Preparation And Sintering Mechanism Of Nano Metal Paste For Power Electronic Packaging

With the development of electronic device towards miniaturization,high integration and multi-function,the power density is gradually increasing,which brings more serious challenges to chip packaging and system cooling.Due to the wide-band-gap,hightemperature resistance,and high energy conversion efficiency,the third-generation semiconductor materials are widely used in aerospace,new energy vehicles,transportation,photovoltaic,and 5G communication fields.Traditional Sn-based solders possess low melting point and poor thermal conductivity,which cannot meet the requirements of heat dissipation and high temperature service for high-power devices.Therefore,it is urgent to develop high temperature resistant and thermal conductivity die-attach materials.Due to the excellent electric and thermal conductivity and small size effect,nano-metal paste can be prepared as high-temperature resistant bonding joints at low temperature to satisfy the requirement of efficient heat dissipation for high-power devices.However,high porosity and easy electromigration in sintered-Ag result in reduction of the bonding reliability and cause circuit failure.Compared with nano-Ag paste,nano-Cu paste not only has excellent electrothermal performance,but also possesses resistance to electromigration and costeffectiveness.However,the spontaneous oxidation of nano-Cu in air increases the difficulty of practical application.This work focuses on the process optimization and novel material fabrication in view of the shortcomings of the nano bonding paste.Nano-metal sintering technology for power device packaging was studied.The bonding performance and the sintering mechanism of nanoparticles were deeply investigated.The main research achievements are depicted as follows:1)Based on the issues of high porosity and poor reliability of sintered-Ag,the Snassisted low-temperature bonding technology of nano-Ag was proposed,which combined with transient-liquid-phase bonding to obtain low porosity and high strength Ag-Sn bonding joints.The reliability strengthening mechanism of Ag-Sn bonding joint was explained according to the microstructure evolution.The porosity of Ag-Sn joint was reduced after aging at 200°C for 2000 h,and the shear strength of the joint increased from 23.7 MPa to59.1 MPa,higher than the nano-Ag joint with 8.0 MPa.The Sn diffused to the Cu substrate to form intermetallic compounds during the aging process,which hinder the formation of oxide layers and improve the high-temperature stability of the bonding structure.2)In order to resolve the issues of easy oxidation in nano-Cu and poor sintering performance in micro-Cu,the oxidation-reduction bonding method was proposed to in-situ reduce nano-Cu on the surface of micro-Cu.The micro-nano core-shell Cu paste was obtained and successfully applied to low temperature Cu-Cu bonding.Micro-nano coreshell Cu paste was prepared from micro-Cu powder after thermal oxidation and formic acidethylene glycol reduction.The shear strength of the bonding joint is 23.7 MPa after sintering at 300°C.The nanoparticles can significantly reduce the diffusion activation energy,which contributes to improve the low-temperature sintering performance of micro-Cu.3)A novel method for developing Cu-Ag composite nanoparticles through polyol onestep reduction was proposed.The Cu-Ag composite nanoparticles with average size of 9 nm demonstrate good dispersibility,excellent sintering performance and oxidation resistance.The composite joint porosity is 1.92%,the resistivity is 8.74 μΩ·cm,and the shear strength of the bonded joint is 32.6 MPa under the ratio of Cu to Ag is 1:3.In addition,the preparation process of micro-nano core-shell Cu-Ag particles was optimized through the densest stacking model to improve the oxidation resistance and fabricate the low porosity and high strength bonding joints.4)The low-temperature sintering mechanism of nano-metal particles was investigated by molecular dynamics simulation.Two-ball sintering models of nano-Cu,nano-Ag,Cu-Ag composite and Cu-Ag core-shell were established.The effect mechanism of particle diameter,sintering temperature,atoms ratio of Cu and Ag,and shell thickness of Ag on sintering performance was studied.The molecular dynamics results indicate that small-size particles are easier to sintering under low temperature because of the high surface activation energy and large sintering driving force.At the same particle size,Cu-Ag composite system possesses better sintering performance than pure metal system.Moreover,the Ag shell is beneficial to prevent the diffusion of Cu atoms and effectively inhibit the spontaneous oxidation of Cu nanoparticles.