Processing And Characterization Of Pressureless Low-Temperature Sintered Silver Bonding On Nickel Metallization

The low temperature sintered silver bonding has been increasingly used for dieattach technology in power electronics packaging for high temperature,high frequency,and high power density applications,thanks to its superior melting point,thermal conductivity,electrical conductivity,and reliability.Nickel-plating,as a common metallization on substrate,baseplate,and print-circuit board,has high corrosion resistance and low cost.However,to the date,there is lacking reports in strong sintered silver bonding on nickel metallization.The weak bonding is due to the nickel oxidation during sintering at high temperature,almost completely immiscibility and low interdiffusivity between silver and nickel,it is difficult to form sintered-silver and nickel joints with strong bonding.It will have great significance if we can achieve strong die-attach on low-cost nickel metallization by pressureless silver sintering in air.Therefore,the factors affecting the bonding strength of sintered silver-nickel joints,such as the densification of sintered-silver paste,the surface roughness,chemical compositions,crystal structure,and oxidation mechanism of nickel-plated substrates,the solid-solution and interdiffusion between silver and nickel,and the pressureless sintering profile(sintering temperature and holding time)were investigated in this study.It is demonstrated that the pressureless sintered silver die-attach on nickel substrate at270? for 10 min has high bonding strength of over 40 MPa and excellent thermal and electrical performance.First,compared with the micron silver paste,the trimodal hybrid silver paste comprising of micron-size,submicron-size,and nano-size silver particles has the higher initial particles packing density and silver solid loading,which can enhance the bonding strength from increased sintering densification.When using the same pressureless sintering profile,the porosity and bonding strength of sintered-trimodal silver paste decreases by 2% and increases by 16 MPa than that of sintered-micron silver paste,respectively.Second,a pressureless sintering profile and bonding mechanism of sintered-silver bonding on nickel was investigated in this study.The effects of sintering temperature and holding time on the sintered-silver microstructures,interdiffusivity between silver and nickel,nickel oxidation during sintering,and bonding strength was discussed in detail.In addition,the effects of surface roughness,chemistry compositions,crystal structure,and oxidation conditions of electroless-plated nickel and electro-plated nickel substrate on bonding strength of sintered-silver joints were analyzed and compared.It is found out that the electroless-plated Ni(P)has higher resistance to oxidation thanks to its amorphous structure,whereas the electro-plated Ni is more easily oxidized due to its crystal structure.As a result,the bonding strength of sinteredsilver joints on electroless-plated Ni(P)is 15?25 MPa higher than that on electroplated Ni substrate.Third,the thermal and electrical performance of pressureless sintered silver dieattach on nickel-plated substrate was characterized.Compared with pressureless sintered silver die-attach on gold-plated substrate and the FEA simulation results,the sintered silver die-attach on nickel-plated substrate has excellent transient and steadystate thermal,and static and dynamic electrical performance.The results prove the feasibility of pressureless silver sintering on nickel metallization.