Study On The Size Effects Of Mechanical Properties And Fracture Behavior For Microscale Lead-free Solder Joints At Low Temperature

In the process of deep space exploration,electronic without temperature control equipment have to experience the harsh environment of extreme low temperature,which seriously threatens the reliability of solder joints in aerospace electronic devices.The solder joints used for electrical conduction,signal transmission and mechanical support are considered to be the most vulnerable parts of electronic equipment.Once the solder joints fail,the equipment cannot be used normally.The reliability of microscale solder joints used for interconnection in electronic equipment is largely controlled by two factors,which is the mechanical properties of the solder matrix and the solder joints,and the evolution of the internal microstructure.Compared with the mechanical properties of solder joints at low temperature,there are few reports on the mechanical properties and failure mechanism of micro solder joints at low temperature.Therefore,if the microstructure and mechanical properties of solder joints under low temperature can be studied,it will further deepen the understanding of the damage mechanism and performance of solder joints in service cryogenic electronic equipment,so as to provide more support on effective data and theoretical guidance for research and development,design and life prediction of cryogenic electronic devices.First,size effects on tensile properties and fracture mechanism for line-type Cu/Sn3.0Ag0.5Cu(SAC305)/Cu joints were investigated by tensile test at decreasing temperature ranging from-120 to 25 ℃.The experimental results showed that the tensile behavior of solder joints was greatly influenced by temperature and solder joint size.The tensile strength of the solder joint increased with decreasing temperature.At the same temperature,the joint tensile strength increased with decreasing thickness-to-diameter ratio R value(R = t/d,1,1/2 and 1/4).In addition,at a same R,the joint with a smaller diameter had a higher tensile strength.In general,the tensile strength showed an inversely proportional function of solder volume.Moreover,as the test temperature decreasing,it was found that the fracture position of solder joints changed from inside of the solder matrix to the interface between the solder /IMC layer,presenting a ductile-to-brittle transition.The ductile-to-brittle transition temperature increased with decreasing R in the solder joints with a same diameter,and it decreased with decreasing joint diameter in the solder joints with a same R.Then,the shear performance of microscale ball grid array structure Cu/SAC305/Cu,Cu/SAC305/Ni,Ni/SAC305/Cu,and Ni/SAC305/Ni solder joints was investigated at decreasing temperatures(25 ℃,0 ℃,-25 ℃,-50 ℃,-75 ℃,-100 ℃,and-125 ℃).The results showed that the shear performance of the solder joints was influenced significantly by both the temperature and substrate materials.The shear strength of the Cu/SAC305/Cu solder joint monotonically increased with decreasing temperature,whereas the shear strength first increased and then decreased for the other solder joints.Meanwhile,when the temperature was lowered,the fracture positions of all solder joints shifted from the inside of the solder matrix to the interface between the solder/IMC layer,resulting in a change in the fracture mechanism,that is,from ductile to brittle,ductile-to-brittle transition temperature of approximately-75 ℃.Moreover,for the solder joints formed by two different Cu/Ni substrates,when the interfacial fracture occurred,it was prone to occur at the Ni substrate side.Finally,the size effects on shear properties and fracture behaviors for BGA Cu/SAC305/Cu solder joints at different heights were investigated at temperatures in the range of-125 to 25 ℃.The results show that the shear strength of BGA Cu/SAC305/Cu solder joints increased with the decrease of temperature at the same size.The shear strength of solder joint was improved with smaller solder joint height at constant temperature.In addition,the finite element results show that the change of solder joint height has little effect on the stress triaxiality,but the decrease of solder joint height will promote the uniform distribution of equivalent stress and plastic strain energy in the solder alloy.As the solder joint height increases,the strain and strain energy concentration at the solder joint and substrate will be intensified,and the solder joint will easily fracture at the interface between the solder matrix and IMC layer.Meanwhile,when the temperature decreases,the stress concentration at the interface is further intensified due to the mismatch between the thermal expansion coefficient of the SAC305 solder and the IMC layer.As a result,brittle fracture occurs at a higher temperature at the solder joint with a higher height,that is,the ductile-brittle transition temperature increases with increasing joint height.