Deformation induced phase rearrangement in near eutectic tin–lead alloy Original Research Article

Deformation induced microstructural changes in the near eutectic tin (Sn)–lead (Pb) alloy are investigated. This study is motivated by the thermomechanical reliability of solder materials used in microelectronic packages, with the primary objective of gaining fundamental understandings of pure mechanical effects without the influence of diffusion related phenomena. Bulk specimens, having an initial microstructure of equiaxed Pb-rich particles within the Sn-rich matrix, are subject to relatively fast deformations of tension, compression and bending. Microhardness indentation is employed to characterize the sliding behavior along grain boundaries and interphase boundaries. It is observed that phase rearrangement induced by boundary sliding commences when the overall strain is still relatively small, giving rise to the gradual development of aligned regions free of the Pb-rich phase. These Pb-free zones, or equivalently Sn grain clusters, are aligned in the direction of macroscopic normal stresses. A micromechanical model, supported by further experimental evidences, is proposed to explain the observed microstructural changes. This study also illustrates the possibility of mechanically induced phase coarsening in Sn–Pb solder alloys.