Effect Of SACBN Interface Layer On Microstructure And Mechanical Properties Of SnBi/Cu Solder Joints

With the continuous improvement of society's awareness of environmental protection,Many countries and regions have promulgated regulations prohibiting the use of lead in electronic products,such as the WEEE Directive and the Restriction of Hazardous Substances Directive.Therefore,a large number of traditional Sn-Pb solders have been replaced by lead-free solders.At present,our country has developed many types of lead-free solders,such as Sn-Ag,Sn-Cu,Sn-Bi,Sn-Zn,Sn-Ag-Cu and other solders.Among them,SnBi solder is the most widely used in the electronic packaging industry.Because it has the advantages of low cost,good wettability,low melting temperature,good tensile strength and resistance to degeneration.However,during the packaging process,a dense Bi-rich phase is formed in the SnBi eutectic alloy.It has been proven that this microstructure is hard and brittle.This brittleness will cause the ductility of the SnBi solder to be insufficient for mechanical shock or drop.Under the conditions,brittle fracture occurs,which reduces the reliability and integrity of the solder joint.In order to reduce the hardness and brittleness of traditional solder joints Sn58Bi/Cu,this paper uses a two-step soldering process to add Sn0.7Ag0.5Cu3.5Bi0.05N(SACBN)interface layer to Sn58Bi/Cu to obtain SnBi/SACBN/Cu with overlapping structure Compound solder joints.First,a comparative analysis of the significant influence of the addition of the SACBN interface layer on the formation and growth of?-Sn crystal grains in the SnBi bulk was analyzed.Compared with the SnBi/Cu joint,the SnBi bulk in the composite joint shows larger?-Sn dendritic crystal grains.Meanwhile,the interface IMC layer changes from Cu₆Sn₅ to(Cu,Ni)₆Sn₅.The addition of the SACBN interface layer improves the shear strength of the SnBi/Cu solder joints.The interface layer can act as a barrier to crack propagation,so that the failure mode of the SnBi/SACBN/Cu composite solder joint becomes a mixed failure mode with both brittle fracture and plastic fracture.In addition,the addition of the SACBN interface layer has shown a positive effect in reducing the hardness of the SnBi/Cu solder joints.Subsequently,the SnBi/SACBN/Cu composite solder joint was subjected to isothermal aging treatment,the structure of the composite solder joint after the treatment was significantly coarsened,and the volume of the SACBN interface layer was significantly reduced.Moreover,the shear failure mode of composite solder joints changed from the original mixed fracture to brittle fracture.Although the plastic deformation capacity and hardness of the SnBi/SACBN/Cu composite solder joints after aging are reduced,the shear strength and bulk solder hardness of the SnBi/SACBN/Cu composite solder joints in the three solder joints shows the highest stability before and after aging.Finally,changing the soldering time of the second step to make SnBi/SACBN/Cu composite solder joints with different soldering times.The results show that as the soldering time increases,the interface IMC layer gradually becomes thicker,and the hardness shows a downward trend.When the soldering time is 40 s and 160 s,the SACBN interface layer suppresses the propagation of fracture in the SnBi bulk and improves the shear strength of the solder joints.As the soldering time reaches 280 s,the shear strength of the SnBi/SACBN/Cu solder joints decreases,and the fracture is mainly located in the SnBi bulk.