This thesis delves into the bonding mechanisms of multilayer copper foil combinations, specifically Electrolytic Tough Pitch (ETP) copper, utilizing Ultrasonic Metal Welding (USMW). The study explores the influence of key weld parameters such as weld time, weldforce, and weld energy, on microstructure.The experimental phase involves the design of a welding setup, with multiple samples subjected to varying parameters based on preliminary weld quality. Specific sets of samples representing different layers of copper foils are selected for detailed analysis using light optical and digital microscopes. Micrographs are scrutinized to classify welds into three categories: 'under-weld,' 'good-weld,' and 'over-weld.' Defects such as delamination, voids, and unbonded foils characterize 'under-weld,' while fractured foils and severe thinning define 'over-weld.' Samples displaying continuous bonding with minimal defects are categorized as 'good-weld. 'Results from micrographs reveal distinct classifications for different layer configurations. Further insight into microstructures is gained using an etchant (Ferric Chloride, FeCl3), aiding in visualizing bonding mechanisms and fine grains formed during the welding process. Light optical microscopy identifies various joint behaviors and weld defects. Notably, swirls and wave-like features on foils in direct contact with the sonotrode and anvil are observed, attributed to plastic deformation induced by weld energy and elevated temperatures. Increased weld parameters result in convoluted waves and plastic deformation propagating through successive foil layers, facilitating material mixing and interfacial wave generation. Additional bonding mechanisms include microbonds and grain formation. Under-welded samples exhibit intermittent gaps, voids, and delamination as observed weld defects.This research provides valuable insights into the intricacies of USMW for ETP copper foils, contributing to a better understanding of bonding mechanisms and potential applications invarious industries.