Connection Behavior And Microstructure Evolution Of SAC 305 / Tinned Copper Pillar Of Embedded Friction Welding

In order to get rid of the influence of molds in the process of Copper Column Grid Array（CuCGA）packaging,our research group proposed a non-mold assisted friction embedded welding method to connect copper columns.The method produces heat by rotating friction and plastic deformation between the welding column and the solder,so that the solder reaches a viscoplastic state and flows around the welding column,and then forms a welded joint after cooling.On this basis,homogenization welding design of welding pillar and solder was proposed,which solved the problems of poor friction welding performance and low joint strength caused by the heterogeneous connection of pure copper welding pillar and solder.In this paper,the homogenization of embedding friction welding is studied in depth.By comparing the microstructure and mechanical properties of the solder joints,the most suitable tin immersion parameters are selected,and the effect of 120℃isothermal aging on the homogenized solder joints is analyzed.At the same time,the flow behavior of in the solder joints before and after homogenization is examined.After tin immersion at 260°C and 280°C for different time,the surface of the coating is smooth and bright white,without scabs and holes.After bending test,it has good bending processability.The thickness of IMC layer and pure tin layer after tin-dipping was measured.The results showed that the thickness of IMC was proportional to the temperature and time of tin-dipping.The thickness of pure tin layer is inversely proportional to the tin dipping temperature and proportional to the tin dipping time.The composition of IMC layer is Cu₆Sn₅.The pull-off load of solder joints with different tin immersion parameters was measured.The average pull-off load of solder joints with 30 s,60 s,90 s and 120 s at 260°C was 53.05 N,65.01 N,72.63 N and 60.5N,respectively.At 280°C,the average pull-out load of solder joints of 30 s,60 s,90 s is 79.03 N,68.5 N,62.82 N.Compared with pure copper solder joints,the average pull-off load of tin-immersed solder joints can be increased by up to 46.53 N.Hot-dip tin can significantly improve the friction welding weldability of copper pillar/solder.In order to obtain higher solder joint strength,the most suitable IMC thickness after tin dipping should be about 2.3μm.The microstructure of the solder joint starts from the interface between the Cu column and the solder,which can be divided into three regions:Stir Zone（SZ）,Thermo-mechanically affected Zone（TMAZ）and Base Material（BM）.The average width of SZ is about 100μm,the average width of TMAZ is about 55μm,and BM is155μm away from the welding column.Compared with pure copper solder joints,the range of SZ is expanded by 25%,the range of TMAZ structure is reduced by 45%,but the overall range of SZ and TMAZ structure is reduced by 14%.The heating rate of the solder joint of the tin-immersed solder column is also higher during the welding process.The temperature of the solder joint is measured at a position of 1 mm from the top of the solder ball and 0.5 mm from the solder column.The peak temperature can reach 97.9°C,which is 12.9°C higher than that of the pure copper solder joint.After isothermal aging at 120°C for 9 days,16 days,25 days and 36 days,the thickness of IMC layer is 3.37μm,4.76μm,6.26μm and 8.44μm.Compared with Cu column/SAC305 solder joint,the growth rate of IMC layer becomes slower.With the extension of aging time,the IMC particles in different regions are aggregated and grow up,and theβ-Sn matrix of TMAZ begins to change from elongated to massive.The average pull-out load is 82.83 N,87.69 N,75.46 N and 67.96 N after aging for 9 days,16 days,25 days and 36 days.A Sn58Bi/SAC305 half-stacked composite solder ball was designed,and the radial flow behavior of the solder during the welding process was studied by using the solder column to form the solder joint.The circumferential flow areas of Cu pillar/SAC305 solder joints at 0.6 mm,1.2 mm and 1.8 mm from the top of solder ball are557703.327μm²,651720.676μm² and 540335.936μm²,respectively.The circumferential flow area of tin-immersed Cu pillar/SAC305 solder joint at 0.6 mm,1.2 mm and 1.8 mm from the top of solder ball is 672521.926μm²,551036.884μm²and 526623.023μm².A Sn58Bi/SAC305 axial superimposed solder ball was designed,and the axial flow behavior of the solder during the welding process was studied by using the solder column to form the solder joint.The maximum axial upward migration distances of tin-immersed Cu pillar/SAC305 solder joints at 0.6 mm,1.2 mm and 1.8mm from the top of solder balls are 379μm,336μm and 302μm,respectively.It can be seen that hot-dip tin can increase the temperature of the friction interface of solder joints,improve the bonding strength of solder joints and change the flow range of solder.