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July 2014 • SMT Magazine 19 plating chemistry is prone to creating an envi- ronment that creates greater organic inclusion and higher stress level in tin crystal structure. The nature of substrate, external mechanical force, and temperature have been found to af- fect tin whiskering as well. Concerns and potential impact If/when tin whisker occurs, concerns and impact primarily fall in the following four cat- egories (SMT Magazine, November 2013). 1. Short circuits When a whisker grows to a length that bridg- es the adjacent lead or terminal, this conductive whisker can cause an electrical short. However, if a whisker is formed but does not bridge its neighbor, there will not be an electrical short. To complicate the phenomena, there are occa- sions where whiskers may not cause failure, or a failure may not be detected even when the whisker physically touch the adjacent lead due to lack of electrical current flow. 2. Tin metal arcing Under high levels of current and voltage that is able to vaporize the whisker and ion- ize the metal gas, metal arc can occur. A NASA report attributed a satellite failure to tin metal vapor arc as the suspected root cause. It is ex- pected that tin arc is more likely to occur under reduced atmospheric pressures or vacuum envi- ronments. 3. Break-off debris The whiskers, being brittle and conductive, can break off from the base of its substrate sur- face, which may create functional issues. This is particularly a concern for sensitive electronic devices, such as optical and computer disk driv- er applications. The break-off behavior varies with the service conditions and the characteris- tics of the whisker. 4. Unwanted antenna Tin whiskers can act like miniature anten- nas, which affect the circuit impedance and cause reflections. In this case, the most affected areas are in high-frequency applications (higher than 6 GHz) or in fast digital circuits. Causes and Contributing Factors Regarding causes and factors, physical met- allurgy is the place to go to. Fundamentally, tin TIN WHISKERS: CAPSulIzATIoN continues fEATURE figure 2: Tin whiskers—organic inclusions and grain size (four months aged at 50°C).