The role of delayed elasticity and stress relaxation in Sn Bi Cu lead free solders solidified under permanent magnet stirring
Abstract 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 degC. 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 ee, delayed elastic strain ed and viscous strain ev 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 b Sn dendrites. Moreover, n value is found to increase with increasing the ratio of ed/ev and decrease with the amount of ev. 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.