basic info on tin whiskers

8/8/2019 Basic Info on Tin Whiskers

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NASA

Basic Info/FAQ

Basic Information Regarding Tin Whiskers

1. What a re 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 a re 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 elementsmaintained 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 ofseveral 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.

Tin "Whisker" shown above growing between pure tin-plated hook

terminals of an electromagnetic relay similar to MIL-R-6106 (LDC8913)

Photo Courtesy of Andre Pelham (Intern)

NASA Goddard Space Flight Center

"Dendrites" are NOT "Whiskers"

"Dendrites" shown above are NOT the same

phenomenon as "whiskers"


nepp.nasa.gov/whisker//index.htm 1/7


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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 havebeen studied for many years. A single accepted explanation of the mechanismshas NOT been established. Some theories suggest that tin whiskers may grow in response to a mechanism of stress relief (especially"compressive" st ress) 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 factorsthat 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 bemost susceptible to whisker formation reportedly because bright tin plating processes can introduce greaterresidual 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 intermetalliccompounds (such as Cu6Sn5 for a Sn over Cu system) that alter the latt ice 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 introducedby torquing of a nut or a screw or clamping against a t in-coated surfacecan 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 byhandling, probing, etc.

6. Coefficient of Thermal Expansion Mismatchesbetween the plating material and substrate




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

Tin whiskers pose a serious reliability risk to electronicassemblies. Several instances have been reported where tin whiskers have caused systemfailures in both earth and space-based applications. To date, there are reports of at least three tin whisker induced short circuits that resulted incomplete 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 DESTRUCTIVEphenomenon known as a Metal Vapor Arccan 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 AM PERES. 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 k ind 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 t in 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 Initia ted Vacuum Metal Arcing in Spacecraft Elect ronics," 1992 Government Microcircui t Appli cation s 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




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Why the Recent Attention to Tin Whiskers?The current worldwide initiative to reduce the use ofpotentially hazardous materials such as lead (Pb) is driving the electronics industry to consideralternatives 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 theindustry 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 sys tems.

Continuing reports of tin whisker-induced failures coupled with the lack of an industryaccepted understanding of tin whisker growth factors and/orproven 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

control ling tin whiskers. The following list is intended to provide a very basic overview of some of the observed characteris tics of tin whiskers.

1. Shapes: Whiskers may be straight, kinked, hooked or forked. Their outer surfacesare often grooved. Some growths may form as nodules orpyramidal structures.

2. Incubation (Dormancy) Period: Experimenters report the incubation period may range from days to years. This attribute of whisker growthis particularly concerning because meaningful experiments to determine the propensity for a particular process to form whiskers may need to

span very long periods of time.

3. Growth Rate: Growth rates from 0.03 to 9 mm/yr have been reported. Growth is highly variable and is likely to be determined by a complexrelationship of factors inc luding plating chemistry, plating thickness, substrate materials, grain structure and environmental storage conditions.

4. Whisker Length: Whiskers as long as a few millimeters are not uncommon with some experimenters observing whiskers in excess of 10mm (400 mils) in length. Only a few researchers have measured the distribution of whisker lengths for specific specimens. Invariably, these




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researchers report the length distribution fits a lognormal distribution.

5. Whisker Diameter: Typical diameters are a few microns with some reports in excess of 10 um and rarely less than 100 nm.

6. Environmental Factors: There is a great deal of contradictory information regarding environmental factors that might affect whisker

formation. Several organizations are attempting to devise accelerated test methods to determine a particular plating process's propensity to

form tin whiskers. However, to date, there are no accepted test methods for evaluating whisker propensity. Indeed, much of the experimental

data compiled to date has produced contradictory findings regarding which factors accelerate (or retard) whisker growth.

Temperature: Some experimenters report that ambient temperatures of approximately 50C are optimal for whisker formation, while

others

observe that room temperatures (22C to 25C) grow whiskers faster. Reportedly, whisker growth ceases at temperatures above 150C

Pressure: Whiskers will grow in vacuum as well as earth based atmospheric pressure.

Moisture: Some observe that whiskers form more readily in high humidity (85% RH) whereas others report moisture is not a

contributing factorThermal Cycling: Some experimenters report that thermal cycling increases the growth rate of whiskers, but others report no effect

due to thermal cycling.

Electric Fie ld: Whiskers grow spontaneously without requiring an applied electric field to encourage their growth. Some recent

observations of tin whisker induced field problems in the commercial sector seem to suggest that an electric field could stimulate

whisker growth, but more analysis is required to confirm these effects (if any). GSFC has demonstrated that whiskers can bend due to

the forces of electrostatic attraction thus increasing the likelihood of tin whisker shorts ( ref. to GSFC experiment #4).

