Interface Structure Design And Bonding Characteristic Research Of Soldered Al Alloy And Sn-based Solders

The low temperature soldering Al alloys using Sn-based solders is an unsolved key technology in electronic industry,and the key scientific issue is the weak Al/Sn interfacial bonding.This paper designed and studied the interfacial structure by doping O,Zn and Ag elements,and also clarified the strengthening mechanism for the Al/Sn interface through interlayers of oxide,solid solution and intermetallic compound using first principle calculation and experiments.The soldering processes of fine-grained 7034 Al alloy and sintered Al electrode at low temperature were optimized and joints with high strength and good electric conductivity were obtained.The hybridization energy range of sp orbital electrons of Al and Sn was large,but the hybridization degree was low.The bond formed at the Al/Sn interface was mainly metallic bond,and also had covalent and ionic bond.The bonds were weak,which leads to low Wsep（work of separation）of only 1.06J/m2.The direct Al/Sn interface was obtained by vacuum pressure bonding,and the joint fractured at th e interface with shear strength of only 20 MPa.The Al/Sn interface could be strengthened through oxide interlayer forming by doping O to the interface.O atoms from the atmosphere could enter the Al/Sn interface through triple line,and then diffuse along the solid/liquid interface rapidly,and the O atoms would react with Al to form an interlayer of Al₂O₃.The electrons of Al and Sn could form strong hybridization with the electrons of O.There were both electron transfer and sharing electrons.Al and Sn formed strong ion-covalent bond with O-terminated surface of Al₂O₃,and the Wsep reached 5.17J/m2 and 2.83J/m2,respectively.When the Al alloy was soldered in the atmosphere,a transitional layer of Al₂O₃ was formed at the interface with thickness of dozens to hundreds nm,and the shear strength of the joint was raised to 40 MPa.Long ultrasonic action and holding time were applied further,and large amount of octahedral primary α-Al particles crystallized at the Al/Sn interface,which increased the interfacial bonding area and mechanical occlusal effect,and the tensile strength of joint could reach up to 63 MPa.The Al/Sn interface could be strengthened further through solid solution interlayer formed by doping Zn and O to the interface at the same time.The doped Zn formed typical metallic bond with Al,and formed metal-covalent bond with Sn.The Wsep of Zn/Al and Zn/Sn interfaces reached 1.49J/m2 and 1.45J/m2,respectively.The Al/Zn/Sn joint fractured inside the solder layer or base metal.The strength coefficient of joints was up to 100%.In addition,1060 Al was soldered using Sn Zn alloy in the atmosphere,the interfacial structure transformed to Al/Al Zn（Sn）/Al₂O₃/Sn Zn,and the Zn would segregate to the Sn Zn/Al₂O₃ interface.Zn could form strong metal-covalent bond with Al-terminated surface of Al₂O₃,and the Wsep reached 2.25J/m2.The Zn-Al bond at the interface was mainly metallic,and the interface could be also significantly strengthened.Based on the combination strengthening of Zn and O,the Sn-Zn solders were used to ultrasonic solder 1060 Al alloy.The interfacial bonding was strong and the joints fractured inside the solder layer.The strength coefficient of joints also reached to 100%.The hypoeutectic Sn-4Zn solder was used to join fine-grained 7034 Al alloy in the atmosphere.An interlayer of Al₂O₃ formed at the interface.The joint fractured through both β-Sn and Sn-Zn eutectic phases,and the tensile strength reached up to 201 MPa.Pure Zn layer was used to ultrasonic TLP bond the fine-grained 7034 Al alloy at 400°C base on that Zn and Al could form strong metallic bond.Joints with full α-Al solid solution were obtained with the ultrasonic time of 60 s and the shear strength of joints could reach 223 MPa.The Zn-Al based solders were also used to join fine-grained 7034 Al alloy at 420°C.The interfacial bonding was strong,and the eutectic phase could be reduced by increasing ultrasonic action time and adding Cu into the solder.The maximum tensile strength of joint could reach 249 MPa.The Al/Sn interface could be strengthened and electronic conductivity could be raised through intermetallic compound interlayer by doping Ag to the interface.The Ag atoms were doped to the interface by adding Ag into pure Sn.Ag and Al formed Ag2 Al compound layer at the interface,the Wsep of compound layer and Al,Sn reached 5.16J/m2 and 1.72J/m2.1060 Al alloy was soldered using Sn Ag solder.When the ultrasonic action and holding time were short,the atom ratio of Ag and Al atom was about 2:1 in the compound layer.The joint fractured at the interface of Sn Ag solder and Ag-Al compound layer,and the shear strength was raised to about 30 MPa.When the ultrasonic or holding time was prolonged,the atom ratio o f Ag and Al atom in the compound dropped to about 1.6:1.The shear strength of joint reached up to 40 MPa,and the joint also fractured at the interface of Sn Ag solder and Ag-Al compound layerThe lead-free Sn Ag solder was used to join rear sintered Al layer assisted by ultrasonic base on the strengthening of Ag to the Al/Sn interface.When the ultrasonic action time was 6s,the Al paste residual layer was dissolved and the solder bonded with Al-Si eutectic layer directly.The electrical resistivity was low and the peeling force of joint was high.The open-circuit voltage,conversion efficiency and fill factor were all higher than the solar cell soldering Ag rear electrode.