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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 important part in the natural recrystallization and grain growth process. The process can be halted by grooves on the surface where grain boundaries meet the surface and anchor the boundaries to these locations. Different orientations of grains possess different surface energies, favoring 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 released through other growth mechanisms.
An oxidized surface or a surface with absorbed impurities from the atmosphere can also alter the surface energies of different crystal planes. The anisotropic properties of tin inherently 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 propensity of a tin deposit to grow whiskers strongly depends on its structure, including surface condition, grain size, grain boundary structure and the relative crystallographic orientation of grains in the deposit.
During tin plating, energy is stored in the deposit as a mechanically stable but thermodynamically unstable dislocation cell structure. 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.
Together, the three stages of the process describe the formation of a new microstructure of lower free energy in the solid state. Recovery stage, sensitive to point defects, reduces lattice strain but does not involve any microstructural change. In contrast to recovery, the ability to make structural changes that occur in recrystallization by decreasing dislocation density generates a new set of fine strain-free grains.
Recrystallized grains are often the result of preexisting regions that are highly misoriented in relation to the material surrounding them. This high degree of misorientation offers the needed growth mobility for the region from which the new grains originate. If the temperature is higher than that required for recrystallization, grain growth continues. Fundamentals of physical metallurgy indicates that, as discussed in Part 2, the nucleation and growth of stress-free grains comprises four steps:
- Nucleation: incubation time
- Growth of nuclei: high rate growth of new grains
- Impingement of grains: with limited space, some nuclei at some point get to touch each other, which prevents subsequent growth
- Conventional growth: when new grains fill all the volume, the process of classical growth starts, and the remainder follows the conventional growth rate (the rate is proportional to the square root of time). Its driving force is to decrease interfacial energy
The second step of grain growth—the growth of nuclei—proceeds until the driving force for this process diminishes and at that point the recrystallization is complete.
Editor's Note: This article originally appeared in the November 2015 issue of SMT Magazine.