Joining Process And Properties Characterization Of High-Temperature Resistant Bondlines Based On Porous Silver Infiltrated With Indium

With the tendency of miniaturization and larger power density for power devices,the ambient temperature is increased by the heat production of power chips and poor heat dissipation conditions.SiC,considered as the next generation material for semiconductor,received a lot of attention due to advantages of the wide forbidden band gap,breakdown voltage,response speed and operation temperature.However,lacking matched die attach materials for high temperature applications?>300??is still one of bottle-neck problems for the development of SiC.In this project,a low temperature bonding process based on In infiltrating porous Ag is proposed.The bonding time was decreased by the huge specific area of porous Ag.In addition,the temperature of bonding process was decreased to 200?due to the low melting-point In,reducing the thermal stress during the bonding process.With the formation of Ag₉In₄ and?Ag?of the bonding and aging processes,respectively,the melting point of the bondline was increased to 660?and 695?,respectively.Thus,porous Ag infiltrated with In is possible to be a die attach material for next-generation SiC devices.The result of the mercury intrusion test showed that the porous Ag sheet used to bonding process is 28.03%,indicating a calculated specific area of 78.9 cm²/g,9.5times larger than conventional TLP methods.In addition,the final phase combinations were calculated to be related to the porosity of porous Ag.The dimensions relationship between pores and ligaments of porous Ag were adjusted by applying mechanical pressure.Further microstructure investigations showed that the structure of porous Ag was totally permeable.The bonding process was optimized by adjusting temperature,pressure,time,flux and substrates.When using Cu substrates,the bonding temperature could be decreased to 160?due to the low eutectic point.However,serious reliability problem occurred during the further aging tests for Cu substrates.Thus,Ag substrates were used for bonding.With a process of 200?-10 min-0.5 MPa,low meting-point In was consumed totally and the bondline was composed of Ag₉In₄ and residual porous Ag skeleton.No apparent voids were observed inside the bondline,whose thickness was 300?m with a melting point of 660?in maximum.By analyses of SEM and XRD,a mechanism of short-distance mutual diffusion in Ag-In reaction was proposed to explain the reaction process inside the bondline,which explained the inhomogeneous compositions well.For the as-reflowed bondline,the shear strength at room temperature reached 37.6 MPa with a Z type intergranular fracture.For testing the reliability in the aging process at elevated temperature,the aging tests at 300?were conducted.Furthermore,shear tests at RT and ET,Vickers hardness tests and electrical resistivity tests were applied to the as-aged samples.SEM and XRD analyses showed that the compositions of the bondline changed from Ag₉In₄ to?Ag?,increasing the melting-point and shear strength?RT?to more than695?and 49.7 MPa.With the formation of?Ag?,electrical resistivity increased,shear strength at ET and Vickers hardness decreased.Even though,shear strength of the as-aged samples reached 37.9 MPa,26.2 MPa and 13.0 MPa at 300?,400?and 500?,respectively.A lot of ductile shear bands were observed at the fracture surfaces of the as-aged samples.In addition,grain-boundary sliding features were observed at the fracture surfaces of the samples sheared at 500?,indicating a weaken grain boundary at such a high homologous temperature.