Preparation And Properties Of Flake Graphite/(Al-30Si) Composites For Electronic Packaging

With the rapid development of integrated circuits,electronic components are constantly developing in the direction of miniaturization and high performance.Electronic components have a strong demand for thermal management materials with high thermal conductivity,low thermal expansion coefficient and certain strength to improve their reliability and service life.Graphite flake has the advantages of low density,high thermal conductivity and low price,so it is suitable to be used as a reinforced material in metal matrix composites for electronic packaging.Graphite/aluminum composites have become a research hotspot of new electronic packaging materials.However,there are some problems such as dispersion and interface compatibility between graphite and aluminum,which affect the properties of the composites,and the weldability of the composites is very poor due to the special microstructure of graphite/aluminum composites.Therefore,in this paper,the reinforcement and matrix of composites are graphite and Al-30Si,respectively,the graphite is modified by electroless copper plating process to promote interfacial wettability.Gr/（Al-30Si）and Cu-Gr/（Al-30Si）composites were prepared by vacuum hot pressing sintering.Optical microscope（OM）,scanning electron microscope（SEM）,energy dispersion spetra（EDS）,X-ray diffractometer（XRD）and thermal/mechanical properties test were used to study the effects of graphite flake surface modification and different Cu-Gr mass fraction on the density,thermal conductivity,thermal expansion coefficient and bending strength of the composites.In the process of practical use of electronic packaging materials,in addition to considering their thermal and mechanical properties,weldability is one of the key factors to expand the scope of application,so it is necessary to study the weldability of the prepared composites.Then the diffusion welding was carried out between the prepared Cu-Gr/（Al-30Si）composite and the Cu substrate,and the diffusion process of the interface layer at different temperatures and holding time was analyzed,and the surface of the Cu-Gr/（Al-30Si）composite was treated with electroless copper plating to study the effect of the coating on the interface between the composite and the Cu substrate.The main results of this paper are as follows:30%Gr/（Al-30Si）composites and 30%Cu-Gr/（Al-30Si）composites were prepared under the conditions of hot pressing sintering parameters:temperature 575℃,pressure 30 MPa,holding time 1 h.Copper plating on the graphite surface increased the density of 30%Gr/（Al-30Si）composites from 91%to 93.1%.The thermal expansion coefficient of X-Y plane decreases from 13.95×10^-6/K to 11.73×10^-6/K,while the thermal expansion coefficient of Z direction decreases from 12×10^-6/K to9.81×10^-6/K.The bending strength increased from 15 MPa.However,the thermal conductivity of X-Y plane and Z direction of the composite has not been improved.Cu-Gr reinforced Al-30Si composites with different mass fraction were prepared by hot pressing sintering with the same parameters.With the increase of Cu-graphite content from 25%to 40%,the density of the composites decreased from 94.4%to91.2%.The thermal expansion coefficient of X-Y plane and Z direction show a downward trend.The thermal expansion coefficient of X-Y plane decreases from12.17×10^-6/K to 10.96×10^-6/K,Z direction from 11.03×10^-6/K to 5.59×10^-6/K.The thermal conductivity of the composite X-Y and Z direction show different trends.The thermal conductivity of the composite on the X-Y plane gradually increases from 88.9W/（m·K）to 164.6 W/（m·K）,and the thermal conductivity of the Z direction decreases gradually,from 52.3 W/（m·K）to 21.5 W/（m·K）.The bending strength of the composites is also decreases from 25 MPa to 13 MPa.The diffusion bonding of 30%Cu-Gr/（Al-30Si）composite with Cu substrate was carried out at 550℃,560℃and 570℃for 1h,respectively.The micro-morphology of the interface and the formation of intermetallic compounds were observed.There was no obvious phenomenon at the interface at 550℃.Al₄Cu₉ phase mainly forms on the side of Cu substrate at 560℃and 570℃.With the increase of temperature,the island or cellular phase gradually grows into a continuous phase,and the growth direction of the phase is mainly along the interface,and then along the vertical interface.The interface phase also gradually changes from loose to dense.When the holding time of diffusion welding is prolonged from 1h to 3h at the same temperature（570℃）,a new phase forms on the Cu side and a delamination appears at the interface,and from the Cu side to the composite side,the phase composition is Al₄Cu₉ phase,Cu Al phase, primary Cu Al₂ phase andα-（Al）+Cu Al₂ binary eutectic phase.When the holding time is extended from 3 h to 5 h,a new Cu₃Al₂ phase forms between Al₄Cu₉ phase and Cu Al phase,and with the extension of holding time,the diffusion layer of Cu Al₂ phase becomes thicker and theα-（Al）phase decreases gradually.In order to improve the interfacial wettability between Cu-Gr/（Al-30Si）composite and Cu substrate,the composite surface is treated by electroless copper plating.The Cu coating on the composite surface is well connected with Cu substrate at 570℃for 5 h,and almost no defects are produced.