Tin Whiskering

     Based on the evidence of whisker growth relegated to the tin filled mechanical interface for the ring lugs shown below, I have begun an investigation into the actual mechanism of whisker growth from pure metal coatings. 

     The current premise is that plated or hot dipped coatings are not allowed to form large crystals but instead are forced to cool so quickly or be deposited such that many more smaller crystals are formed resulting in a relatively higher density versus a predominantly crystalline solid; perhaps the metal is forced cool so quickly that an amorphous solid forms with no crystal growth at all.  Rapid cooling for the ring lug example could be attributed to a lug at room temperature being dipped into molten tin where the thermal mass of the bare copper part relative to the tin plating results in an equilibrium temperature well below the melting point of the tin (232C) and through which the transition temperature between solidus and liquidus is quickly passed.  Subsequent mechanical energy imparted to the metal, primarily as a function of thermal cycling of the substrate metal and plating, supplies the energy needed for randomly ordered atoms in the plating to combine into larger, more ordered crystalline structures.  As is the case for water and many other materials, the now more highly ordered atoms in the crystal matrices require more volume to than the original mass of the randomly ordered material.  This increase of structure within the tin coating then results in an increased internal pressure that can only be relieved by extrusion of metal at points of surface weakness.  Striations along the length of the whiskers can then be attributed to a "die" formed from crystals adjacent to the extrusion column which would also account for the shape of the tip of the whiskers:  The whisker end nodules are simply the crystals pushed out at the end of the extrusion when the whisker breaks through the surface.

     By adding alloying metals as in the case on tin-lead solders, the allow metal may then prevent formation of large crystalline structures and therefore prevent or at the very least lessen to a tolerable level the effects of internal state change.  Using the previous example of water, it has been well documented that crystals forming from pure water or out of a material in super saturated solution will generally drive foreign atoms, molecules and even particles away from the building crystal lattice.  Atoms within an alloy would not be expected to have the same mobility as in the water example as well as the fact that multiple crystals forming simultaneously within the material will tend to trap the interfering atoms within the matrix.
 

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1. Two samples of pure tin were melted in aluminum cans and allowed to cool at different rates.  The one on the left was convection cooled and took tens of seconds and the one at the right was immersed in liquefied freeze spray to cool in a fraction of a second.

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