Semi-solid Compression Brazing Of Particle Reinforced Metal Matrix Composites For Electronic Packaging

AlSiC composite with high volume fraction has many advantages,such as high specific strength,high specific stiffness,high thermal conductivity and adjustable coefficient of thermal expansion.It is an ideal electronic packaging material.At present,there is no mature joining technology for AlSiC composite,which leads to great limitations in practical engineering applications.Therefore,the key to the large-scale application of AlSiC composite is to develop the joining technology for AlSiC homogeneous and heterogeneous materials.Based on the existing joining technology at home and abroad,aiming at the characteristics of AlSiC composite materials,a new semi-solid compression brazing technology of homogeneous and heterogeneous materials between AlSiC and AlSi is proposed in this paper.By designing the composition of the filler metal and studying the microstructure evolution and compression deformation behavior of the filler metal,interaction between the filler metal and base metal in the brazing process,the disruption mechanism of the oxide film on the surface of the base metal and the metallurgical bonding mechanism of the joint interface systematically,the optimal process parameters and methods were finally obtained,and the feasibility of the technology was verified.Two kinds of Zn-Al-Cu filler metals with as-annealed and as-rolled states were designed and prepared.By means of semi-solid isothermal holding,the transformation process of spherical microstructure of the two kinds of filler metals during brazing was studied,and the evolution mechanism of spherical microstructure was clarified.The effects of initial state of the filler metal,heating temperature and holding time on spherical microstructure were studied.It is found that compared with as-annealed filler,the spherical microstructure of as-rolled filler after spheriozation is finer and more uniform,and the sphericity degree of the solid grains in the filler metal is higher.Moreover,the shape factor of the solid grains is the highest at 392℃,20min and the solid fraction is 63%.The deformation behavior of the filler metal in semi-solid state was studied by hot compression experiment.Through the analysis of the strain-stress curve and microstructures after compression,the change law of semi-solid deformation behavior of the filler was obtained.For as-rolled filler metal,the compression deformation is dominated by sliding between solid particles mechanism at 382℃.With the increase of temperature,the initial deformation of the filler is mainly dominated by flow of liquid incorporating solid particles or liquid flow mechanism,and then transformed to sliding between solid particles mechanism.For annealed filler metal,it is difficult to deform due to the intercrossing and interlacing of dendrites,which leads to the decrease of strain.In the process of compression deformation,the interface between filler metal and base metal is a mixture of compression-extrusion and friction-shear.The solid grain fraction,solid grain shape factor,heating temperature,holding time and initial state of filler metal are the main factors affecting the two effects.Semi-solid compression brazing of AlSiC/AlSiC were studied by the systematic process tests and the metallurgical joining of AlSiC/AlSiC was realized.Compared with as-annealed filler metal,as-rolled filler has stronger solid grain sliding ability,obvious friction shear effect on the base metal surface during brazing,so the oxide film removal is better and the joint strength is higher.At 392℃,10MPa,there is no oxide film on the interface.The shear strength of the joint reaches 110MPa,which is about 82%of the strength of AlSiC base metal.With further increasing brazing temperature,the solid fraction decreases which causes the removal of the oxide film and the strength of the joint to decrease.In addition,with increasing volume fraction of SiC particles in AlSiC,the proportion of Al alloy matrix and oxide film on the surface of the base metal decreases,which cause the difficulty of the oxide film breaking to decrease and the breaking effect to increase.Aiming at the problem that the oxide film removal is more sensitive to brazing temperature,an optimized semi-solid compression brazing is proposed.Based on the study of the deformation behavior of filler metal during the continuous heating process,the disruption mechanism of the oxide film on the base metal surface under the optimized process was revealed.The solid grains in the filler metal gradually spheroidize with increasing temperature,and the process of aggregation and depolymerization are repeated continuously,resulting in strong compression and friction effects on the base metal surface,so that the oxide film will be broken gradually.When a large amount of liquid appears,the liquid phase produces a certain scouring effect on the base metal surface,which further peels off the broken oxide film.Under the optimized process,the shear strength of the joint first increases with the increase of temperature and then tends to be stable.After 392℃,the joint strength basically remains at about 100MPa.Semi-solid compression brazing of AlSi/AlSi were studied by the systematic process tests.It is found that the continuous oxide films on the surface of AlSi base metal and filler metal are difficult to remove effectively when AlSi/AlSi brazing is directly conducted by semi-solid compression method.However,when semi-solid compression brazing assisted by SiC particles is adopted,the oxide film on the surface can be broken successfully by the extrusion of SiC particles,and the reliable joining of AlSi/AlSi can be realized.When SiC particles size is 1μm and the concentration increases to 3g/m~2,the shear strength of the joint reaches 68MPa.As SiC concentration further increases to 4g/m~2,SiC particle colonies with non-wetting areas of filler metal form at the interface,resulting in a decrease in the bonding strength.For 5μm SiC particles,SiC colonies forms at a high SiC concentration.The clearances among particles are large enough to provide a highly effective infiltration of the filler metal and thus good interface bonding is still obtained at the high SiC concentration.At 16g/m~2,the bonding strength reaches the maximum value of 92MPa,and fracture occurs at the interface between SiC particle layer and the filler.High residual stress at the SiC particle layer/filler metal interface is responsible for the fracture.The bonding of AlSiC/AlSi was realized by SiC particle assisted semi-solid compression brazing.The joint strength is up to 80MPa,which is significantly higher than that of the joint without SiC particles.SiC particle assisted semi-solid compression brazing of AlSiC socket and AlSi layer was carried out.The joint has a high bonding rate,but the air tightness cannot meet the requirement of the electronic packaging.Semi-solid compression brazing of AlSiC water-cooled plate was carried out and the joint well meet the water tightness requirements of the electronic packaging.