Research On Process And Mechanism Of Bonding Using Nano-scale Silver Decorated RGO Paste At Low Temperature And Pressure

In order to meet the demand of high working temperature and environmental protection,the development focused on the sintering of nano-particles of third-generation semiconductor based electronic devices are thriving.Nano-particle sintered joints exibit high electrical and thermal conductivity as well as high thermal mechanical reliability even at relatively high temperature,whereas its processing temperature can be lower than 300°C.In the present study,the Ag-RGO hybrid structure was synthesised by adhering the citric acid coated nano-silver particles onto the in-situ reduced graphene oxide(RGO).The Ag-RGO hybrid was then prepared as a composite paste and the high-strength(42.5MPa)Cu-Cu joint under low pressure and low temperature was achieved.The silver nanoparticles with the smallest particle size of 5 nm coated with citric acid were prepared by chemical reduction method,which effectively prevented the oxidation and agglomeration of nano-silver-particles in air and aqueous solution.The structure of reduced graphene oxide was complete and the surface oxygen deficiencies have been effectively removed during the reduction process.The effects of temperature and the addition of RGO in the Ag-RGO hybrid structure were investigated.Its hardness and thermal diffusion coefficient both increased as the sintering process temperature rises.When the RGO mass fraction reaches 20%,the thermal diffusion coefficient of the Ag-RGO paste sintered layer reached 59.835 mm2/s.The effects of RGO addition,sintering temperature,heating rate and holding time on Ag-RGO paste were studied.When the sintering temperature reaches 300°C,the strength of the joint were up to 42.5MPa.The optimum technological parameters were as follows: RGO addition amount 10 wt.%,sintering temperature 300°C,heating rate 5°C/min and holding time 60 min.The mechanism of sintering process of nano-silver particles is divided into three main parts.The first part is the surface melting mechanism of single nano-silver particles under the influence of temperature factors.The second part is the mutual sintering mechanism of the nano-silver particles on a small scale.The whole sintering process is divided into three main stages: the initial stage,the middle stage and the final stage.The Ag-RGO hybrid structure with organic coating layer on the surface prepared is also divided into three stages: the volatilization of the coating layer,the sintering process,the expansion stage.When the temperature reached 260°C and above it,the interfacial layer appeared at the interface between the copper substrate and sintered nano-silver of the joint.EDS line scan of the interface indicates that there was no oxide layer formed on the surface of Cu substrate.Further analysis of the fracture surface of the joint indicates that the surface of Cu substrate produced an oxide layer as the temperature increases,and this oxide layer was then reduced to the submicron Cu particles by the decomposition of the citric acid coated onto nano-silver particles,where the diffusion between spherical submicron Cu particles and nano-silver particles leaded to a formation of a diffusion layer.