The Stress Generation Driving Force in Sn Whisker Growth

In Fig. 5(c), a cross-sectional SEM image, prepared by focused ion beam, of pure Sn finish on Cu leadframe is shown. While the layer of CugSns compound can be seen between file Cu and Sn, there is almost no Cu6Sn5 precipitates in the grain boundaries of Sn. The grain size in the Sn finish is also approximately several microns. The lack of grain boundary Cu6Sn5 precipitates is the most important difference between the eutectic Sn-Cu and the pure Sn finish. 

The origin of the compressive stress can be mechanical, thermal, and chemical. But the me- 

Since whisker growth is a kinetic process in which stress generation and stress relaxation oc- 

In diffusional processes, such as the classic Kirkendall effect of interdiffusion in a bulk diffusion couple of A and B, the atomic flux of A is not equal to the opposite flux of B. If we assume that A diffuses into B faster than B diffuses into A, we might expect that there will be a compressive stress in B, since there are more A atoms diffusing into it than B atoms diffusing out of it. However, in Darken s analysis of interdiffusion, there is no stress generated in either A or B. But Darken has made a key assumption that vacancy concentration is in equilibrium everywhere in the sample. To achieve vacancy equilibrium, we must assume that lattice sites can be created and/or annihilated in both A and B, as needed. Hence, provided that the lattice sites in B can be added to accommodate the incoming A atoms, there is no stress. The addition of a large number of lattice sites implies an increase in lattice planes if we assume that the mechanism of vacancy creation and/or annihilation is by dislocation climb mechanism. It further implies that lattice planes can migrate. 

For the fixed volume of V in the finish (Fig. 6) (Eq I) to absorb the added atomic volume due to the indiffusion of Cu, we must be able to add lattice sites in the fixed volume. If we assume Darken s mechanism and allow the volume to expand, we should have no stress. If not, stress will be generated. In a later section, when we discuss the oxide on Sn surfaces, we shall explain why the volume cannot expand and why stress is generated. 

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