Abstract
To investigate the effect of external loads arising from differential thermal expansion between a substrate and a surface mount component, specimens with a simulated surface mount component (nickel) on a copper substrate having a 1 mm2 joint area and solder thickness of about 100 µm were prepared to induce extrinsic shear in joints undergoing thermomechanical fatigue (TMF) cycling. The specimens were fabricated stress free and later clamped to a copper block to cause a significant reversal in sign of the shear imposed on the solder joint during TMF cycling for 20 minutes at 150°C and 3.5 hr at -15°C. The evolution of surface damage and microstructure was examined using SEM and Orientation Imaging Microscopy (OIM). The joints were almost single crystals. However, the orientations of the tin in each joint is different, leading to different resolved stresses on a given slip system. The joint with the largest resolved shear aligned with the crystal caxis showed the most damage. Low angle tilt boundaries developed, and sliding was observed on boundaries near 7 and 14° that have a coincident site lattice. Schmid factor analysis was carried out in regions that showed ledges or grain boundary sliding. Slip on (110) planes correlated well with some of the ledges.
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