Improved flatness in cold rolling of copper foils

Abstract

Cold-rolled copper foils in thicknesses between 10 μm–100 μm have many ranges of applications, as for example in the electronic industries for flexible printed circuits boards (PCB) and for cable wrapping and shielding. A good flatness of the foils is important, especially for the PCB-foils which are surfaced after rolling. Normal shape defects, such as centre buckles and long edges, are today detected efficiently by commercial shape meters. Flatness is controlled by, for example, work-roll bending and thermal crowning of the rolls. However, in the rolling of copper foils a special kind of out-of-flatness occurs, which is impossible to detect and control by normal systems for shape control. This kind of defect is characterized by an evenly distributed pattern of small buckles. In the present work it is supposed that this phenomenon is related to a variation in forward slip along the barrel of the roll. This in turn is due to various friction conditions at different locations along the contact between the rolls and the foil. According to a theoretical model advanced by the authors, the variation in forward slip can be decreased for certain combinations of entry- and exit-tensions. The applicability of the model that has been developed for improving the flatness has been confirmed in industrial production.