Shear And Fatigue Performance Of Cu/Sn-3.0Ag-0.5Cu/Cu Joints With Different Height-to-Diameter Ratios Under Electro-Thermo-Mechanical Coupled Loads

With the development of the times,electronic equipment are closely connected with people’s daily lives,and the reliability of products has become a research focus in the electronics industry.Solder joints play the role of electrical connection so that electrical signals can be transmitted between circuit components and electronic devices.Hence,the reliability of solder joints has been a concern throughout the electronic packaging field.The service environment is one of the main factors affecting the reliability of solder joints.At present,one of the main service environments of solder joints is the electro-thermal-mechanical(ETM)coupled load.In addition,warpage in electronic packaging will lead to concave and convex deformation,resulting in different joint heights with same solder volume.Therefore,if the shear behavior and fatigue behavior of solder joints with the same solder volume and different heights under the electro-thermal-mechanical coupled field can be studied and related to the mechanical behavior of solder joints in the package,it will further deepen the understanding of the damage mechanism and performance of solder joints in electronic equipment under the electro-thermal-mechanical coupled field.More efficient data support and theory foundation are provided for the structure design,performance assessment and life prediction of electronic products.First,the shear performance and fracture behavior of microscale ball grid array(BGA)structure Cu/Sn–3.0Ag–0.5Cu/Cu joints with same solder volume and different heights at increasing current density were systematically investigated by experimental characterization,theoretical analysis and finite element simulation.The results showed that the shear strength of the solder joint decreased with increasing current density,while it increased with decreasing joint height.These changes were mainly due to Joule heating,the non-thermal effect of current stressing and mechanical constraint in the solder joint.As current density increased,both Joule heating and the non-thermal effect of current stressing aggravated solder joint shear strength more and more severely.At the same current density,Joule heating’s deterioration on the shear strength was less serious in the smaller height joint,and the non-thermal effect was insensitive to the change in joint height.The higher shear strength of the smaller height joint was due to the larger mechanical constraint in the solder matrix induced by the substrate.Moreover,as current density increased,the fracture position changed from the solder matrix to the solder/IMC layer interface.The fracture mode shifted from ductile one to ductile-brittle mixed one,and the critical current density corresponding to the fracture mode transition increased with decreasing joint height.The above findings indicate that the BGA structure solder joint with a smaller joint height is more reliable.Then,the low cycle fatigue behavior,fracture performance and lifetime prediction model of ball grid array(BGA)structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different shear amplitudes and different joints heights at increasing current density were described by experimental characterization,theoretical investigation,and numerical simulation.The experimental results showed that the fatigue lifetime of solder joints decreased with increasing shear amplitude and current density.As current density increased,the fracture position changed from the fracture in the solder matrix to the partial fracture at the solder/IMC layer interface and then to the whole fracture at the solder/IMC layer interface.And,the shape of the fracture path shifted from arc-shape to flat-shape.In addition,the fracture model changed from ductile fracture to ductile-brittle mixed fracture and then to brittle fracture mode.This change was mainly due to the strain mismatch at the solder/IMC layer interface aggravated sharply.Moreover,a fatigue lifetime prediction model considering non-thermal effect of current based on plastic strain was proposed by finite element simulation.Finally,the warpage deformation of the package at the board level was simulated numerically,and the model loaded with fatigue loads after warpage deformation was studied.It was found that the shapes of solder joints were different from the deformation field after warpage of board-level package.The solder joints in the middle has higher height due of solder joints were elongated.The solder joints of the four vertices has lower height due of solder joints were compressed.This indicates that the warpage leads to the phenomenon of the same volume and different height in solder joints.Then,the dangerous solder joints of the package structure under fatigue loads were evaluated by von Mises stress,plastic strain and plastic strain energy.The results show that the solder joints at the diagonal ends are higher in both stress and strain and strain energy,which means that they are more likely to fail.In addition,due to the interconnection of the solder joints,the plastic strain of the dangerous solder joints in the board-level structure was much smaller than that of a single solder joint under the same fatigue load,resulting in a much higher fatigue life compared to a single solder joint.