The Theory Behind Tin Whisker Phenomena, Part 4

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In this fourth installment of the series, we will continue discussing the likely key processes engaged in tin whisker growth. These key processes include:

  • Grain boundary movement and grain growth
  • Energy dynamic of free surface
  • Role of recrystallization
  • Solubility and grain growth in response to external temperature
  • Lattice vs. grain boundary diffusion
  • Reaction and dynamic of intermetallic compounds
  • Crystal structure and defects

In Parts 2 and 3, we have discussed the first five processes: grain boundary movement and grain growth; energy dynamic of free surface; solubility and grain growth in response to external temperature; and the role of recrystallization. Now, we will outline the next two processes—lattice vs. grain boundary diffusion, and reaction and dynamic of intermetallic compounds.

Lattice vs. Grain Boundary Diffusion

Diffusion is a key part of crystal growth process; and whisker phenomenon is primarily a diffusion-controlled process. Since diffusion is a necessary path to grow whiskers, slowing the rate of diffusion of tin intra-granularly or along grain boundaries should be an effective approach to slow down the whisker growth.

The actual monitoring or controlling the processes between the lattice diffusion and grain boundary diffusion is a complex challenge. Nonetheless, understanding the main factors between these two diffusion processes will help navigate the practical solutions.

Diffusion through tin matrix is expected to be impeded by alloying element. One vivid example is SnPb. This binary system comprises two phases—a Pb-rich phase and Sn-rich phase. The fact that SnPb alloy alleviates tin whisker propensity largely attributes to that Pb slows the diffusion of Sn in the matrix and allows for rapid stress relaxation, which in turn, reduces or prevents continuing whisker growth.

Diffusion rates are also grain boundary (g.b.) sensitive; the greater the number of g.b., the faster the diffusion rate is. For a grain boundary rate-determining process, the greater the number of g.b. could lead to a faster rate of whisker growth. Increasing the grain size of the tin plating will reduce the number of grain boundaries, slowing the diffusion. Intermetallic compounds (IMCs) with “suitable” sizes tend to form preferentially along grain boundaries. Fewer grain boundaries offer fewer sites for non-uniform IMC growth, resulting in the reduced stress in the system.

Additionally, the diffusion rate varies with the crystallographic orientation. The interstitial diffusion of Cu/Ni along the c-axis of tin grains is much faster than along a,b-axes. Accordingly, grain orientation can affect diffusion rates in tin matrix. Controlling the grain orientation can alter the rate of diffusion. However, tin's anisotropic properties can be reduced by refining its microstructure.

Overall, increasing the grain size of the plating or controlling the grain orientation can slow the rate of diffusion.

Both lattice diffusion and grain boundary diffusion are participants in tin whisker growth. The relative rate of lattice diffusion and grain boundary diffusion varies with temperature, which plays a role to the actual mechanism of the process, as well as to the growth of whisker. However, as the temperature rises, in the case of tin to above 75°C, the lattice and grain boundary diffusion rates start to converge to a similar rate.

The relative diffusion rates between lattice and grain boundary in relation to the size, morphology, crystal lattice structure and external conditions (e.g., temperature) are more intricate than the first glance—indeed, a complex challenge to "conquer and control!"

To read this entire article, which appeared in the September 2016 issue of SMT Magazine, click here.


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