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  for Metal Finishing since 1989
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Zinc "whiskers"

Have you ever heard of zinc whiskers? I haven't.

A colleague is claiming that the SCRATCHED zinc plated low carbon steel enclosure of an AC power supply is causing zinc whiskers to be found within the product that the AC power supply is located in.

The exterior surface of the power supply is slightly scratched, but there is one rather large scratch, 1/8" x 1/2", present. This scratch has caused bare metal to be exposed with no signs of corrosion present. All other scratches do not cause bare metal to be exposed.

Any information is appreciated.

Thanks.

Peter S [last name deleted for privacy by Editor]
- Tucson, Arizona
1995

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Yes!

Zinc Whiskers are unfortunately real, and have been involved in a product liability case where a patient on an apnea monitor died. See 'The Phenomenon of Zinc Whisker Growth and the Rotary Switch' by J.R.Downs and R.M.Francis in the Aug. '94 issue of Metal Finishing magazine.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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We've also found this to be true with Cadmium plated parts. Required conditions included that the base metal (steel) was not stress relieved and that the atmosphere in the system was relatively high in RH (moisture content). On samples we worked with several years ago, whiskers to 1/4 inch length could be caused to grow under these circumstances within days. Stress-relieving the base metal solved the problem.

Wolf Penzel
1995

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1995

There was a question earlier about zinc whiskers. I was wondering if anyone has heard of whiskers appearing with tin-zinc plating?

Thanks

Mike Ulinski

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Hello,

We are a sizable Electronics company, using a lot of Zinc plated chassis for our Internet routers. We have been plagued by Zinc Whiskers lately, and we would like to learn more about them. Where would we find books, publications, information about them. Where do we start ?

Thank you.

Noel V [last name deleted for privacy by Editor]
Internet Routers
1996

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Tin whiskers have been a well documented phenomenon for decades, but I hadn't heard of zinc whiskers until the above-referenced law case written up in the Aug. '94 issue of Metal Finishing magazine.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1996

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1998

I'm a hardware engineer for a large computer company and we too have been plagued with this problem. I found microscopic zinc whiskers coming from the sheet metal frame that holds our electronic backpanels in place.

The whiskers would cause (in some instances 500 to 2500 ohm shorts to ground ) raising havoc with our electronics. We are currently going back to our sheetmetal supplier to try and find any reason for this. At this point it appears to be only on some of the sheetmetal parts. I have found quite a bit of info on-line by searching for tin plating. I still can't believe that this was found back in the 1940's by Bell Labs and we are still seeing this today!

Willis D [last name deleted for privacy by Editor]

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All the discussion above is so interesting! I am surprised that such a major phenomenon is coming into light just recently, especially in the electronics industry where it could cause disasters.

Has anyone done any study as to why this happens? Do whiskers grow even if the surface is chromated or painted? I recall one experience 15 years ago where I had faced a similar problem on 5 micron wide Al interconnects on ICs. Everything was fine until we deposited PVD SiO2 to seal Al lines, and we found chunks of metal disappearing, and never found where it went. We used to call them mouse-bites. My study indicated that it was stress induced mass transfer. After modifying the process to minimize stress, the bites disappeared.

Mandar Sunthankar
- Fort Collins, Colorado
1998

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Indeed, tin whiskers have been recognized for decades and well understood for many years. Stress reduction and/or a small percentage of lead (or possibly bismuth) can stop them. Whiskers of zinc and cadmium may be more recent discoveries, although their cause is apparently the same, as is their cure. I have it on pretty good authority from a manufacturer of zinc alloy plating processes that the new alloy plates are less susceptible to whiskers than plain zinc, for the same reason than tin-lead is less susceptible than pure tin, but I don't know of anything being published in that direction yet.

