Isothermal cyclic bend fatigue test method for lead free solder joints

Isothermal three-point and four-point cyclic bend fatigue test method have been developed for Sn-Ag-Cu solder joints. Reported bend tests from the literature were conducted at room temperature (25degC) and there is lack of data for lead-free solder joints. In this study, VQFN assembly with Sn-Ag-Cu lead-free solder was tested under three-point and four-point cyclic bending load at room temperature (25degC) and high temperature (125degC). Two different board side surface finishes including Ni/Au and OSP were investigated. One daisy chain loop containing all 48 solder joints was designed for in-situ measurement of resistance and solder joint failure detection. Fatigue resistance of VQFN with OSP finish is slightly better than Ni/Au case. The fatigue failure behavior at high temperature (125degC) to room temperature (25degC) is compared. FEA modeling and simulation were performed for different testing conditions to investigate the stress strain behavior of solder joint. Volume-averaged energy density accumulation is used as a fatigue damage parameter. Energy-based fatigue models were developed for Sn-Ag-Cu lead free solder subjected to cyclic bending load at 25degC and 125degC