Interconnection Behavior And Reliability Of Low Temperature Sintering Nano-silver Joints

With the continuous innovation of microelectronic technology and urgent requirements for industrial development,power electronic devices are developing towards the trend of high freaquency,high integration and high intelligence.Especially in some specific areas,the power electronic devices are supposed to serve under the environment of high temperature,strong radiation and high current density.Great challenges are imposed on the packaging technology of power chips in the devices.Low temperature sintering Ag technology is the most promising solution of chip packaging for high temperature application.Due to the size effect,metallurgical bonding and densification among nano/micro Ag particles occur at a temperature much lower than the melting point of bulk Ag while the sintered Ag exhibited similar physical properties with its bulk counterpart,such as high melting point,high thermal and electrical conductivity and high m echanical strength.Although the study on the preparation of low temperature sintering Ag paste and the fabrication of sintered Ag joints has achieved great success,it is significant to deeply understand the physical properties of Ag sintered bulk and its interconnection joints,especially the mechanical property at different service periods.Hence,the sintered Ag joints were investigated and the dependence of mechanical property on the microstructure at different service periods was studied in details.In this dissertation,a thermo-compression sintering method with gradient pressure and one-step forming was proposed,by which the Ag nanoparticle（NP）sintered bulk was prepared with no crack and defect.The microstructure and mechanical property of the Ag NP sintered bulk were investigated.In the respect of microstructure,the density of sintered bulk is mainly determined by sintering pressure,the size distribution and shape of pores are affected by sintering pressure and temperature and the coarsening behaviour is controlled by sintering temperature.In the repect of mechanical property,the Young’s modulus is determined by density and the yield strength and elongation are affected by porosity and intergranular microstructure,while the size distribution and shape of pores,grain size and ligament size have a only second order of the importance.In addition,the mechanical property of Ag NP sintered bulk,Ag microflake+nanoparticle（MF+NP）sintered bulk and Ag MF sintered bulk were compared and discussed.The application of sintering Ag technology is restricted by the long processing time.In this paper,the Ag sintered joints were prepared by a two-step method including low temperature drying and isothermal thermo-compression sintering.The microstructure and shear strength of joints,the bonding mechanisms between Ag NPs and Cu substrate and the mechanical reliability during thermal shock test were studied.The results show that the mean shear strength of Ag NP sintered joints is161.7 MPa when the sintering parameters are set as 225℃,5 MPa and 10 min.The superior mechanical strength is contributed to the low porosity and fine grain strengthening.Ag/Cu₂O/Cu and Ag/Cu interfaces formed between Ag NPs and Cu substrate during the sintering process.Besides,the Ag MF+NP sintered joints exhibited excellent structural stability during the thermal shock test,which was attributed to low coefficient of thermal expansion and Young’s modulus.The oxidation issue seriously threatens the reliability of sintered Ag joints on Cu substrate.First,the mechanical reliability of Ag NP sintered joints was evaluated during high temperature storage test.Then,the local transient liquid phase bonding was induced to improve the high temperature stability of Ag NP sintered joints.The results show that the hermetic microstructure forms in the sintered Ag,which guarantees long-term service at 200℃and short-term service at 300℃in air.The main reasons for the strength degradation of the aged joints are the growth of Cu oxide and the formation of loose microstructure due to nonequal interdiffusion of Ag and Cu atoms.The composite joints exhibited superior high temperature resistance.Ag-Sn intermetallic compounds（IMCs）formed at the edges of sintered joints that could inhibit the penetration of oxygen and the microstructural coarsening behaviour of sintered Ag.Cu-Sn and Ag-Sn IMCs formed at the bottom of the joints simultaneously.This IMC barrier hindered the diffusion of Cu atoms towards the sintered Ag layer and the formation of Cu oxides effectively.A high-temperature（300℃）storage test in air demonstrated that the shear strength of this kind of joints maintained above 60 MPa for 1000 h.