Study On Microstructure Evolution And Tensile Mechanical Properties Of Cu/Sn(Sn+Foam Cu)/Cu Solder Joints

The solder joints are the weak link in electronic packaging,statistics have show that70%of failures in electronic components are caused by solder joint failures.The miniaturization,intelligence and integration of electronic products have brought great convenience to people’s daily lives,but also put forward higher requirements for the reliability of the solder joints.It is urgent to develop a new generation of Sn base lead-free solder with high reliability and adaptable to the development direction of electronic products.In this paper,the Cu/（Sn+Foam Cu）/Cu structure solder joints（hereinafter referred to as Cu/（Sn+Foam Cu）/Cu joint）were prepared by using the foam Cu that is electroplated with Sn and then soaked with molten Sn as solder,for comparison,a Cu/Sn/Cu"sandwich"structure soldering joints（hereinafter referred to as Cu/Sn/Cu joint）were prepared using the same process.The microstructure evolution,tensile mechanical properties and fracture morphology of two kinds of joints were studied and analyzed at different soldering temperatures（300℃and 320℃）and different holding times（10min,30min,60min and 120min）.In addition,high temperature aging experiments were carried out on two kinds of joints,the microstructure evolution,tensile mechanical properties and fracture morphology of the two joints were compared after high temperature aging.In the soldering process,two kinds of intermetallic compounds（IMCs）,scallop-type Cu₆Sn₅ and planar-type Cu₃Sn are generated at the interface of Cu/Sn/Cu solder joint.However,due to the existence of temperature gradient,IMC at both cold and hot ends appear asymmetric growth phenomenon,the growth rate and thickness of IMC at cold and hot ends are not consistent.The thickness of the IMC layer at two ends increase significantly with the extension of holding time,the number of Cu₆Sn₅ scallop humps decrease but the volume increase,and the Cu₃Sn layer grow in a flat way.Soldering temperature can affect the growth rate of IMC at both hot and cold end.The tensile strength decreases with increasing holding time under two soldering temperature.In addition,the increase of holding time lead to the increase of IMC layer thickness,and the fracture mode of Cu/Sn/Cu joints gradually change from ductile fracture to brittle fracture under tensile load.Observing Cu/（Sn+foam Cu）/Cu joints can find,the scallop-type Cu₆Sn₅ and planar-type Cu₃Sn IMCs can be observed at the interface and around the foam Cu skeleton inside the joint.The asymmetric growth of IMC at the interface is improved because foam Cu hinder the diffusion of Cu atoms and Sn content in solder is reduce.With the extension of holding time,the number of Cu₆Sn₅ humps decrease and the volume increase,while Cu₃Sn maintain flat growth,the total thickness of IMC layer at both cold and hot ends gradually thicken.The growth rate of IMC layer at Cu/（Sn+foam Cu）/Cu joints interface is significantly lower than that at Cu/Sn/Cu joints interface.With the prolong of holding time,the tensile strength of Cu/（Sn+foam Cu）/Cu joints decrease,but the tensile strength is always higher than that of Cu/Sn/Cu joints.In addition,the tensile strength varies with different soldering temperature,the tensile strength at 320℃is higher than that at 300℃.With the increase of IMC layer thickness,the fracture mode change from ductile fracture to ductile and brittle mixed fracture under tensile load.Different from the soldering process,the growth of Cu₆Sn₅ layer evolve from scallop shape to plane shape during high temperature aging stage,and the thickness decrease with the increase of aging time,while Cu₃Sn thicken on the basis of plane shape.The total thickness of IMC layer increase with the increase of aging time,but the growth rate slow down compare with the soldering process.In addition,the tensile strength of Cu/Sn/Cu joints in the process of aging show a downward trend,but the decline is slow.The fracture mode of Cu/Sn/Cu joints in the aging stage are ductile fracture or ductile and brittle mixed fracture.The ductile fracture occurs when the aging time is 0 h,72 h and 168 h,and then the mixed fracture occurs.With the increase of aging time,the total thickness of IMC layer and the thickness of Cu₃Sn layer at both cold and hot ends of Cu/（Sn+foam Cu）/Cu joints increase,but the growth rate slow down,while the scallop-type Cu₆Sn₅ gradually flatten and its thickness gradually decrease.In addition,the growth rate of IMC layer at the cold end is slightly slower than that at the hot end,and the growth of IMC layer is mainly diffusion-controlled.The tensile strength of Cu/（Sn+foam Cu）/Cu joints decrease slowly with the increase of aging time.According to the fracture morphology,the fracture mode of Cu/（Sn+foam Cu）/Cu joints is ductile fracture or ductile and brittle mixed fracture.