A novel preplated leadframe (PPF), i.e. Sn-PPF, is developed by electrodepositing a 3∼4nm thick Sn layer between the Cu base leadframe and the Ni plating. The nano-thick intermetallic compound (IMC) layer forms automatically through interdiffusion and reaction between Cu, Ni and Sn. High resolution transmission electron microscopy (HRTEM) observation reveals crystallography relationship between the IMC and Cu base. A drastic reduction of Cu out-diffusion is confirmed by an EDX based oxidation test carried, indicating a significant enhancement in the protection of the leadframe against oxidation and corrosion attack. The Cu diffusion coefficients in the IMC layer were estimated to be about at 250°C and m2/s at 300°C. Results of the wire pull test, the solderability test and the bending test exhibit Sn-PPF's quality with a Ni layer less than 200nm satisfying industrial requirements.