Investigation of Stress Evolution Induced by Electromigration in Sn-Ag-Cu Solder Joints Based on an X-Ray Diffraction Technique
The critical current density and temperature endured by solder joints are identified as the two most important parameters that determine the occurrence of electromigration (EM). It is well known that movement of metal atoms/ions during current stressing can induce obvious stress concentrations at the anode and cathode interfaces of the solder joint. Therefore, the global average effects of EM on stress evolution at both the anode and cathode interfaces were investigated with different current densities by employing a nondestructive x-ray diffraction technique. The ultimate results indicated that the compressive stress increased gradually at the anode side under low current density. However, stress fluctuation could be observed under high current density due to an obvious Joule heating effect. For further understanding of the mechanism of stress evolution in the solder joint during current stressing, a four-stage model is considered. First, Cu expansion led to increase of the compressive stress at both electrodes. Second, stress relaxation compensated the compressive stress and caused it to decrease. Third, the EM effect promoted compressive stress and tensile stress at the anode and cathode, respectively. Finally, a steady state of stress was achieved at the anode, while the state of tensile stress at the cathode remained transient.