Evolution Of Microstructure And Mechanical Properties Of Friction Welding Joint Of SAC0307/Cu Column

The Column Grid Array（CGA）with less internal stress is popular in the field of large-size electronics packaging.The traditional Cooper Column Grid Array（CuCGA）needs additional auxiliary devices to achieve interconnection.The research group has developed a new CuCGA interconnection method without auxiliary positioning device.CuCGA solder joint was formed between Cu column and solder ball by micro bench drill.In this paper,CuCGA solder joints are prepared by SAC0307 solder ball,0.8mm diameter pure Cu column and PCB with 2.4 mm diameter Cu disk.In this paper,the friction welding spot is reflow treated by simulating the form of double side welding.The reliability of friction solder joints was studied by using low temperature aging to simulate the product under low temperature aging environment.The effect of high cost Ag content on welding spot was studied.The friction welding spot of SAC305/Cu column was prepared for comparative analysis.Under the action of thermal coupling,the friction welding spot of SAC0307/Cu column is divided into three areas:Stir zone（SZ）is about 0-120μm from the interface;Thermo-mechanically affected zone（TMAZ）is approximately 120-220μm from the interface;Base material（BM）is located about 220μm away from the interface.A Cu-Sn diffusion layer with an average thickness of about 1.01μm is generated at the solder joint interface.The average tension load of the solder joint is32.21 N.The hardness of the average indent hardness of each area inside the solder joint is higher the farther it is from the Cu column.SZ is 155.65 MPa,TMAZ is172.76 MPa and BM is 206.51 MPa.High Ag content will improve the indentation hardness inside the solder joint,but it has little influence on the solder joint strength.The average indentrification hardness of SZ,TMAZ and BM inside the friction welding spot of SAC305/Cu column is 195.65 MPa,240.94 MPa and 260.21 MPa,and the average tension load is 31.62 N.After reflow,the internal structure of friction welding spot of SAC0307/Cu column grows.Cu₆Sn₅layers with an average thickness of about 2.12μm were generated at the solder joint interface,and the average pull-off load increased to104.82 N.The mean indent hardness of SZ,TMAZ and BM inside the solder joint is188.91 MPa,202.49 MPa and 216.08 MPa.The friction welding spots of SAC0307/Cu column were aged for 25 days at75℃and 90℃,and the composition of the interface IMC layer was Cu₆Sn₅,without the generation of Cu₃Sn.The average thickness of IMC layer at 75℃for 25 days is about 7.53μm.The average pull-off load increased to 70.03 N at 16 days of aging and decreased to 66.72 N at 25 days of aging.The average thickness of IMC layer is about 9.98μm when aging at 90℃for 25 days.The average tension load increased to74.54 N at 16 days of aging,and decreased to 71.31 N at 25 days of aging.After aging,the internal structure of friction welding spot of SAC0307/Cu column grew up,and the density of IMC particles onβ-Sn matrix decreased.Due to the low aging temperature,there is no great change in SZ microstructure.No reticular structure was observed in BM tissue.This is because Ag content in the solder is relatively low,the network distribution inside the solder joint is sparse,the number of particles is small and the size is large,the grain density becomes further sparse during aging,and it is difficult to show a clear network structure.After aging,the hardness of SZ inside the solder joint is improved.The hardness of TMAZ is increased in the early aging stage,while the hardness of BM is lower than that of the unaging stage.After aging at 90℃,the friction welding spot of SAC305/Cu column is thinner than the interface IMC of SAC0307/Cu column.The average thickness of the interface IMC layer at 25 days of aging is about 3.02μm.Ag content has little influence on the connection strength of aging solder joints.The average tension load increases to 72.35 N at 16 days of aging,and decreases to 70.44 N at 25 days of aging.After aging,coarsening also took place in the internal structure of the solder joint,but the distribution of reticular particles was still maintained in BM structure.Under the same aging condition,the hardness of each area inside the welding spot is higher than that of friction welding spot of SAC0307/Cu column.