Preparation Of Cu-Ag Core-shell Nanoparticles For Low Temperature Sintering Of Power Chip

With the emergence of wide band gap(WBG)materials,electronic devices are required to work under high temperature,high frequency and high power conditions.Traditional Snbased interconnect materials are difficult to adapt to service requirements such as power cycling and temperature cycling due to the growth of intermetallic compound layers at the interface.This paper has developed a core-shell structure silver-coated copper powder that can be used in low-temperature sintering technology for electronic packaging.After it is formulated into solder paste,the nano-effect of metal nano-materials is used to achieve lowtemperature sintering interconnection to meet the service requirements of power devices.The Cu-Ag core-shell solder paste maintains the excellent electrical and mechanical properties of the silver paste and reduces the industrial cost.Compared with the copper paste,it has excellent oxidation resistance and is suitable for low-temperature sintering interconnection of electronic packages.In this paper,the Cu-Ag core-shell nanoparticles were prepared by the compound method of replacement reaction and chemical reduction reaction.During the preparation process,fully covered Cu-Ag core-shell nanoparticles were obtained by changing the types,amount of chemical reagents,and reaction time.The changes of copper nanoparticles during electroless silver plating were studied,and the nucleation and growth mechanism of silver atoms during was analyzed.The interconnection experiments between bare copper substrates were carried out under the thermo-compression sintering by using the Cu-Ag core-shell paste.The effects of different substrate roughness,applied pressure,and sintering temperature on the reliability of the interconnection structure were investigated.The results show that the particle size distribution of the Cu-Ag core-shell nanoparticles was 3 nm to 19 nm,and the average diameter was 8 nm.HRTEM was used to characterize the special core-shell structure.Compared with pure copper nanoparticles,Cu-Ag core-shell nanoparticles have excellent oxidation resistance due to the coating of silver atoms on its surface.Investigation of the changes of copper nanoparticles during electroless silver plating revealed that silver atoms were gradually deposited on the surface of copper nanoparticles in a layer-island mixed growth mode after heterogeneous nucleation,and eventually forming fully covered Cu-Ag core-shell nanoparticles.The Cu-Ag core-shell nanoparticles after thinning the surface organic dispersant were formulated into a solder paste,which can form a dense sintered structure under a certain thermo-compression sintering.On this basis,it is found that the sintered microstructure of the Cu-Ag core-shell paste on the rough substrate was denser than that of the smooth substrate,and has high shear strength.The results were related to the contact area between nanoparticles and substrate and mechanical occlusion.Observing the microstructure of the interconnect joints obtained at different sintering temperatures,it is found that increasing the temperature would cause the copper nanoparticles coated with silver atoms to turn into bare copper nanoparticles,resulting in oxidation of the copper nanoparticles,which was not conducive to atomic diffusion process.The crosssection morphology of the joint revealed that the fracture occurred at the interface,indicating that the reason for the low joint strength was the atomic diffusion reaction between the nanoparticles and the substrate.