Tin whiskers is a major problem of RoHS Lead-free design, whose components are having pure Tin or Matte Tin. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does NOT require either dissolution of the metal NOR the presence of electromagnetic field. The mechanisms by which tin whiskers grow have been studied by NASA for many years. A single accepted explanation of the mechanisms has NOT been established. Some theories suggest that tin whiskers may grow in response to a mechanism of stress relief (especially “compressive” stress) within the tin plating. Other theories contend that growth may be attributable to re-crystallization and abnormal grain growth processes affecting the tin grain structure which may or may not be affected by residual stress in the tin plated film.

Some commonly accepted factors that cause whiskers:

  • Residual stresses within the tin plating caused by factors such as the plating chemistry and process. Electroplated finishes (especially “bright” finishes) appear to be most susceptible to whisker formation reportedly because bright tin plating processes can introduce greater residual stresses than other plating processes.
  • Inter-metallic Formation: The diffusion of the substrate material into the tin plating (or vice versa) can lead to formation of inter-metallic compounds (such as Cu6Sn5 for a Sn over Cu system) that alter the lattice spacing in the tin plating. The change in lattice spacing may impart stresses to the tin plating that may be relieved through the formation of tin whiskers.
  • Externally Applied Compressive Stresses such as those introduced by torquing of a nut or a screw or clamping against a tin-coated surface can sometimes produce regions of whisker growth.
  • Bending or Stretching of the surface after plating (such as during lead-formation prior to mounting of an electronic component)
  • Scratches or nicks in the plating and/or the substrate material introduced by handling, probing, etc.
  • Coefficient of Thermal Expansion Mismatches between the plating material and substrate