Grain boundary sliding on near-7°, 14°, and 22° special boundaries during thermomechanical cycling in surface-mount lead-free solder joint specimens

To investigate the effect of external loads arising from differential thermal expansion between a substrate and a surface-mount component during thermomechanical cycling, specimens with a nickel surface-mount component on a copper substrate were prepared. Specimens consisted of two 100 μm thick 1 mm2 solder joints about 9 mm apart, with two designs. In one specimen (denoted “dual-shear”), the as-fabricated joints were not stressed due to differential contraction during solidification and cool down. In the other specimen (denoted “component”), a continuous copper substrate between the joints caused the nickel component to be put in compression during cool down, which imposed shear on the joints. To impose differential thermal shear strains, the “dual-shear” specimen was clamped to a copper block to cause a significant reversal in sign of the shear imposed on the solder joint during cycling. In the “component” specimen configuration, the existing compressive strain in the component increased with cooling, but the stress was reduced with heating. Both joints were polished on the side, and then subjected to thermomechanical cycling for 3.5 h at −15 °C and 20 min at 150 °C. The effect of the two strain histories on microstructural evolution and heterogeneous straining was compared using scanning electron microscopy and orientation imaging microscopy. Each joint was made up of one to three very large grains containing low-angle boundaries and embedded minority orientations that evolved with cycling. The “component” specimen exhibited less heterogeneous strain than the “dual-shear” specimen that had reversed shear stresses. Since the dominant tin orientation was different in each joint, the resolved shear stress on various slip systems differed. In both specimens, the joint with the largest resolved shear parallel to the c-axis showed the greatest amount of heterogeneous strain arising primarily from sliding on [1 1 0] tilt boundaries misoriented by 7°, 14°, or about 20°. These boundary misorientations are close to misorientations with a coincident site lattice geometry.