done
at +/- 1g the etching process is fairly isotropic so the etchant
undercuts at a similar rate to etching down. A 1oz (~35um) foil will
therfore undercut by >=35um (on each side) so the feature size could
be 70um (3thou/mil) down on the artwork dimension.

David Whalley