The present study unveils a comprehensive analysis of ultrasonic welding bonding mechanisms for multilayer copper foil combinations, notably Electrolytic Tough Pitch (ETP) copper, in pouch cell-based battery packs. Pouch cell-based battery packs utilize multilayer copper foils for their tabs, requiring a strong connection to the busbar to meet strict requirements for joint strength and electrical conductivity. Ultrasonic welding is employed for this purpose, specifically focusing on analyzing the bonding mechanisms of multilayer copper foil combinations. This study investigates the effects of key weld parameters like weld time, weld force, and weld energy on microstructure. Micrographs are analyzed to categorize welds as ‘under-welded’, ‘well-welded’, or ‘over-welded’. Light optical microscopy identifies various joint behaviors and weld defects such as swirls, interfacial waves, microbonds, interfacial gaps, voids, and delamination. A pattern in the distribution of certain joint features is noted, dividing the weld nugget into three regions: top, middle, and bottom. The top and bottom regions show signs of material mixing, while the middle region is characterized by microbond formation.