We previously proposed an elastic-plastic-creep model to estimate the fatigue strength of lead-free solder joints subjected to cyclic thermal loading. The proposed model requires detailed experimental data regarding the time-independent plastic strain and the time-dependent creep strain during cyclic thermal loading. This paper proposes an experimental method for determining the characteristics of both the plastic and creep strains generated during cyclic loading that employs stepped ramp waves. This method is applied to cyclic tension-compression loadings using a Sn–3.0Ag–0.5Cu lead-free solder for several loading conditions. The method can separate between the time-independent plastic strain and the time-dependent creep strain in cyclic inelastic deformation of solder alloys.