In the field of electronics, it is crucial to guarantee long-term joint reliability of the Sn-Ag-Cu solder alloy. Creep tests in low-stresses are necessary to guarantee the reliability. However, a very long period of time is required for the creep tests. To decide a steady-state creep strain rate, an approximate equation of a creep curve was newly applied to the helical spring creep test known as a method of a low-stress creep test. Obtained results are as follows. (1) The torsional strain component is decreased with the deformation of the helical spring shaped specimen. Therefore, the steady-state creep rate cannot be detected in the helical spring creep test. The approximate equation of a creep curve introduced by Li's group was found to be effective to obtain the steady-state creep rate for Sn-based solder alloy. (2) The helical spring creep testing method that uses the torsional deformation of the test specimen is more effective for the Sn-based solder alloy with a strong anisotropy of strength than the uniaxis creep testing method. (3) In the heat-treated Sn-3.0 mass%Ag-0.5 mass%Cu solder alloy, the creep stress exponent changed from 19 into 1.1 on the boundary of about 14 MPa. This result implies that the creep deformation mechanism was changed from the dislocation creep with dispersion-strengthening to the grain boundary sliding. This phenomenon was found for the first time by using the helical spring creep testing method.