7. Whisker Prone Processes: There is tremendous debate in the industry regarding which plating processes are prone to whiskerformation. Most of the literature agrees that "pure tin" electroplated surfaces (especially those that employ brighteners in the plating process)

are the most susceptible to whisker formation. There are also reports that tin-lead plating can grow whiskers; however, such whiskers are

generally reported to be less than 50um long.

Suggestions for Reducing Risk of Tin Whisker Induced Failures: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 restrict ions against theuse of pure tin plating is highly recommended. Most (but not all) of the commonly used military specifications currently have prohibitionsagainst pure tin plating. Studies have shown that alloying t in with a second metal reduces the propensity forwhisker growth. Alloys of tin andlead 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 s ignificant risk for the geometries of today's microelectronics.




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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 ofthe 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, t in 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.

1. Solder Dip the platedsurfaces sufficiently using a t in-lead solder to completely reflow and alloythe tin plating. Obviously, special precautionsarerequired to prevent thermal shock induced damage,to prevent loss of hermeticity and to avoid thermal degradation. This approach mayhave limited success since it may be difficult to ensure that the entire surface is properly reflowed. See the April 2004 Photo of the Month for

one example of the limitations associated with this particular mitigation strategy.

2. Replate the whisker prone areas. Some manufacturers may be willing tostrip the pure tin plate from finished products andre-plate using asuitable alternate plating materialsuch as t in/lead or Nickel. Caution is advised if considering use of an external plated finish (e.g., Sn-Pb orCu) on top of an existing pure tin deposit. There is some evidence that whiskers may s till form from the pure tin layer and protrude through the

thin external deposit.

3. Conformal Coat or foamencapsulation over the whisker prone surface can significantly reduce the risk of electrical short c ircuits causedby whiskers. The choice of coating material, thickness and possible degradation with time/environmental exposure can impact the

effectiveness of the coating. NASA GSFC experiments have shown that use of Arathane 5750 (formerly Uralane 5750) conformal when applied

uniformly to a nominal 2 to 3 mils thickness can provide significant benefit by containing whisker growth outward through the coating. This

coating is also resistant to being penetrated by whiskers attempting to puncture the coating from the outside.

"Metal Whiskers: A Discussion for CALCE Symposium", Symp. on Part Reprocessing, Tin Whisker Mitigation and Assembly Rework,

Nov. 12, 2008

J. Brusse, H. Leidecker, L. Panashchenko (especially pages 19-28)

See also the following research by Dr. Tom Woodrow (Boeing) in which he evaluated 6 different conformal coat materials for purposes of

inhibiting whisker formation and subsequent shorting hazards.

"Evaluation of Conformal Coatings as a Tin Whisker Mit igation Strategy, Part 2", SMTAI, Sept. 2006

"Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy", IPC/JEDEC 8th International Conference on Pb-Free

Electronic Components and Assemblies, San Jose, CA, April 18-20, 2005

technical paper (~11 Mbytes) and

presentation (~19 Mbytes)

Ithas also been observed experimentally that conformalcoating can restrict the availability of tinsufficiently to minimize the risk of plasmaformation during a shorting event. However,such factors as the minimum thickness of conformal coatingnecessary to prevent plasmaformationhave not been determined. Similarly, it hasbeen shown that foam can prevent sustained arcingbut the effects of foam type, foamdensity, pore sizeetc. have not been evaluated.

4. Evaluate Application Specific Risks. A variety of application specific considerations may be used to assess the risk of whisker inducedfailures and assist in making "use as-is" or "repair/replace" decisions. These factors include circuit geometries that are sufficiently large to

preclude the risk of a tin whisker short, mission criticality, mission duration, collateral risk of rework, schedule and cost. To assist in

evaluating application specific risks, David Pinsky (Raytheon) has developed a tin whisker risk assessment algorithm which can be reviewed

Note: reference to this algorithm herein DOES NOT imply endorsement by NASA

In 2002, Dr. Mike Osterman (CALCE Center at the University of Maryland) published a white paper outlining pros and cons of assorted

strategies for mitigating risks associated with tin whiskers.




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Responsible NASA Officials:

Michael Sampson/NASA GSFC Code 306 Dr. Henning Leidecker/NASA GSFC Code

562

Additional Researchers:

Jay Brusse/Perot Systems

Jong Kim/Orbital Sciences Corp.

Last Updated:

June 16, 2009

DISCLAIMER:This website provides information about tin whiskers and related research. The independent research performed

during the past 50+ years is so vast that it is impractical to cover all aspects of tin whiskers in this one

resource. Therefore, the absence of information in this website about a particular aspect of tin whiskers should

NOT be construed as evidence of absence.

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