Understanding and mitigating tin whiskers
“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.
(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.
How and why do they grow?
The industry does not really know what causes tin whiskers, nor do we really understand how they form. We cannot predict their appearance other than to say whiskers are likely to form on pure tin. Studying them with accelerated life tests has proven useless because they do not grow any faster or sooner in the simulated environment.1
So tin whiskers grow at random and interfere with system/subassembly performance. What can you do? To develop a mitigation strategy, the first thing we need is an understanding of what causes tin whiskers. Unfortunately, there is no accepted explanation of how they form, but a number of theories exist. Some postulate that the whiskers form in response to residual stresses within the tin plating and are caused by the chemistry of the plating. They point to the residual stress that results from the bright (small grain) electroplate process finishes as making those finishes prone to whiskers, yet large-grain finishes (matte) are also known to grow whiskers. Other theories hold that recrystallization and abnormal grain growth may impact the lattice spacing leading to whiskers.
Stresses can come from many places and are accepted in the lead world, but these same stresses seem to induce whiskers in the pure tin world. Sources of stress include compressive forces from external activity like tightening a fastener; bending or stretching that might occur in the formation of the leads; and even nicks or scratches created in normal handling. Finally, a seemingly mundane difference in the coefficient of thermal expansion between the lead-frame base material and the tin-plating material has been cited as a possible source for stress that causes the whisker problem.1 Annealed matte tin seems to be the most successful finish for reducing stress and thus is often used by component companies as a lead-free finish.3
Where does that leave us? Many experiments have been conducted with inconsistent results. The present consensus is that influences that increase the stress or promote diffusion tend to induce whisker formation. In summary, the industry really does not know what causes tin whiskers to form.
Is lead really the problem?
Changing pace just a bit, consider the lead question from a different perspective. How much lead is really being consumed each year? According to the International Lead and Zinc Study Group, worldwide usage of lead in 2010 was 9.595 million metric tons, up from 8.966 million metric tons in 2009.4 (This increase is understandable, given the slowed economy in 2009.) Of that lead usage, 80% is consumed in lead-acid batteries, an application that remains exempt from RoHS compliance. Note also that before the RoHS directive, only 0.5% of lead was consumed in electronic solder and a mere 0.05% was consumed in electroplate for ICs.
What do all these statistics tell us? The 2010 usage of lead, in all applications, was approximately 21 million pounds. Of that, 16.8 million pounds was consumed in batteries and only about 10,500 pounds would have been consumed in IC lead finish if the RoHS directive were not in force for electronics.
Recall that the expected environmental harm from lead in electronics was the impetus behind the RoHS legislative action. Lead was feared as a contaminant to groundwater. Many well-intentioned people overlook one important fact, however: Elemental lead is not water soluble. Other sources concur: "Lead does not break down in the environment. Once lead falls onto soil, it usually sticks to the soil particles."5 When burned in an open-fire recycling operation, lead was feared to cause a poisonous vapor if inhaled. From NASA6, the facts are:
- An open-fire temperature is approximately 1000°C, but lead boils at 1740°C.
- Thus, the vapor pressure of lead would be negligible, presenting little possibility of lead-vapor poisoning.
- Workers who solder with tin lead (SnPb) solder do not have high lead levels in their blood.
In the end, there is no evidence that lead in electronics presents a health risk or causes environmental harm. Ironically, many of the proposed lead-free solutions do pose environment problems and many are much worse for the environment.
Options for lead-free electronics
The move to lead-free products meant that the electronics industry has had to develop lead-free solders and terminal finishes compatible with those solders. Manufacturers have tried a number of different lead-free alloys and some very sophisticated binary, ternary, and quaternary alloys and discovered that these alloys are both expensive and hard to use. Additionally, several tin-silver alloys like tin-silver-copper, tin-silver-bismuth, tin-silver-copper-bismuth, and various other combinations have also been investigated. Bismuth-209 is slightly radioactive, so it posed its own set of issues.
