Ratcheting and creep responses of SAC solder joints under cyclic loading

To investigate the fatigue response of Pb-free Sn3.8Ag0.7Cu (SAC3807) solder, cyclic double lap shear tests consisting both loading ramp and dwell periods under isothermal conditions were performed on ball grid array (BGA) SAC3807 solder joints. Factors including test temperature, shear load amplitude and load dwell time were considered in the experiment for determining the damage acceleration effects. From the experiment it was observed that, during the cyclic shear load ramping stages, ratcheting still occurs even though the peak load is below the yielding point of solder. Transient and steady-state creep responses were also observed during the dwell stages of the cycling profile. Both ratcheting and creep responses become more significant as temperature and peak load increases. An important finding of the study is that the contribution of creep to the overall load-displacement hysteresis is more significant than the contribution of ratcheting. The corresponding inelastic energy dissipation under the cyclic double lap shear experiments were compared numerically to that of a typical wafer-level package under board-level temperature cycling (T/C). The comparison can be used for developing acceleration factors between the cyclic shear and board-level T/C tests.