Study On Microstructure Evolution And Interconnection Properties Of Ag-Sn Composite High-temperature Resistant Solder Joints

With the rapid development of the wide band gap semiconductor high power devices such as SiC and GaN in the aerospace,energy and other industrial fields,the working environment of the corresponding devices is becoming increasingly harsh.In order to ensure the stable performance of the power devices,the soldering temperature and soldering time should also be kept within reasonable limits,in order to reduce the thermal stress in power device module.After soldering,the solder joint must also have a high melting point,high thermal conductivity,high electrical conductivity and good thermo-mechanical properties.however,the matched soldering materials are still lacking.Despite significant progress in the research of interconnection technology represented by transient liquid phase connection and low-temperature nano or micron Ag sintering,there are still some issues such as long soldering time and low density,which cannot fully meet the packaging requirements of wide band gap semiconductor power devices.While,Ag has superior electrical conductivity,thermal conductivity and oxidation resistance,and Sn is the most commonly used metal in the electronic interconnection.Therefore,the kinetic parameters of the Ag-Sn reaction were firstly calculated,and then,based on this theory,Ag@Sn,Cu@Ag@Sn and nano Ag-Sn composite particles were prepared as a new preform or paste packaging material.The composition,microstructure,interconnection properties and internal relationship of Ag-Sn packaging materials were systematically studied.The microstructure evolution and performance changes of Ag-Sn solder joints under thermal cycle（-55-200℃）and high temperature（200℃）storage were also investigated.The first phase of the Ag-Sn reaction is Ag₃Sn,which is then transformed into Ag₃Sn and Ag₄Sn at the Ag-Sn interface.Ag₃Sn is volume diffusion controlled growth in the solid-solid reaction（100-200℃,60-1500 min）,while it is grain boundary/melting channel diffusion growth in the solid-liquid reaction（250-350℃,10-240 s）.Ag₄Sn is volume diffusion controlled growth in both solid-solid and solid-liquid reactions.During the Ag-Sn solid-solid and solid-liquid reactions,large Kirkendall pores are not formed in neither Ag nor Ag-Sn IMCs,which contributed to the improvement of the density of Ag-Sn solder joints.Based on the special structure of core-shell,the reaction interface area is increased and the diffusion distance is shortened,the solder joints based on 30μm Ag@Sn preforms（the average Ag particle size is 30μm and the thickness of Sn plating layer is 1μm）with uniform microstructure can be obtained after soldering at 250℃for 5 min.The remelting temperature of the solder joints is 480℃,and the high-temperature（400℃）shear strength is 27.8 MPa.The resistivity of the solder joints is 4.34μΩ·cm,and the thermal conductivities are 231.49 W/m·K,202.07 W/m·K and 192.89 W/m·K at 30℃,150℃and 300℃,respectively.The higher density solder joints can be obtained based on Cu@Ag@Sn（the average Cu particle size is 25μm,the thickness of Ag plating layer is 0.5μm and the Sn plating layer is 1.5μm）or 5μm Ag@Sn（the average Ag particle size is 5μm,the thickness of Sn plating layer is 0.4μm）preforms than 30μm Ag@Sn preform.It is discovered that the changes of Ag₃Sn morphology（granular→layered）produced by Ag-Sn reaction will be advantageous in filling the void between numerous particles during the soldering process.Due to the increased density of the solder joints based on Cu@Ag@Sn preforms,the high temperature shear strength at 400℃increase to 31.9 MPa.The nearly full density solder joints based on 5μm Ag@Sn preforms can be obtained,in which a full Ag₃Sn structure and high-density twins inside are observed,and the high-temperature（400℃）shear strength of which reaches up to 37.2 MPa.Numerous shear bands extend throughout the fracture surface due to the plastic deformation of Ag₃Sn.The composite paste is prepared by 70-100 nm Ag-Sn particles with a 5 nm Ag₃Sn coating.The phases in the solder joints based on the composite paste after soldering are Ag_6.7Sn and Ag₄Sn.Due to the effects of the pre-melting and the latent heat of phase transformation of Ag₃Sn coating,the density,electrical conductivity,thermal conductivity,shear strength and electrochemical migration resistance of the nano Ag-Sn composite solder joints are better than those of nano Ag solder joints.After soldering at260℃for 20 min,the shear strength of nano Ag-Sn composite solder joints is 2.4 times that of nano Ag solder joints,the electrochemical migration time（leakage current of the path between electrodes reach 1 m A）of this nano Ag-Sn composite solder joints is prolonged to be 9.6 times of that of sintered nano Ag solder joints.Due to the excessive interfacial stress between Cu₆Sn₅ and Ag-Sn IMCs,it is discovered that when Cu₆Sn₅ exists in the Ag-Sn solder joint,the thermal cycling performance will be somewhat impaired.The formation of Cu₆Sn₅ can be avoided by plating a layer of Ag on the direct bonding copper（DBC）substrate.There is no crack in the typical solder joints based on 5μm Ag@Sn preform after 1500 thermal cycles,and the room temperature shear strength reaches up to 32.2 MPa.The microstructure of solder joints based on 5μm Ag@Sn and nano Ag-Sn composite paste both transform into Ag（Sn）solid solution with uniform microstructure when stored at high temperature,and their room temperature shear strength of solder joints firstly increase and then decrease with storage time.After 1200 h storage,their room temperature shear strength remains 64.5%and 58.1%of the initial value respectively.However,the room temperature shear strength of solder joints based on 25μm Cu@Ag@Sn（the Sn plating layer is 0.5μm）remains 68.3%of the initial value,due to the delayed coarsening of Ag（Sn）solid solution grains by the dispersed Cu₃Sn particles.