NASA
Basic Info/FAQ

1. What are Tin Whiskers?

2. What are the Mechanisms by which Tin Whiskers Form?

3. What are the Risks/Failure Mechanisms Associated with Tin Whiskers?

4. Why the Recent Attention to Tin Whiskers?

5. What are the Commonly Reported Characteristics of Tin Whiskers?

6. Suggestions for Reducing Risk of Tin Whisker Induced Failures

What are Tin Whiskers?

Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish.  Tin whiskers have been observed to grow to lengths of several millimeters (mm) and in rare instances to lengths in excess of 10 mm.  Numerous electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials.

Tin whiskers are not a new phenomenon.  Indeed, the first published reports of tin whiskers date back to the 1940s and 1950s. Tin is only one of several metals that is known to be capable of growing whiskers.  Other examples of metals that may form whiskers include some tin alloys, zinc, cadmium, indium, antimony, silver among others . 

People sometimes confuse the term "whiskers" with a more familiar phenomenon known as "dendrites" commonly formed by electrochemical migration processes.  Therefore, it is important to note here that whiskers and dendrites are two very different phenomena. A "Whisker" generally has the shape of a very thin, single filament or hair-like protrusion that emerges outward (z-axis) from a surface.  "Dendrites", on the other hand, form in fern-like or snowflake-like patterns growing along a surface (x-y plane) rather than outward from it.  The growth mechanism for dendrites is well-understood and requires some type of moisture capable of dissolving the metal (e.g., tin) into a solution of metal ions which are then redistributed by electromigration in the presence of an electromagnetic field.  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. 

More Examples of Metal Whiskers on EEE Parts and Associated Hardware

What are the Mechanisms by which Tin Whiskers Form?
The mechanisms by which tin whiskers grow have been studied 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 recrystallization 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.  

Those advocating "stress" as crucial for metal whisker formation point to some commonly accepted factors that can impart additional residual stress:

1. 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.

2. Intermetallic Formation: The diffusion of the substrate material into the tin plating (or vice versa) can lead to formation of intermetallic 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.

3. 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.

4. Bending or Stretching of the surface after plating (such as during lead-formation prior to mounting of an electronic component)

5. Scratches or nicks in the plating and/or the substrate material introduced by handling, probing, etc.

6. Coefficient of Thermal Expansion Mismatches between the plating material and substrate

What are the Risks/Failure Mechanisms Associated with Tin Whiskers?
Tin whiskers pose a serious reliability risk to electronic assemblies.  Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications.  To date, there are reports of at least three tin whisker induced short circuits that resulted in complete failure of on-orbit commercial satellites.  There have also been whisker-induced failures in medical devices, weapon systems, power plants, and consumer products.

The general risks fall into four categories:

1. Stable short circuits in low voltage, high impedance circuits. 
   In such circuits there may be insufficient current available to fuse the whisker open and a stable short circuit results. Depending on a variety of factors including the diameter and length of the whisker, it can take more than 50 milliamps (mA) to fuse open a tin whisker.
2. Transient short circuits.
   At atmospheric pressure, if the available current exceeds the fusing current of the whisker, the circuit may only experience a transient glitch as the whisker fuses open.
3. Metal Vapor Arc 
   If a tin whisker initiates a short in an application environment possessing high levels of current and voltage, then a VERY DESTRUCTIVE phenomenon known as a Metal Vapor Arc can occur.  The ambient pressure, temperature and the presence of arc suppressing materials also affect metal vapor arc formation. In a metal vapor arc, the solid metal whisker is vaporized into a plasma of HIGHLY CONDUCTIVE metal ions (more conductive than the solid whisker itself). This plasma can form an ARC capable of carrying HUNDREDS OF AMPERES. Such arcs can be sustained for long duration (several seconds) until interrupted by circuit protection devices (e.g., fuses, circuit breakers) or until other arc extinguishing processes occur. This kind of arcing is happening in the metal vapor.  When an arc quenching agent (e.g., air) is present, more power must be installed into the event to replace power lost to the non-interesting processes happening in the quenching agent.  Therefore, as air pressure is reduced, less power is required to initiate and sustain a whisker-induced metal vapor arc.  For example, past experiments^** have demonstrated that at atmospheric pressures of about 150 torr, a tin whisker could initiate a sustained metal vapor arc where the supply voltage was approximately 13 Volts (or greater) and supply current was 15 Amps (or greater).  Tin (or other materials) from the adjacent surfaces can help to sustain the arc until the available material is consumed or the supply current is interrupted. Metal vapor arcs in vacuum are reported to have occurred on at least three commercial satellites resulting in blown fuses that rendered the spacecraft non-operational.
   **  J.H. Richardson, and B.R. Lasley, "Tin Whisker Initiated Vacuum Metal Arcing in Spacecraft Electronics," 1992 Government Microcircuit Applications Conference, Vol. XVIII, pp. 119 - 122, November 10 - 12, 1992.
4. Debris/Contamination. 
   Whiskers or parts of whiskers may break loose and bridge isolated conductors or interfere with optical surfaces

