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During the solidification of solder joints comprised of near eutectic Sn-Ag-Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder Ball Grid Array (BGA) joints, 900 µm in diameter, are comprised of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, 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. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.
By: Donald W. Henderson, James J. Woods, Timothy A. Gosselin, Jay Bartelo, David E. King, T. M. Korhonen, M. A. Korhonen, L. P. Lehman, E. J. Cotts, Sung K. Kang, Paul A. Lauro, Da-Yuan Shih, Charles Goldsmith, Karl J. Puttlitz
Published in: Journal of Materials Research, volume 19, (no 6), pages 1608-12 in 2004
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