The Bonding Interface Evolution Of Sintering Silver Paste On Bare Cu Substrate At Elevated Temperature

Nowadays,with the miniaturization,integration,and applications in harsh environments of electronics,both solder alloys?leaded or lead-free?could not meet the stringent requirements for higher temperature applications due to their low melting and operating temperatures,such as in the automotive,aviation,space,and nuclear industries.The alternative die-attachments materials,which could be used in high temperature application,have been increasingly attracted by researches.Silver paste has been reported as a promising die-attach material due to its high melting temperature?960??,low sintering temperature?275??,high electrical/thermal conductivity(4.1×10⁷ S·m^-1and 240 W·m^-1·K^-1,respectively),and high reliability.However,the application of external pressure used to increasing the sintering driving force has a drawback that it makes the implementation of automation in the manufacturing a very difficult process,thereby increasing the manufacturing cost.A wide size distribution silver paste was introduced to use for pressureless and low temperature sintering.The silver paste was characterized by DSC,Raman.The effect of processing parameters?sintering temperatures,sintering time,sintering atmosphere and thickness of sintered Ag paste?on microstructure and performance was systematically discussed.The results showed that strong joints for large area IGBT chip?>100 mm²?with high bonding strengths?higher than 20 MPa?were obtained at low temperature 270^? for 30 min using the silver paste sintering under99.99%N₂ due to the ultrahigh dense silver film?porosity is around 12%?.Meanwhile,strong metallic bond and hydrogen bond formed between bare Cu substrate and silver paste under 99.99%N₂ by preventing the copper oxidized.To evaluate the practilcle application of the nano silver paste in electronic industry,the aging conditions was chosen based on JESD22-A103-A.The sandwich strcutrue?IGBTs chip/silver paste/bare Cu?was conducted aging tests in air and coarse vacuum atmosphere?simulated vacuum sealing module?.The effect of aging conditions on thermal impedance,bonding strength and microstructure of sandwhich structure?especially for the porous sintered Ag and Cu/porous sintered Ag bond line?was studied.The results showed that the silver paste could be an alternative die-attach material for solders,whether in air or coarse vacuum atmosphere due to its low thermal resistance and reliable bonding strength.However,the bonding strength began to decrease due to the copper oxide in the bare Cu substrate/porous sintered Ag bondline.On the contrary,the bonding strength and thermal impedance was improved after aging at 250^? in coarse vacuum.Because the bondline of bare Cu and sintered Ag are still simple inter-diffusion bands,and the sintered Ag was densified after aging.The necks grow up with the disappearance of the small particles adjacent to the big ones,abnormal grain growth occured and the thickness of silver flake increased.According to the results of interface reaction?bare Cu substrate/porous sintered Ag?,microstructure of sintered silver,this paper established the microstructure evolution model of the bare Cu substrate/porous sintered silver after aging.The diffusion process of bare Cu/porous sintered Ag bondline was studied after aging at elevated temperatures.The morphology evolution mechanism of porous sintered Ag was also studied.