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A low temperature wafer-to-wafer bonding technology for 3D packaging/integration based on Cu-Sn isothermal solidification (IS) technology is introduced in this paper. The fluxless bonding technique using Cu-Sn multilayer composites to produce higher re-melting temperature bonding layer is presented. The structure of the intermediate multilayers and bonding patterns are designed, and the bonding process is optimized. The microstructure of bonding layer was investigated by SEM (Scanning Electronic Microscopy) and EDS (Energy Dispersive X-Ray Spectrometer). The compositions of the bonding layer show that there are intermetallic compounds (IMCs) with higher melting points. The bonding layers consist of Cu6Sn5 and Cu3Sn phases. High strength of bonding layer has been detected, with average shear strength of 37.5MPa.