In all, there had been many serious problems converting to lead-free electronics, but I will not go into a diatribe on all of them today. There are, however, two solutions worth mentioning.
- Pure Tin (Sn) is inexpensive and readily available, is not chemically hazardous, and is easy to use. Most lead-free terminal finishes today are annealed matte tin as compared to bright tin or small-grain tin. The known and anticipated issue with pure tin has been presented above: whiskers. They will form over time, at random, and can eventually cause shorts or worse. Whiskers grow fairly slowly at sea level but more rapidly at higher altitude. There are mitigation techniques which I will discuss momentarily.
- Nickel Palladium Gold (NiPdAu) is a popular lead-free finish material being used more and more widely. Maxim Integrated Products offers it on over 5000 different part numbers today. It is more expensive than pure tin and requires high-temperature lead-free solder.
When considering the finish on components, it is also important to evaluate the ultimate required reliability versus risk. Work presented at the 2010 International Symposium on Tin Whiskers at the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland groups the ultimate reliability into three categories and suggests the level of risk for different lead finishes (see table 1):7
- Level I: the product has an expected field life of less than five years.
- Level II: the product requires a very high level of reliability; a failure may be tolerable where redundancies exist or if the failed component/subassembly can be repaired or replaced.
- Level III: the product must be ultra-reliable; there is no easy way to repair or replace a component/subassembly which must have a long planned service life.
Manufacturing problems and surprises
Of course, when the electronics industry moved to eliminate lead, problems were anticipated, like the tin-whisker effect. Nonetheless, there were also some complete surprises. Electronics manufacturing engineers knew that if they mixed leaded and lead-free parts in assembly, they would have to use two passes through the solder machines. Yes, it was a problem, but not a surprise. What was a surprise was the way the thin PCBs sagged at the higher temps required for the lead-free solders. Manufacturers had acknowledged the possibility of tin whiskers and it still remains a major concern, but they did not always think about the additional thermal load created by the higher temperature pass and the number of cycles through the reflow hardware.
A real surprise was the fracturing of the lead-free solders in high-vibration environments. SnPb solder is not brittle, but many of the newer lead-free options are, which can create a problem in some applications. Aircraft, for example, have both many frequencies of vibration and fairly rapid temperature cycling as they move from the earth’s surface (at perhaps +25°C to +40°C) to 30,000 feet (at -60°C). Lead-free solders experienced fractures that, in turn, caused intermittent contacts in circuits. And to be sure, that is not a good thing on a fly-by-wire airplane, is it?
Finally, satellites grow whiskers very rapidly. Recall that the higher the altitude, the more rapid the whisker formation. Consequently, the various satellite agencies now require a minimum of 3% ead in the finish of the leads. Most IC makers actually supply a more conventional 85/15 SnPb finish.
Mitigation, not elimination
At the outset, let me state the obvious: Mitigation is not elimination; it is merely a reduction in severity. Tin whiskers will still grow. In fact, many metals, including zinc, cadmium, indium, silver, aluminum, gold, and yes, even lead, grow whiskers. It’s a fairly widespread phenomenon, but the most common and most dangerous manifestation is tin whiskers.1 If we find ourselves forced to use solders containing tin, how do we address the concern for high-reliability applications?
Here are some suggestions for reducing the risk of tin whiskers:
- Do not use pure tin. That seems simple enough. Instead use a tin-lead alloy with at least 3% lead. Yes, even SnPb has been shown to grow whiskers, but they were observed to be much smaller than pure tin whiskers.
- Do not rely on the order paperwork. Use x-ray fluorescence (XRF) to verify finish on all critical parts.
- Refinish a pure tin-finished part with a hot-solder dip. Maxim offers this as an option on all of its lead-free devices.
- Use some type of encapsulation or conformal coating. NASA has shown that Arathane 5750 (formerly Uralane 5750) can be effective in preventing tin-whisker shorting when applied with a nominal thickness of 2 mils to 3 mils on the pure tin surface.
