Microstructurally Induced Evolution of Creep Strain in SnAgCu (SAC) Solder Joints During Thermal Cycling

SAC solder joints contain few anisotropic viscoplastic grains and each grain contains pro-eutectic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\beta$</tex>-Tin dendrites surrounded by a Tin-Silver eutectic region. Grains are separated by viscoplastic grain boundaries. Over the past decade, crystal viscoplasticity (CV) models developed by this group have considered deformation mechanisms at each length scale due to dislocation creep, to determine viscoplastic deformation of single grains. The effect of the evolution of the microstructure on creep behavior of solder grains has also been captured using a combination of multi-scale-CV modeling and finite element grain-scale modeling. Studies conducted by multiple researchers indicate that the evolution of solder joint microstructure takes place during thermal cycling in two ways: (i) ripening of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Ag}-\text{Sn}$</tex> intermetallic structure in the eutectic region and breakdown of dendritic structure in the pro-eutectic region within each grain, due to higher diffusion rates at elevated temperature; and (ii) localized recrystallization of larger grains to form finer grain structure in regions that see a combination of cyclic stress and elevated temperature. Effect of factor (i) above on creep behavior of solder has been previously presented by the authors. The current paper presents simulation of localized recrystallization of single crystal solder joint into finer grains. Progressive thermal cycling, causes progressive increase in the region of recrystallization, which reduces the overall creep resistance of oligocrystalline solder joints. Reduction in the creep resistance causes progressive reduction in the fatigue resistance of solder joint.