Study On Ultrasound-assisted Soldering Al And Cu Substrates With Sn-Zn-Bi-Ag Lead-free Solders

It has important engineering application value to solder aluminum and copper substrates at low temperature,but with problems,i.e.poor mechanical properties.In this paper,the soldering performance of aluminum and copper is improved by designing the chemical composition of solder alloys and optimize the process of ultrasonic-assisted soldering parameters.First,based on the cluster-plus-glue-atom（CPGA）model,the Sn-Zn based solder is optimally designed to obtain the chemical compositions of Z1,Z2 and Z3;Then,aluminum-copper dissimilar metal soldering process is optimized using ultrasonic-assisted soldering technology with Z2 solder alloys,and the dissolved amount of Al element from the aluminum substrate to the solder,the microstructure of the aluminum-copper interface and the mechanical properties of the solder joints are studied under different ultrasonic-assisted soldering parameters.The main conclusions of this paper are as follows:（1）With the addition of Ag element,the elongated Zn-rich（Al）phase in Sn-Zn based solder is obviously refined,and the AgZn₃ phase forms.Thus the oxidation resistance of the solder is improved.The addition of Bi in the Sn-Zn based solder alloys improves the wetting properties on the aluminum and copper substrates,and lowers the melting points of the solders.（2）At the interface of the aluminum side,the solder and the Al substrate are connected with the Al-Zn-Sn solid solution;while at the interface of the copper side,the solder and the Cu substrate are connected with Cu₅Zn₈ intermetallic compound（IMC）.（3）After ultrasound-assisted soldering of aluminum and copper substrates,the phase components do not change significantly at the interface of the aluminum side,but the interface irregularities increased,which is beneficial to the improve the mechanical properties of solder joints.At the interface of the copper side,the solder and the Cu substrate are connected with Cu₅Zn₈ and Al_4.2Cu_3.2Zn_0.7 IMC.The shear strength of the joint is improved for about 10 MPa.（4）With the increase of ultrasonic temperature,ultrasonic time,and ultrasonic power,the dissolved amount of Al element from the aluminum substrate to the solder increases continuously and shows a linear positive correlation（5）The optimized ultrasound-assisted soldering process parameters:300 ^oC,10 s-30 s,and 6 W-9 W.Under the optimized ultrasonic-assisted soldering process,the soldering strength is higher,the fracture position is within the soldering materials,and the maximum shear strength reaches 38.57 MPa.