Ultrathin copper foils fabricated by accumulative pack rolling and surface roughening investigation

Ultrathin copper foils (thickness less than 50 μm) with controlled surface roughness are essential in modern electronics, improving performance and reliability in applications such as lithium-ion batteries and printed circuit boards (PCB). Proper surface roughness enhances resin adhesion in PCB lamination, while thin foil thickness reduces weight and increases energy density in battery collectors. Accumulative pack rolling (APR) is a novel severe plastic deformation process capable of imposing large strain to fabricate ultrathin copper foils with tunable surface roughness. Using a 0° pack arrangement, APR successfully produced ultrathin copper foils of 2.2 μm thickness, whereas a 90° pack arrangement obtained thicker copper foils (11 μm) with high surface roughness (Ra ~4.5 μm). Differences in foil thickness were attributed to through-thickness pinholes caused by increased roughness, reported here for the first time in this processing type. Surface roughness increase is linked to Cube soft orientations with weaker tensile properties. During deformation, these orientations remain or increase, becoming strain incompatible with harder orientations such as S, Brass, and Dillamore. This incompatibility leads to shear bands, acting as geometric softening mechanisms that distort lamellar structures, propagate across interfaces, and transmit strain localization into adjacent foils, resulting in out-of-plane displacement. This intrinsic deformation-induced roughening mechanism allows copper foils to achieve customizable surface properties without additional roughening treatments. These advantages position APR as a competitive method for electronic materials manufacturing. • Ultrathin copper foil (2.2 μm) achieved using accumulative pack rolling (APR). • 0° pack yields smoother surface (0.35 μm) than 90° pack (4.5 μm) with ridging. • Surface roughness of the foils influenced by strain incompatibility between hard and soft grains. • Severe pinhole formation due to through-thickness shear bands.