In this installment of the series, we will complete the discussion of key processes likely 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, 3, and 4, we discussed the first six; now we will outline the last processes for crystal structure and defects.
Crystal Structure and Defects
Crystal structure is the bedrock of properties and behavior of materials.
Crystal structure defects and the dynamics of defects including dislocation structure and its movements account for the characteristics and performance of materials. Because the stress needed to move dislocations increases with the spacing between the planes, dislocations are expected to move along the densest planes of atoms in a material. Let’s take two primary parameters of crystal lattice into consideration: type of unit cell and atomic packing factor (APF).
Face-centered cubic (FCC) and body-centered cubic (BCC) metals have more dense planes than other crystal structures, so dislocations move relatively easy and the materials possessing these crystal structures impart high ductility. Lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. It is easier for planes of atoms to slide by each other if those planes are closely packed. For example, lead (Pb) with FCC lattice structure exhibits higher ductility than tin (Sn) with tetrahedral lattice. However, when obstacles are introduced into the lattice structure, such as interstitial atoms or grain boundaries, dislocations can be pinned and their movements hindered. In addition, if more dislocations are produced they will get into each other’s way and impede their own movements.
Within cubic lattice, a FCC crystal structure will exhibit more ductility (deform more readily under load before breaking) than a BCC structure. The BCC lattice, although cubic, is not closely packed and forms strong metals (e.g., alpha-iron and tungsten). The FCC lattice is both cubic and closely packed and forms more ductile materials (e.g., silver, gold, and lead).
Comparing between cubic-lattice (FCC, BCC) and non-cubic lattice (HCP, tetragonal, orthorhombic, monoclinic) structures, cubic-lattice structures allow slippage to occur more easily than non-cubic lattices because their symmetry provides closely packed planes in several directions.
In comparison, hexagonal close packed (HCP) lattices are closely packed, but not cubic. HCP metals (e.g., cadmium, cobalt and zinc) are not as ductile as the FCC metals. The FCC and HCP structures both have an APF of 0.74 and a coordination number of 12, consisting of closely packed planes of atoms (vs. APF of BCC = 0.68). The difference between the FCC and HCP is the stacking sequence. The HCP layers cycle among the two equivalent shifted positions whereas the FCC layers cycle between three positions.
So how does crystal structure affect tin whisker?
Tin possesses a non-cubic crystal structure (tetragonal), thus it does not allow agile slippage to readily occur and cannot proceed deformation easily. Indium also has a tetragonal crystal structure. This “inconvenient slippage” contributes to the driving forces in forming whiskers. Zn has a HCP structure. Comparing Sn with Zn, Sn’s lower APF (0.54) further facilitates whisker process as the result of more free diffusion distance, thus it is expected that Sn is even more prone to whisker than Zn.
From crystal structural perspective, among the common metals used in electronics, what is the relative whisker propensity?
If the role of crystal structure is a pure play in whisker process, tin and indium are more prone to whisker than zinc. In turn, Zn is more prone to whisker than Pb, Ag, Au and Cu.
Part 6 will conclude the series by summarizing the theory behind tin whisker phenomena.