Wang-Yun Li

Creep behavior of Cu/Sn-3.0Ag-0.5Cu/Cu solder joints under tensile stress coupled with DC current stressing

Creep behavior of microscale Cu/Sn-3.0Ag-0.5Cu/Cu joints with different thicknesses under electric current stressing was studied in comparison with those without current stressing. The effect of current stressing on creep mechanism was characterized by calculation of the stress exponent (n) of the steady-state creep rate and fractographic analysis of fractured joints. Results show that creep curves of solder joints under current stressing consist of three distinct stages, namely the primary, secondary and tertiary stages. With increasing current density, the steady-state creep rate increases significantly while the creep lifetime decreases. The higher the electric current density is, the higher the steady-state creep rate is and the lower the creep lifetime is. The decrease of joint thickness leads to decrease in steady-state creep rate and increase in creep lifetime under current stressing. The steady-state creep rate of solder joints with a large thickness is more sensitive to the change of the applied tensile stress than that with a small thickness. The value of the stress exponent (n) varies with the current density and joint thickness. Decrease in joint thickness brings about the change of the fracture location from the middle of the solder matrix to the transition region between the solder/Cu6Sn5 interface and solder matrix, and correspondingly the fracture mode tends to transform from ductile to a mixed ductile-brittle mode of fracture.

The influence of imposed electric current on the tensile fracture behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu solder joints

The fracture behavior of microscale lead-free Sn-3.0Ag-0.5Cu solder joints under electrotensile load was characterized, in comparison with those under pure tensile load. Experimental results show that under electrotensile load the stress-strain and strain-time curves of joints exhibit three distinct stages, i.e., the fast deformation stage at the beginning of loading, linear deformation stage and the accelerating fracture stage. No significant difference in strain feature between the electrotensile loaded joint and pure tensile loaded joint was observed. The solder joints under electrotensile loading and tensile loading exhibit the same fracture mechanism at the same thickness-to-diameter ratio of joints, while the fracture strength of solder joints under electrotensile load is decreased greatly compared with that under pure tensile load, even lower than that of the bulk solder. Moreover, the orientation of β-Sn grains may tend to rearrange along the direction of current stressing under electrotensile loading.

Size effect on creep deformation and fracture behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu solder joints

Creep of solder joints usually takes place in serving electronics due to the high homologous temperature of lead-free solders. With scaling down the dimension of electronics, the size of solder joints shrinks sharply, which may influence significantly the mechanical performance of solder joints, including creep behavior. In this study, the creep behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints with a diameter of 300 μm and decreasing thicknesses of 200, 100 and 50 μm was investigated. Results show that the typical “three-stage” feature of the creep curve remains regardless of the change in solder joint thickness. On the other hand, there is an obvious size effect on creep deformation and fracture behavior of solder joints, i.e., when the joint thickness decreases, the steady-state creep rate decreases and the creep lifetime increases, and the fracture mode changes from ductile to the mixed mode of ductile and brittle. However, the creep mechanism is independent of joint thickness. The creep stress exponents are 4.2, 3.8 and 3.1 for solder joints with thickness of 200, 100 and 50 μm respectively, and the corresponding creep activation energies are 90.0, 62.0 and 66.0 kJ/mol respectively, indicating that the creep behavior of all the solder joints is dominated by lattice diffusion mechanism.

Abnormal creep behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints with different joint thicknesses under electro-thermo-mechanical coupled loads

Creep deformation and fracture behavior of solder joints with a constant diameter (300 µm) and the decreasing joint thickness (from 300 to 25 µm) under electro-thermo-mechanical coupled loads were characterized using a dynamic mechanical analyzer assembled with a DC power source. The results show that the feature of creep curves of solder joints remains unchanged with the decreasing joint thickness, which consists of the primary, secondary and tertiary stages. The steady-state creep rate increases with increasing stress level and temperature, while hardly presenting a monotonical decrease with the decreasing joint thickness, compared with that of joints without current stressing. When the joint thickness decreases from 300 to 25 µm, the steady-state creep rate first decreases and then increases with an alternate change feature, showing an abnormal creep behavior. Moreover, fracture position shifts from the solder matrix to the interface between the solder matrix and interfacial Cu6Sn5 layer in solder joints with thicknesses of 300 and 200 µm when the temperature is increased, while the creep fracture occurs preferentially only at the solder/Cu6Sn5 interface of all the solder joints with thicknesses of 150, 100, 75, 50 and 25 µm. Both the abnormal variation of the steady-state creep rate and the fracture behavior are influenced by Joule heating, constraint effect, electromigration, electromigration-induced back stress, grain size of interfacial Cu6Sn5 layer, the number and orientation of grains in the solder matrix.

