Abstract: The failure process of soldered joint in SMT was investigated by electrical resistance measurement method and crack observation method.The characteristics of electrical resistance value variation of lead-tin and lead-free soldered(SAC305) joints during the thermal fatigue test were obtained.And at the same time the crack propagation in soldered joint was observed.According to these measurements, the failure rules of lead-tin and lead-free soldered joint were compared.The relationship between electrical resistance value variation and crack propagation of soldered joint during thermal fatigue test was studied by FEM, and an empirical criterion to estimate the failure of the soldered joint in the thermal fatigue test was obtained based on electrical resistance value variation.The experimental results show that the lead-free soldered joint has a higher resistibility in thermal fatigue than the traditional lead-tin soldered joint.The criterion based on electrical resistance value variation was founded from the experimental and simulation results.