Effect Of Bi On Mechanical Properties And Interfacial Microstructure Of Sn-1.0Ag-0.5Cu Solder Joints

With the development of electronic packaging technology,the microelectronics field puts forward new requirements for the reliability of micro solder joints.The excessively thick intermetallic compounds(IMCs)formed between the lead-free solder and the pad will reduce the reliability of the solder joint.Therefore,it is necessary to effectively control the growth of IMCs.In this study,a small amount of Bi element was added to the Sn-1.0Ag-0.5Cu solder to analyze the IMC growth law of the solder/Ni substrate interface during the solid-state aging process.At the same time,the solder joints were tested for shear performance,and the influence of a small amount of Bi element on shear force is analyzed.In addition,the growth law of IMCs and mechanical properties of Cu/Sn-Ag-Cu/Cu,Cu/Sn-Ag-Cu/Cu-10Ni,Cu/Sn-Ag-Cu/Ni interconnected solder joints under the condition of long time liquid-solid reaction were studied.The research results show that a prismatic(Cu,Ni)₆Sn₅ layer and a slender rod-shaped(Ni,Cu)₃Sn₄ layer are formed between the Sn-1.0Ag-0.5Cu solder and the Ni substrate after reflowing at 250? for 60s,and(Ni,Cu)₃Sn₄ is close to the side of the substrate,while(Cu,Ni)₆Sn₅ is close to the side of the solder..The thickness of IMC increases with the extend of aging time,but with the increase of Bi content,the thickness and crystal grains of IMC decrease,and the Bi element is distributed in the solder in the form of solid solution and does not participate in the interface reaction.In addition,with the increase of Bi content,the shear force of solder joints increases.However,due to the brittleness of Bi,when the Bi content is greater than 1.5wt.%,the shear fracture mode changes from ductile fracture to ductile-brittle mixed fracture.Combined with the shear force,fracture morphology and IMC microstructure,this experiment shows that the optimal amount of Bi added is about 1.5wt.%.The growth rules and mechanical properties of the three interconnected solder joints are analyzed.The results show that the IMCs on both sides of the Cu/Sn-Ag-Cu/Cu solder joint grows symmetrically,and both sides of the IMCs are Cu₆Sn₅ and Cu₃Sn.With the extension of the liquid-solid reaction time,the growth of IMC is dominated by the growth of Cu₆Sn₅,and the fracture mode of the solder joint changes from ductile fracture to ductile-brittle mixed fracture.After the Cu/Sn-Ag-Cu/Cu-10Ni solder joint,the Cu side IMC is Cu₆Sn₅,and the Cu-10Ni side IMC is(Cu,Ni)₆Sn₅.After a long time of liquid-solid reaction,the IMC on both sides of the Cu/Sn-Ag-Cu/Cu-10Ni solder joint is(Cu,Ni)₆Sn₅,and the IMC on the Cu-10Ni side grows abnormally quickly,and there is a lot of looseness in the solder IMC.After a long period of liquid-solid reaction,the shear force of the solder joint decreases,and the solder joint fracture is located at the IMC of the Cu side interface.The fracture mode changes from ductile fracture to ductile-brittle mixed fracture,but dominated by brittle fracture.After the Cu/Sn-Ag-Cu/Ni solder joints are soldered,the Cu side IMC is Cu₆Sn₅,and the Ni side IMC is(Cu,Ni)₆Sn₅.As the liquid-solid-reaction time continues to increase,the Cu side IMC of the Cu/Sn-Ag-Cu/Ni solder joint gradually changes from Cu₆Sn₅ to(Cu,Ni)₆Sn₅,and the IMC growth rate of the Ni side is gradually greater than that of the Cu side.The fracture mode of the solder joint changes from ductile fracture to ductile-brittle mixed fracture,dominated by ductile fracture.Considering the shear force and fracture mode of the three interconnected solder joints,Cu/Sn-Ag-Cu/Ni has the best mechanical properties.When 1.5wt.%Bi is added into Sn-Ag-Cu solder,the thickness of IMC between Cu/Sn-Ag-Cu-1.5Bi/Ni interface is decreased,the growth rate of IMC is slowed down,the shear force of solder joints is increased,and the reliability of solder joints is improved.