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During processing and qualification, packages are subjected to significant temperature changes. Thermal cycling test is commonly applied for reliability qualification to determine the ability of components and interconnects to withstand mechanical stresses induced. Thermal oxidation of metal surface, especially on copper leadframe, is unavoidable. In this study delamination between Epoxy Molding Compound (EMC) and copper leadframe is investigated using the button shear test and the X-ray Photoelectron Spectroscopy (XPS) technique. The button shear samples were subjected to thermal cycling in accordance to JEDEC standard. The shear test was conducted on the sample after the thermal cycling. A thin copper oxide film is formed on the leadframe across the EMC-leadframe interface after molding. The change in adhesion between oxide film and EMC has been investigated as a function of the thickness and the chemical content of the copper oxide by other investigators. The oxide thickness in the samples was extremely thin and did not reach the critical value reported in previous studies. The chemical composition is examined by the XPS technique after the shear test showing only cuprous oxide at the intact interfacial area, while both cuprous and cupric oxides were observed in other areas of the sample. The button shear test results showed an increase in critical shear force at the initial phase of the thermal cycling test followed by a gradual fall in value. The increase in shear force indicates that chemical in addition to mechanical factors are significant in thermal cycling. After analyzing both the button shear and XPS results, a linear relationship between the cuprous oxide content and the adhesion force is found. This leads to the observation that the change in interfacial adhesion is related to the growth of cuprous oxide content on the leadframe.