Preparation And Property Study Of Nano-silver Paste Modified By Tin Nanoparticles For Interconnect Of Power Chips

As the operating temperature of chips used in power electronic converter continues to increase,how to ensure stable and reliable operation of the chips at high temperatures has become an important research topic.Nano-silver paste,as a new type of electronic packaging material,has several advantages,such as low sintering temperature,good electrical and thermal conductivity as well as high operation temperature.However,thus far,the existing nano-silver paste still needs higher sintering temperature（>250oC）than traditional solder,while the bonding strength and density of sintered joints are low.The main purpose of this thesis is to decrease the sintering temperature and meanwhile increase the bonding strength of nano-silver paste.Aiming at above,nano-silver paste is modified by tin nanoparticles to improve its overall performance.Firstly,silver nanoparticles and tin nanoparticles were prepared by wet-chemical method and characterized,and then the preparation process was optimized.Subsequently,the sintering process of the prepared nano-silver paste modified by tin nanoparticles（NSP-TNP）under different conditions was studied.Finally,the sintering process parameters of the NSP-TNP bonding joints are optimized to achieve high shear strength at low sintering temperature without pressure.The results show that tin nanoparticles coated by the PVP-PVSA block copolymer can be successfully synthesized in the aqueous phase by chemical reduction and stored in air for 3months,which implies that the tin nanoparticles have strong anti-oxidation ability.Tin nanoparticles with an average particle size of 23 nm and a melting point of 200oC were prepared after optimizing the concentration of the coating agent and reducing agent.When the addition amount of tin nanoparticles is less than 5 wt.%,the phase constituent of the sintered NSP-TNP is Ag-Sn solid solution without any brittle Ag₃Sn.In addition,the melting of tin nanoparticles during the sintering process makes the sintered NSP-TNP have a denser microstructure.The densification phenomenon and solid solution strengthening effect lead to significant increase in Vickers hardness of the sintered NSP-TNP.Finally,the NSP-TNP bonding joints were prepared using optimized sintering process with taking into account tin nanoparticles content,substrate roughness,sintering temperature and holding time;under the condition of sintering at 206oC for 30 min,the joints reach a shear strength of 34.9 MPa.