Abstract
The authors have proposed a parameter of "creep strain amplitude" as an alternative to the inelastic strain amplitude to estimate the fatigue life of solder alloys. To evaluate the fatigue strength of the solder joints using the creep strain amplitude, a constitutive model which can accurately predict the creep deformation under cyclic loadings is needed to conduct the FEA of the solder joints. Especially, the model must be able to describe the recovery behavior of the creep strain hardening due to the stress variation under cyclic loadings. However, such a model for solder alloys has not been proposed because of the lack of the information about the recovery behavior. Then, in this work, we investigated the behavior by conducting creep tests including stress variation process using Sn-3.0Ag-0.5Cu (SAC) solder. In the tests, the stress was maintained constant at 20 MPa for 1000 s at first (Stage I), and then the stress was decreased to a lower level and maintained constant at the level for a time period. Five kinds of stress levels were chosen for the lower stress, and five kinds of time periods were used for the stress maintenance at each stress level. After the process, the stress was increased to 20 MPa and maintained constant again for 1000 s (Stage II). The recovery behavior of the creep strain hardening was discussed by comparing the creep curve and the creep strain rate between the Stage I and the Stage II.
