Eric J. Cotts

Abstract: The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.

TL;DR: In this article, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size.

TL;DR: In this article, the cyclic growth twinning of Sn grains during solidification from the melt was observed in Sn-Ag, SAC, and Sn-Cu solders.

TL;DR: In this article, the microstructure of the Sn-Ag-Cu solder was examined by optical microscopy and scanning electron microscopy (SEM) for various compositions near the ternary eutectic for different cooling rates from the solder melt.

TL;DR: In this article, the effects of alloy composition variations and cooling rates on microstructural stability and deformation processes were investigated in a pure-tin ingot and a reflowed sample.