Preparation Of Submicron Silver Particles And Research On Sintering Interconnection Behavior

The application of the third-generation semiconductor power electronic devices requires the interconnect materials to have good thermal conductivity and high temperature service performance.Nano silver paste is the most potential interconnecting material in the field of power electronics devices.Due to the size effect,silver nanoparticles can form a porous sintered network at a temperature significantly lower than the melting point of its bulk counterpart(961?),resulting in the sintered joint with high electrical and thermal conductivity,excellent mechanical properties and high temperature stability.On account of the decomposition and volatilization of the organic coating layer on the surface of silver nanoparticles and the volume shrinkage during the sintering process,macroscopic cracks would appear in the sintered microstructure,which would seriously affect the reliability of joints.Based on these problems,the submicron silver paste was prepared,the influence of sintering process on its sintering behavior was studied,and the influence of gas corrosion environment on the reliability of the pressureless sintered joints was investigated.Silver nitrate was successfully reduced to submicron silver particles by chemical reduction with ascorbic acid as reducing agent.The influence of the molecular weight of the dispersant poly(vinylpyrrolidone)(PVP)and the mass ratio of PVP to AgNO₃on the size and particle size distribution of the submicron silver particles was studied.When the average molecular weight of PVP is 58k and 360k,the morphology of the prepared silver particles is uniform without agglomeration.As the mass ratio of PVP to AgNO₃ increases,the average diameter of silver particles decreases,the monodispersity increases,and the particle size distribution is concentrated.When the mass ratio of PVP-58k to AgNO₃ is 1:4,the average size of the prepared submicron silver particles is 189nm,and the exothermic peak temperature ranged from 155 to190?,which can realize low-temperature sintering.The influence of sintering temperature,holding time,joint area and interface metallization type on the sintering behavior,shear strength and fracture mode of pressureless sintered joints were studied.The submicron sliver paste successfully forms qualified joints between Ni/Ag metallization DBC substrates.The increase of sintering temperature and holding time promotes the coarsening of the sintering neck and the sintered structure,and increases the shear strength of joints.The fracture of the joint shifts from the near upper interface to the near lower interface inside the sintered silver layer,and the plastic deformation of the fracture sintered microstructure is intensified.The shear strength of joints is higher than 70MPa under the sintering temperature of 250?and the holding time of more than 30 min.As the joint area increases,the shear strength of the joint decreases.The progressive microstructure evolution is observed in the sintered joints of all sizes,with the center sintered lagging and the edge sintered densification.The shear strength of the sintered joint between Ni/Au metallization DBC substrates is 29.9MPa,and the bonding between the interface of Au and sintered Ag is poor.Due to the seriously oxidization and the cracks at the interface,the shear strength of pressureless sintered joints between DBC substrates is only 9.9MPa.The influence of corrosive environment and test period on the reliability of pressureless sintered joints was studied.The corrosion of Ag sintered microstructure by neutral salt spray test is very light.After 1000 h of neutral salt spray test,the shear strength of pressureless sintered joints can be maintained.When the sulfide gas corrosion test period is 120 h,the Ag sintered microstructure exposed to the sulfide gas environment is corroded to form black Ag₂S.With the extension of the test period,the corrosion in the direction perpendicular to the joint extends along the sintering neck to the interior of the sintered silver layer,but it does not corrode horizontally to the Ag sintered microstructure inside the joint.In this case,the shear strength of the joint does not decrease.Thus,pressureless sintered joints still have high reliability in salt spray and sulfuration environments.