Study On The Joining Mechanisms And Mechanical Properties Of Cu/Sn/Cu Joints By The Ultrasonic-Assisted TLP Soldering

In recent years,power electronic devices are developed continuously toward small size,high power,and high frequency.The energy density per unit area of the chip is getting higher and higher,resulting in a sharp rise of its operating temperature.In addition,in the automotive,aerospace,nuclear power,and other fields,some power devices have operating temperatures as high as 350°C to 500°C.The traditional connection method using low melting point lead-free solder has been unable to meet the high-temperature reliability requirements of high-power electronic devices.Therefore,the development of chip bonding technology which can form high melting point connection joints has become one of the most important research directions in the field of packaging technology.Due to the fact that the Transient Liquid Phase（TLP）soldering process can form high-melting IMCs joints at low temperatures,which attracts cause a high degree of attention,but a long process time brings reliability problems to the device.Therefore,how to prepare reliable IMCs joints quickly and efficiently is one of the urgent problems to be solved.In this paper,IMCs joints were fabricated by ultrasonic-assisted TLP soldering in a short time（10s）.The growth characteristics and evolution law of Cu-Sn IMCs,fast connection mechanism and ultrasonic grain refinement mechanism during the forming process of IMCs joints were described in detail by combining experimental and theoretical analysis.The mechanical properties of IMCs joints were evaluated in detail.The microstructure stability of Cu₃Sn joints after high temperature aging was also studied.Through the study of effects of ultrasonic amplitude on the interface reaction,it was found that only when the ultrasonic amplitude exceeds 5μm,strong cavitation would occur in the liquid Sn solder,and cavitation corrosion would occur in the Cu substrate.Cu-Sn IMCs showed"non-interface"growth characteristics under the action of ultrasonic waves,i.e.Cu₆Sn₅ or Cu3Sn nucleated and grew in the liquid solder.The ultrasonic action in the liquid soldering filler metal would become more significant with the increase of ultrasonic amplitude,the decrease of connection gap and the increase of soldering temperature,which would shorten the time to finally obtain the full Cu3Sn joint.The ultrasonic wave produced acoustic cavitation and streaming in the liquid solder,which caused a large amount of Cu to be stripped from the substrate and rapidly transferred and dissolved into the liquid solder.At the same time,cavitation caused the liquid Sn solder to be in a supersaturated state of Cu,so that Cu-Sn IMCs could be rapidly formed in the cooling process.Cu₆Sn₅ and Cu3Sn grains were refined under the action of ultrasonic waves,showing equiaxed shapes of different sizes.The larger the ultrasonic amplitude was,the higher the grain refinement degree and the smaller the size of Cu-Sn IMCs grains were.Cu particles were stripped from the substrate under acoustic cavitation and then fully dispersed into the liquid solder under acoustic streaming.Due to the dissolution of Cu in the liquid solder,some of the Cu₆Sn₅ crystal nuclei woul also be precipitated and fully dispersed in the liquid solder under the acoustic streaming,both undissolved Cu particles and precipitated crystal nuclei would become nucleation particles in the cooling stage,and the dispersed nucleation particles would refine the final crystal grains.Through the analysis of the mechanical properties of the Cu-Sn IMCs joints,it was found that,the Cu₆Sn₅ layer containing a limited number of grains formed by the the conventional TLP soldering process exhibited uneven mechanical properties.Under the action of ultrasonic waves,Cu₆Sn₅ layer was composed of refined grains and exhibited more uniform mechanical properties.Through the shear test of joints,it was found that the IMCs joints under the action of ultrasonic wave had higher shear strength,and the ultrasonic wave changed the fracture mode of the joint by changing the grain shape and size.Through the high temperature aging treatment of the Cu₃Sn joint,it was found that above 350°C,after a certain holding time,the transformation of Cu₃Sn to Cu₄₁Sn₁₁ occurred in the joint.The higher the temperature was,the faster the transition was;the analysis showed that Cu₄₁Sn₁₁ was formed by solid phase reaction of 11Cu₃Sn+8Cu→Cu₄₁Sn₁₁ at 350°C or higher.The reaction was mainly dominated by the diffusion of Cu atom.The activation energy of the reaction was calculated to be 94.3 KJ/mol.Through mechanical properties tests,it was found that the microhardness and shear strength of the Cu₄₁Sn₁₁ joint could reach 451.5 HV and 65MPa.During the heat preservation process at 340°C,the coarsening behavior of Cu₃Sn grain was studied and the coarsening regulation was found.With the prolongation of holding time,Cu₃Sn grain would go through a rapid growth period and then tend to be stable.At the same time,it was found that the aging treatment could reduce the microhardness and shear strength of Cu₃Sn joints.Cu₃Sn joints maintained a high level in high-temperature shear strength.