A large area of concern with the use of tin-plated or pure tin component finishes is the occurrence of tin whiskers. This condition arises when tin begins to grow tiny filaments known as whiskers. A tin whisker is a spontaneous columnar or cylindrical filament, which can branch, of mono-crystalline tin emanating from the surface of a plating finish. Tin whiskers generally have an aspect ratio (length/width) greater than 2; whiskers have been found to be over several mm in length in rare instances. The whiskers can be kinked, bent, or twisted and may be surrounded by striations/rings. These whiskers are very brittle therefore can lead to intermittent or permanent electrical shorts when broken and re-deposited upon the circuit's surface.
(Note: Tin whiskers are not to be confused with dendrites: fern-like growths formed as a result of dissolved metal ions in the presence of an electromagnetic field.)
Critical precursors that increase the propensity of tin whisker formation include:
- Compressive stresses in the plated coating.
- Intermetallic formation between the tin and other metals, such as copper, silver and nickel.
- External mechanical stresses applied to the tin: Torqued screws, scratches and bending.
- To a degree, the grain structure of the plated tin.
Highly stressed deposits of pure, electroplated, 'bright' tin are probably the most susceptible to tin whisker formation. However, immersion tin deposits, while much thinner and much less prone to be stressed, have still shown evidence of tin whisker formation. Absolutes do not apply to this phenomenon.
Ways of avoiding whiskers in actual field applications include:
- Not using pure tin, especially in a 'brightened' format,
- Reflow of the tin plating to re-fuse/re-crystallize and stress-relieve the deposit and
- Using barrier materials (over-plating or organic post-coatings) to encapsulate any whiskers which have formed since completion of the plating.