The Influence Of Metal Foam On The Microstructure Evolution And Mechanical Properties Of Solder Joints Under The Action Of Current

With the rapid development of science and technology,in order to meet the increasing needs of people in different scenarios,electronic devices are developing towards the direction of package miniaturization and high density,in which electronic components are required to have high reliability,long service life,low power consumption,light weight,low cost and other characteristics.However,conventional Cu-Sn solder joints are subject to electromigration at high current densities,which in the long term will lead to electromigration failure,resulting in significantly shortened solder joint service life.Therefore,the preparation of high-quality solder joints has become one of the most pressing issues in the field of electronic packaging.Based on the principle of solid-liquid interdiffusion of metal atoms promoted by electromigration,this paper constructs a“sandwich”structure composite solder joint and systematically investigates the effect of the addition of Ni foam and Cu foam on the microstructure and mechanical properties of the solder joint.For traditional Cu/SAC/Cu joints,intermetallic compound（IMC）generation increased significantly as current density increased and bonding time extended,grain size continued to coarsen at the same time.The Cu atoms migrated from the cathode substrate to the anode substrate under the action of the electron wind,and the IMC at the cathode and anode exhibited an asymmetric growth pattern,with the IMC thickness on the anode substrate side being significantly thicker than that at the cathode.The tensile strength of the solder joint decreased continuously with increasing current density,the fracture crack extended within the IMC interface and the fracture mode was brittle fracture.After the composite solder joint with the addition of Ni foam was bonded at 320°C,the IMC in the seam was distributed near the Ni foam and on both sides of the substrate,and its composition was mainly（Cu,Ni）₆Sn₅.When the bonding time reached 8 h,the IMC was diffusely distributed in the form of islands,the Ni foam skeleton hindered the migration of Cu atoms in the solder and no asymmetric growth of IMC occurred in the seam.In addition,with the inclusion of Ni foam in the solder,the grain size was effectively refined and homogenized,and the tensile properties of the solder joints were significantly improved,up to 76.89 MPa.However,after 8 h of bonding at a current density of 6.67×10² A/cm²,the IMC occupied almost the entire seam area,the grains were severely coarsened,the grain growth direction tended to be parallel to the current direction,the strengthening effect of the Ni foam was not sufficient to resist the weakening effect of the inherent brittleness of the IMC,so the tensile strength of the solder joint dropped sharply.The composite solder joint with the addition of Cu foam produced Cu₆Sn₅ and Cu₃Sn near the foam Cu skeleton after bonding at 280°C.At a current density of 3.33×10² A/cm²,no asymmetric growth pattern was observed for the IMC at the cathode and anode.At a current density of 6.67×10² A/cm²,the Cu foam was gradually consumed until it disappeared completely as it provided Cu atoms for the Cu-Sn reaction.The Cu atoms diffused smoothly from the cathode to the anode,and the asymmetric growth pattern of IMC reappeared.Furthermore,because of the deformation capacity of the foam Cu,the fracture mode of the solder joint was ductile fracture with improved tensile strength at a current density of 0 A/cm².However,as the current density gradually increased,the Cu foam skeleton was consumed substantially and a thicker IMC layer was generated at the solder joint interface,and the fracture mode of the solder joint gradually changed from ductile fracture to brittle fracture.Therefore,adding Cu foam to the solder at higher current densities is not effective in improving the reliability of the solder joint.