Cyclic bend fatigue reliability investigation for Sn-Ag-Cu solder joints

In this study, VQFN assembly and FBGA assembly with Sn-3.8Ag-0.7Cu lead-free solder and OSP board surface finish were tested under four-point cyclic bending load at high temperature (125degC). Experimental results show that the FBGA component has higher bending fatigue resistance compared to VQFN component at the same bending condition. Failure mode analysis shows that the dominant failure is solder joint fatigue cracking failure for both assemblies. FEA modeling and simulation were performed to investigate the stress-strain behavior of solder joint. Energy-based bending fatigue model has been developed based on experimental data and FEA results for VQFN assembly under high temperature bending condition. Bend fatigue model proposed in this paper for VQFN assembly was compared to energy-based fatigue model derived from bulk solder fatigue test. Bending fatigue model from VQFN specimen results in significant difference in predicting bending fatigue for FBGA assembly compared to experimental result. However, the fatigue model from bulk solder fatigue test is suitable for prediction of FBGA bending fatigue life