Why the Recent Attention to Tin Whiskers?
The current worldwide initiative to reduce the use of potentially hazardous materials such as lead (Pb) is driving the electronics industry to consider alternatives to the widely used tin-lead alloys used for plating. For example, the European Union has enacted legislation known as the Restriction of certain Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) Directives which have set June 2006 as deadlines for electronic equipment suppliers to eliminate most uses of Pb from their products.  It is widely believed (though reasons remain somewhat of a mystery) that Pb when alloyed with tin imparts whisker-inhibiting attributes to the final finish.

With respect to factors such as solderability, ease of manufacture and compatibility with existing assembly methods, pure tin plating is seen by the industry as a potentially simple and cost effective alternative. In fact, many manufacturers have been offering pure tin plated components as a standard commercial (and in some cases high reliability) product for years while others are exploring pure tin alternatives for the very first time.  Many electronics manufacturers have never heard of the phenomenon of tin whiskers and therefore, may not consider the risks of tin whisker growth during the validation of new plating systems.

Continuing reports of tin whisker-induced failures coupled with the lack of an industry accepted understanding of tin whisker growth factors and/or proven and reliable test methods to identify whisker-prone products has made a blanket acceptance of pure tin plating a risky proposition for high reliability systems.  Still, organizations such as NASA and the DoD may soon be faced with few options other than pure tin plating since the desires of the commercial market for environmentally friendly components carry far more weight than the infinitesimally small market share of the high reliability user.

What are the Commonly Reported Characteristics of Tin Whiskers?

The vast disparity in the observations reported by different experimenters is evidence of the complications associated with understanding and controlling tin whiskers.  The following list is intended to provide a very basic overview of some of the observed characteristics of tin whiskers.

The uncertainties associated with tin whisker growth make it extremely difficult to predict if/when tin whiskers may appear. The following list provides some suggestions for reducing the risk of tin whisker induced failures. 

1. Avoid the use of PURE TIN plated components if possible. Utilization of procurement specifications that have clear restrictions against the use of pure tin plating is highly recommended. Most (but not all) of the commonly used military specifications currently have prohibitions against pure tin plating. Studies have shown that alloying tin with a second metal reduces the propensity for whisker growth. Alloys of tin and lead are generally considered to be acceptable where the alloy contains a minimum of 3% lead by weight. Although some experimenters have reported whisker growth from tin-lead alloys, such whiskers have also been reported to be dramatically smaller than those from pure tin plated surfaces and are believed to sufficiently small so as not to pose a significant risk for the geometries of today's microelectronics.

2. Post Procurement It can be dangerous to rely on the part manufacturer's certification that pure tin plating was not used in the production of the product supplied. NASA GSFC is aware of several instances where the procurement specification required "No Pure Tin", but the product supplied was later determined to be pure tin.  In some of these instances, tin whisker growths were also discovered.  Users are advised to analyze the plating composition of the products received as an independent verification.

When simple avoidance of pure tin plating is not a viable option (such as in cases where its use is discovered late in system integration/test), then the following approaches may also be considered to reduce risk.