“Tin whiskers” is not an imaginative, fanciful term for some aspect of electronics manufacturing. Tin whiskers are real, and they pose a serious problem for electronics of all types. When used as a finish material for electronic components, pure tin can spontaneously grow conductive whiskers. These structures can form electrical paths, affecting the operation of the subject device. This article discusses the problems caused by the removal of lead from electronics and describes some techniques to mitigate tin whiskers.
Lead has been banned by the Restriction of Hazardous Substances (RoHS) directive. Although RoHS originated in Europe, its directive now affects virtually every piece of electronics gear manufactured today or planned for the near future. Connectors, passive and active components, switches, and relays now must all be lead-free.
Why such a restrictive mandate? The impetus does not originate with electronics and semiconductors (ICs), but with perceived public health. European safety agencies determined that it was necessary to prevent lead from entering landfills because it is a neurotoxin and is known to inhibit hemoglobin production and affect brain development. Children are clearly more at risk than adults. Wonderfully, the removal of lead from paint and gasoline has measurably improved our environment and has been especially beneficial for children. Unfortunately, the switch to alternative solders in order to achieve RoHS compliance has created some challenges for the semiconductor industry, especially tin whiskers.
Understanding the culprit
Tin whiskers are not a new phenomenon; in fact, they were first reported in papers written in the 1940s. Tin whiskers are almost invisible to the human eye and are 10 to 100 times thinner than a human hair (see figure 1). They can bridge fairly large distances between electrical device leads, and in so doing, can short out the conductors. They can grow fairly rapidly; incubation can range from days to years.1 There is no set timetable for when they commence growing.
Figure 1: SEM image shows an example of a needle-like tin whisker. (Courtesy of CALCE/University of Maryland)
When a whisker grows between two conductors, the whisker usually fuses (disappears), creating a momentary short circuit. In some cases the whisker forms a conductive path, creating false signals at an incorrect location which can, in turn, cause improper operation of the device in question. In very rare cases, rather than disappearing like a fuse link, the whisker can instead form a conductive plasma capable of carrying over 200 A. Whiskers can also break and fall into contact with printed circuit board (PCB) traces and other conductive pieces where they interfere with electrical signals. In optical systems they can disrupt or diminish the transmitted light; in MEMS they can interfere with the intended mechanical function.
Whiskers are real and they cause real problems, but they are also random. How big an issue are they really?
Pure, tin-plated electronics have become ubiquitous over just the past five years. These electronic systems form the backbone of our communications and financial systems, our manufacturing and transportation systems, and, of course, our power plants (nuclear and conventional). Tin whiskers have created conductive paths and other destruction in unintended places. In 2005, a random “full turn-off” signal at the U.S. Millstone nuclear plant in Connecticut was attributed to a tin whisker.2
Because of the potentially dangerous and unpredictable risks of pure tin, it is not presently used in medical devices. Lead is allowed for use in external medical devices until 2014 and for internal medical devices until 2021.