A conformal coating is, as the name implies, a coating with an inert material that can protect electronic circuit boards from the problems related to tin-whisker growth, such as shorts, plasma arcs, and debris. In defining the requirements for a conformal coating consider the following:
- It must slow the formation of tin whiskers. We acknowledge that tin whiskers cannot be stopped until we understand how they form in the first place.
- It must prevent the outward escape of any tin whiskers that do nucleate.
- It must prevent the penetration of whiskers formed outside the conformal coat.
- It must protect the coated circuit board from loose whisker debris.
Many types of conformal coatings have been studied over the years by Boeing, Schlumberger, Lockheed, Raytheon, The National Physical Laboratory (UK), CALCE, and NASA, among others. A summary of the studies shows that no conformal coating meets all the criteria outlined above (see table 2).7 Ultimately, no coating is 100% effective and whiskers still grow. The Arathane coating seems promising when applied sufficiently thick, however, and the conformal coating does prevent shorts from debris. Thermal effects need to be considered if a conformal coating is used on parts which will need to dissipate heat when operating. If necessary, the device may need to be derated.
The shift away from lead solder presents risks for high-reliability applications, particularly in the form of tin whiskers on tin-containing finishes. NiPdAu presents one alternative since it has also proven resistant to whisker formation, but the suitability of NiPdAu for a high-vibration environment is still under evaluation. It is a higher temperature solder and may, indeed, be less ductile than traditional SnPb solder.
When a tin-bearing finish is used, conformal coatings have been somewhat effective and may also be suitable. Although the whiskers are contained, the conformal coating adds processing steps, possible thermal issues, and cannot totally prevent whisker formation.
Either solution above adds cost. When considering finish material for electronics, the SnPb solution is still the best because industry has more experience working with the material. Unlike NiPdAu, which must be plated to the entire lead frame before die bond and encapsulation, SnPb is electroplated to the lead frame after plastic encapsulation. More important, SnPb has been shown not to have a whisker problem and to be very resilient in high-vibration environments.
Both the RoHS directive and the tin-whisker issue present challenges, but they are not insurmountable. With proper choice of solder, manufacturers of high-reliability devices can achieve RoHS compliance while still maintaining reliability, performance, and cost objectives.
References
1. NASA Tin Whisker (and Other Metal Whisker) Homepage, “Basic Info/FAQ.” https://nepp.nasa.gov/WHISKER/background/index.htm.
2. P. Daddona, "Reactor Shutdown: Dominion Learns Big Lesson from a Tiny Tin Whisker," The Day (New London, CT), July 4, 2005.
3. Y. Fukuda, M. Osterman, and M. Pecht, “The Effect of Annealing on Tin Whisker Growth,” IEEE Transactions of Electronic Packaging Manufacturing, Vol. 29, No. 4, pp. 252-258, October 2006.
4. International Lead and Zinc Study Group. “Lead and Zinc Statistics.” https://www.ilzsg.org/static/statistics.aspx.
5. Ohio Department of Health, “Lead, Answers to Frequently Asked Health Questions,” 10 Aug 2009.
6. A. D. Kostic, “Lead-free Electronics Reliability – An Update.” The Aerospace Corporation, Geoint Development Office. August 2011.
7. These third-party test results are cited in the NASA GSFC presentation, “Long Term Investigation of Urethane Conformal Coating Against Tin Whisker Growth,” 7 Dec 2010. https://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-IPC-Tin-Whisker.pdf.
8. NASA GSFC presentation, op cit.
About the author
John O’Boyle is the senior business manager for the Military/Aerospace Business unit at Maxim Integrated Products. O’Boyle has over 30 years of semiconductor industry experience, and is currently responsible for driving the release of the company’s parts for military/aerospace applications. He has also developed an innovative program for the delivery of SnPb-finished parts to military and aerospace customers that require a lead finish. He holds BSEE, MSEE, and MBA degrees from Santa Clara University.
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