Cu-based leadframe sheets were oxidized by a chemical oxidation method, which formed two kinds of oxides on its surface, a brown one and a black one. The oxidation characteristics of each oxide were studied and then to measure the adhesion strength of the leadframe/EMC interface as a function of fracture toughness, the oxidized leadframe samples were molded with an epoxy molding compound (EMC) and machined to form sandwiched double-cantilever beam (SDCB) specimens. SEM and XRD studies on the surfaces of the oxidized leadframe as well as the measurement of fracture toughness showed that the interfacial fracture toughness is directly related to the formation of acicular CuO precipitates on the surface of the leadframe. However, once a continuous layer of CuO precipitates formed on the surface of the leadframe (brown oxide) or on the Cu2O layer (black oxide), the interfacial fracture toughness is found to saturate regardless of a further oxide thickening. The size of the acicular precipitates seems to have a secondary effect on the interfacial Fracture toughness.