Creep deformation and fracture behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints under electro-thermo-mechanical coupled loads

The effect of electric current density on the creep deformation and fracture behavior of micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu joints under electro-thermo-mechanical coupled loads was investigated in this study. Results show that all solder joints exhibit similar creep curves displaying typical three-stage creep characteristics regardless of the current density, which suggests that mechanical stress is the dominant factor controlling creep process of solder joints under electro-thermo-mechanical coupled loads. Moreover, the steady-state creep rate increases and the creep lifetime decreases with increasing current density, testing temperature and mechanical tensile stress. However, the creep stress exponent and activation energy do not show obvious change with the increase of current density, meaning that the creep mechanism of solder joints remains under electro-thermo-mechanical coupled loads with different current densities. The creep mechanism is determined to be dominated by lattice diffusion. Furthermore, the fracture position in solder joints changes from the solder matrix to the IMC/solder interface with the increase of current density, temperature and mechanical tensile stress.

Size effect on the interfacial reactions and microstructural evolution of Cu/Sn3.0Ag0.5Cu-ball/Sn3.0Ag0.5Cu-paste/Cu joints in flip-chip on BGA packaging

Size effect of solder balls on the interfacial reaction and microstructural evolution of BGA structure Cu/Sn3.0Ag0.5Cu-ball/Sn3.0Ag0.5Cu-paste/Cu joints during isothermal aging at 125 °C was systematically investigated. Results show that a large amount of bulk Cu6Sn5 phase distributes in the solder matrix of joints with large solder ball size, resulting from larger outflux Cu atoms from the interface to the molten solder and the low solubility of Cu in the solder matrix. The solder ball size has a significant influence on the interfacial Cu6Sn5 layer thickness at the Sn3.0Ag0.5Cu-ball/Cu interface, which increases with decreasing solder ball size, while showing less effect on that at the Sn3.0Ag0.5Cu-paste/Cu interface. The grain size of Ag3Sn phase in joints decreases with decreasing solder ball size. During isothermal aging, the growth of interfacial IMC layers at both Sn3.0Ag0.5Cu-ball/Cu and Sn3.0Ag0.5Cu-paste/Cu interfaces of joints is mainly controlled by bulk diffusion.

Interfacial reactions and formation of intermetallic compound of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu joints in flip-chip on BGA packaging

The interfacial reactions and formation of intermetallic compound of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu joints in flip-chip on BGA packaging were studied by using a differential scanning calorimeter. Results show that a thin circular Cu₆Sn₅ layer forms first on the surface of Cu substrate due to the diffusion of Sn atoms dissolved in the soldering flux, subsequently the planar-like Cu₆Sn₅ layer forms and covers over the first layer, and many Ag₃Sn particles form in the grain boundaries of Cu₆Sn₅. Then, a slight increase of the soldering temperature from 217°C to 218°C has a significant influence on the morphologies of the interfacial Cu₆Sn₅ and Ag₃Sn, that is, the morphologies of Cu₆Sn₅ and Ag₃Sn are changed into scallop-like and large plate-like, respectively. When the soldering temperature is increased to 221°C, a large amount of plate-like Ag₃Sn phase dissolves into the molten solder and the small network-like Ag₃Sn phase forms, and the grain size of Cu₆Sn₅ is increased. Furthermore, there is almost no trace of Ag₃Sn at the interface layer of Cu₆Sn₅ at the soldering temperature of 227°C. The morphology changes of the interfacial Cu₆Sn₅ and Ag₃Sn are attributed to the increase of interfacial energy and the eutectic reaction between Sn and Ag in the solder matrix, respectively.