The role of delayed elasticity and stress relaxation in Sn-Bi-Cu lead-free solders solidified under permanent magnet stirring

This paper develops methods to explore the influence of permanent magnet stirring (PMS) on solidification microstructure, thermal behavior and creep parameters of Sn-20Bi-0.4Cu alloys predicted from the primary creep stage at 25 °C. A series of short-term stress relaxation test (SRT) and strain relaxation & recovery test (SRRT) are conducted to assess the viscoelastic behavior of tested samples. A three-components consisting of elastic strain εe, delayed elastic strain εd and viscous strain εv are developed and characterized from SRRT for anticipated creep-type constitutive equation. Significant improvement of delayed elastic strain, about 46% of the elastic strain, has been detected after applying PMS. The relative contribution of dislocation mechanism with n values of 5.4–7.7 is discussed in terms of imposed strain and the role of PMS on promoting the columnar-to-equiaxed transition (CET) of lamellar β-Sn dendrites. Moreover, n value is found to increase with increasing the ratio of εd/εv and decrease with the amount of εv. The PMS also significantly decreased the undercooling, pasty range, onset temperature, while maintaining the peak temperature at the same level. These effects have good prospects for lower temperature processed electronic interconnections. [Display omitted] •SRT and SRRT tests generate a significant amount of data on material properties.•PMS leads to the formation of heterogeneous structure that affects the ductility.•The stress dependence of global strain rate was found to obey power-law.•The delayed elastic strain component was increased after applying PMS.•PMS significantly decreased the undercooling, pasty range and onset temperature.