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12 SMT Magazine • November 2015 In this third installment of the series, we will continue discussing the likely key processes engaged in tin whisker growth. energy of Free Surface The energy of free surface plays an impor- tant part in the natural recrystallization and grain growth process. The process can be halted by grooves on the surface where grain boundar- ies meet the surface and anchor the boundar- ies to these locations. Different orientations of grains possess different surface energies, favor- ing the growth of grains that are large enough to overcome the groove anchoring effect. A tin grain structure with grain boundaries ending at the surface tends to impede the transition of a classical recrystallization to grain growth. When this occurs, stored energy must be re- leased through other growth mechanisms. An oxidized surface or a surface with ab- sorbed impurities from the atmosphere can also alter the surface energies of different crystal planes. The anisotropic properties of tin inher- ently have different surface energies of grains that are exposed at the surface. This difference in energy and the mobility of grain boundaries, or lack of, cause different paths of grain growth. In addition to the stored energy, the pro- pensity of a tin deposit to grow whiskers strong- ly depends on its structure, including surface condition, grain size, grain boundary structure and the relative crystallographic orientation of grains in the deposit. recrystallization During tin plating, energy is stored in the deposit as a mechanically stable but thermo- dynamically unstable dislocation cell struc- ture. When temperature is high enough (or increases), the state of energy becomes more unstable, driving the system to proceed into a strain-free process. This energy release process can be separated into three identifiable stages: recovery, recrystallization, and grain growth. coLuMN by Dr. Jennie S. Hwang H-TecHNologieS group SmT ProSPeCTS & PerSPeCTIveS A Look at the Theory behind Tin Whisker Phenomena, Part 3

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