Effects of location and size of Kirkendall voids on mechanical response of Cu/Sn solder joint under tension

The effects of the location and size of Kirkendall voids at the nanoscale on the mechanical response of a Cu/Sn solder joint under a tension test are studied using molecular dynamics simulations. The simulation results show that the ultimate stress and ultimate strain of a solder joint with two pre-existing voids decrease with increasing void radius regardless of the distance (L2) between the centre of void 2 and the interface when the distance (L1) for void 1 is 4 nm. The elastic deformation period of the solder joint shortens with increasing void radius for void radii ≥3 nm. At the end of the tensile test, a collapse occurs at the layer interface when the two pre-existing voids are very small (e.g. void radius = 1 nm) regardless of the L2 value. For two pre-existing voids with different radii, cracks at the interface grow toward the larger void and then merge with it when the two voids are close to the interface (e.g. L1 = L2 = 4 nm). Fracture occurs at the pre-existing voids when the two voids have large radii (e.g. 4 nm) and are very close to each other.