Effect Of Welding Process On Microstructure And Mechanical Properties Of Friction Welding Joints For SAC0307/Cu Column

CGA is widely used in the field of electronic packaging because of its good thermal fatigue resistance.Traditional Cu CGA realizes reflow welding by positioning Cu column with hole mold.The hole mold reduces welding joint quality during disassembly.Therefore,our research group develops friction welding method without auxiliary mold.A micro precision drill bit is used to clamp the Cu column and the solder ball,and the Cu column is inserted at high speed to realize interconnection.In order to optimize the welding process,this paper investigated the influence of welding speed,pressure and auxiliary heating pressure on the joint quality of friction welding joints,and analyzed the microstructure and mechanical properties of welding joints under isothermal aging.Due to thermal-mechanical coupling,the internal structure of welding joints with welding rotation speed in the range of 4000 r/min～8000 r/min can be divided into Stir Zone（SZ）,thermo-affected Zone（TMAZ）and Base Material（BM）.The Cu₆Sn₅ compound layer of 0.9μm is formed on the contact interface by atomic diffusion during the friction between Cu column and solder.When the rotation speed is lower than 6000 r/min,insufficient heat input leads to partial disconnection at the interface and low average pull-out load.When the rotation speed is higher than 6000 r/min,the mechanical force is too large,leading to partial disconnection of the interface and low average pull-out load.The solder joint interface is well connected at 6000 r/min,the average pull-out load is 39.82 N,and the fly-out formation is the least.The IMC particles in SZ were fine at different speeds,and the IMC particles in BM were in a network surrounding theβ-Sn matrix.At rotational speeds 4000 r/min～5000 r/min,TMAZ presents elongated deformation structure.At rotational speeds of 6000 r/min～8000 r/min,the recrystallized grains in the elongated TMAZ were dispersed and the degree of recrystallization was low.When the rotation speed is 6000 r/min,the average indentation hardness of SZ,TMAZ and BM inside the solder joint is 145.80 MPa,183.72 MPa and 225.01 MPa respectively.In order to reduce the flashover of solder joints,a static restraint pressure shoulder is installed on the drill bit.The pressure friction welding joints formed at the optimal rotational speed of 6000 r/min have no flaying,and the average pull-out load is 50.69 N.The solder joint interfacial layer is composed of Cu₆Sn₅,and the friction metallurgical reaction is intensified by pressure.The thickness of the interfacial layer is 1.38μm.Compared with the unpressurized solder joints,the recrystallization degree of TMAZ is higher.The IMC particles in BM form a network surrounding theβ-Sn matrix,and the pressure promotes the aggregation of IMC particles into a cluster network.The average indentation hardness of SZ,TMAZ and BM inside the solder joint is 150.44 MPa,172.32 MPa and 221.43 MPa respectively.The peak temperature of the bottom interface of the pressure friction welding joint is 120℃.During the welding process,the auxiliary heating of the joint at 100℃can increase the peak temperature of the interface to 150℃and promote the atomic diffusion of the interface.The composition of the interface layer is Cu₆Sn₅ and the thickness of the interface layer is 1.61μm,which effectively reduces the thermal stress inside the welding joint.The average pull-out load is 71.61 N at the rotational speed of 6000 r/min.The IMC particles in SZ were fine,and the recrystallized grains in TMAZ were dispersed,while the IMC particles in BM were in a broken network surrounding theβ-Sn matrix.The average indentation hardness of SZ,TMAZ and BM inside the solder joint is 160.06 MPa,153.54 MPa and 197.33 MPa respectively.After aging at 120℃for 4/9/16/25/36 days at 6000 r/min,the average thickness of the interfacial layer is 3.18μm,4.73μm,5.42μm,5.88μm,7.29μm,respectively.After 4/9/16/25/36 days of aging,the average thickness of the interface layer is 2.66μm,3.39μm,3.78μm,4.65μm,6.09μm,respectively.After aging 4/9/16/25/36 days,the average thickness of interfacial layer is 2.14μm,2.73μm,3.66μm,3.95μm and 5.11μm,respectively.Solder joints under different welding processes after aging,the interface layer thickness and the aging time is directly proportional to the square root of interface layer composition were Cu₆Sn₅,district organization IMC particles are merged and coarsening grown up and pulled off the load with the increase of aging time increased after the first reduce trends,SZ indentation hardness increases,TMAZ and BM lower hardness,TMAZ minimum hardness,it is the weakest position in the internal mechanical properties of solder joints.