Preparation Of Nanosilver Paste And Its Low-temperature Pressureless Sintering

With the development of semiconductor industry,semiconductor materials with wide-bandgap like SiC have attracted more and more attention.While the application of SiC high-power devices have brought many challenges to packaging.For example,the chip should have good properties at high temperatures.In addition,excellent electrical,thermal and mechanical performance is required.Traditional joining materials like lead-free solder and conductive adhesive,however,fail to work at high temperatures.In view of these problems,in this paper nanosilver paste which can be used in the packaging of high-power devices is synthesized.And its low-temperature pressureless sintering performance is studied.Nanosilver particles capped with sodium citrate are synthesized using sodium citrate as reductant and silver nitrate as precursor.The reaction is at 100 ℃ under vigorous stirring,and as-prepared nanosilver particles are dispersed uniformly in alcoholic solvents.Flocculating agent is used to flocculate nanoparticles then nanosilver paste with ethylene glycol as solvents is obtained by centrifugation.Nanosilver particles have the particle size of 47 nm.The paste has a solid content of 73 %,and with water content of 5 %.Most of the capping agent will decomposed at 250℃.Junction temperature of the IGBT chip attached by sintered nanosilver paste is 20℃ lower than that attached by Au80Sn20 solder.Sintered nanosilver paste under 250℃ without pressure on it has a resistivity of 8.36 μΩ·cm.The influence of process conditions on porosity,grain size and perimeter of sintered network is studied.The decrease of thickness and heating rate can eliminate crack of the surface after sintering.Higher sintering temperature as well as longer holding time lead to the decrease of porosity,which promotes neck growth in order to form bigger sintered network.When sintering temperature rises from 200℃ to 300℃,porosity and network perimeter will decrease by 67.7 % and 87.9 % respectively,and grain size will increase by 322.0 %.When holding time rises from 0 min to 120 min,porosity and network perimeter will decrease by 36.5 % and 59.0 % respectively,and grain size will increase by 91.7 %.Samples with sintered nanosilver paste joining metal substrates are prepared in order to discuss the effect of heating rate,sintering temperature and holding time on shear strength of the joint.With the decrease of heating rate as well as the increase of holding time and sintering temperature,shear strength of the joint will increase.For the joint of sintered nanosilver paste joining Cu substrates with the size of 1.5 mm×1.5 mm and 10 mm×10 mm,when heated with the rate of 2℃/min to 250℃ and held for 60 min,the shear strength reaches 30.1 MPa.There are 3 fracture positions: metal coating,interface of sintered Ag layer-metal substrate and in sintered Ag layer.When sintered at 250℃ or 300℃ for 60 min,the fraction happens in Ag layer or on metal coating,indicating the joint has excellent properties under these conditions.