Fatigue crack propagation behavior of Sn-Ag-Cu solder interconnects

Lead-free solders are replacing traditional lead-rich solders in the electronic industry. In the present study, the fatigue crack growth behavior of Sn-Ag-Cu solder interconnect has been investigated. An approach based on phase transformation theory and fracture mechanics was applied to predict fatigue crack propagation in a Sn-Ag-Cu interconnect, which consists of solder and intermetallic layers. Fatigue experiments were carried out on plastic ball grid array (PBGA) solder interconnects, different fatigue crack growth phases in interconnects were observed in the experiments. Displacement-controlled shear fatigue was done with a simple strain range from 0.01 to 0.1 (0.005 μm to 0.050 μm displacement) and cycle frequency of 0.1 Hz. It is found in the experiment that the majority of the interconnect lifetime is contained at the crack nucleation and early crack growth stage. Since solder alloys operate at high homologous temperatures, usually above 50% of their melting temperatures, combined creep and plasticity effects play important roles in interconnect failure and need to be considered in the analysis. A finite-element analysis was conducted to predict the required energy U to increase the crack by a unit area. Unified creep-plasticity theory was incorporated in the model to predict the creep and hysteresis effects on fatigue crack propagation in solder. The fatigue crack propagation rate in a Sn-Ag-Cu solder was predicted using phase transformation theory. Reasonable agreement between theoretical predictions and experimental results was obtained.