Stress Release
Mechanical changes in the dimensional stability of copper cladding occur after the etching process removes the structural support of the metal from the dielectric substrate. This copper foil relaxation results in a slight movement of the remaining features as the internal stresses from the original manufacturing of the foil dissipate. Such movements must be predicted to ensure layer to layer alignment.
Material Behavior
Electrolytic copper is deposited onto a drum under tension which creates inherent strain within the crystalline structure of the metal. Once the fabricator etches away large areas of copper, the copper foil relaxation allows the underlying laminate to shift or shrink. This effect is particularly pronounced in thin core materials used for multilayer boards where the copper provides a significant portion of the structural rigidity.
Precise scaling adjustments in the CAM software attempt to neutralize these shifts.
Registration Boundary
Verification of these dimensional shifts happens during the post etch punch or lamination prep stage. If the copper foil relaxation exceeds the predicted tolerances, the registration targets will not align with the subsequent layers. This phenomenon is a property of the material interaction and is not a result of thermal expansion during the soldering process.