Only recently did I find out that cadmium cannot be used in deep space applications because of a sublimation mass transfer phenomenon, Mandar. Can you enlighten us on that one?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1998

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Ted, the explanation for cadmium sublimation in space is relatively simple compared to the zinc-whisker phenomenon. Cadmium vaporizes (sublimes) easily on earth at elevated temperatures from its solid state, without melting. Also, it sublimes at lower and lower temperatures as the surrounding pressure decreases. If its vapor pressure is higher than the surrounding pressure, it will sublime. In space, the air pressure decreases to 10E-8 Torr and below as the distance from earth is increased beyond about 20 miles. At these air pressures, cadmium will just disappear at room temperature (25 °C). The other phenomenon in space is collision with oxygen atoms in thin air when the spacecraft is moving at high speed. This erodes and pits softer metals quickly.

Just to add to my note above about cadmium sublimation in space: there are several dozen elements and compounds or alloys that exhibit the same property as cadmium in space, and consequently are useless in many space applications. These include cadmium compounds and alloys, Zinc and zinc alloys, Magnesium, Selenium, tellurium and alloys, even pure silver and its compounds, just to name a few.

Mandar Sunthankar
- Fort Collins, Colorado
1998

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As an Industrial Hygienist, I have the opportunity to assess numerous health risks. Never have I seen any research about zinc whiskers or zinc needles. A minor search of professional IH groups has not identified any health based documentation. Of course zinc oxide ⇦this on eBay or Amazon [affil links] , zinc stearate, and other zinc forms have been known and assessed for quite some time now. One article I recently read, attempted to draw a comparison between asbestos and zinc whisker exposure. While I can understand the electronic risks, do you know of any health based documentation? I am aware of the court case you directed others towards.

Thanks

Ed Kramer
- St. Louis, Missouri
1999

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Would you please give us that asbestos-zinc whisker article's reference please? Sounds a little far out to me.

  
Tom Pullizzi
Falls Township, Pennsylvania
2002

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I have not read the asbestos-zinc whisker article, but I do know that a relationship could be drawn between them. In at least one state (Michigan or Minnesota, I forget, but I'm sure there's others), the legal definition of 'asbestos' is any inorganic substance that has a 100:1 length-to-width ratio or greater. By this definition, zinc whiskers are classified as asbestos and a manufacturer could be sued on the grounds that the company is exposing their workers to asbestos. This legal definition is different than the true geologic definition of asbestos, which has this description in addition to others but is limited to a small range of silicate minerals. As a side note, 95% of all asbestos does NOT cause lung cancer. They do, however, cause asbestosis, which is the scarring of lung tissue as these crystals get stuck in the lung, which I'm sure that zinc whiskers could cause.

I cannot remember which state this story came from, but some mining company that mined nothing similar to asbestos was sued for contaminating the river exiting the mining grounds with asbestos. After some scientific research was done, it was found that the mining company was keeping the river TOO clean. Some diatoms (aquatic microorganisms) that grow only in the cleanest of waters produced silica shells with a length-to-width ratio of 100:1 or greater, which met the legal definition. Unfortunately, I never heard about the conclusion of the lawsuit …

Chase Watkins
- Purcell, OK
2002

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Yes, certain particle sizes and shapes are troublesome regardless what material they're made of -- think of glass shards and the terrible bodily damage they can do despite how innocuous glass is as a 'chemical'.

Asbestosis is a real disease that had a number of industrial workers as its victims, and who have our deepest sympathy for their tragic fate.

But asbestos became a multi-billion dollar boondoggle, with the American golden goose butchered to feed our gluttonous legal industry. Now that our domestic plating industry is all but gone, it should be very amusing watching our legal industry trying to sustain itself by recovering damages for zinc whiskers from Chinese plating shops :-)

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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2005

I am a co-author of the article referenced elsewhere on this site. It is not for any individual to make a claim as to hazards of any material. I realize that the comparison to asbestos has raised some eyebrows. First, I would like to say that the article being distributed is an excerpt of a much larger article that was prepared to outline a very specific operational function in an attempt to recover a zinc needle (zinc whisker) contaminated specification environment.
The citing of asbestos properties was made for the purpose of highlighting the fact that it is the shape of the debris, NOT what the chemical makeup, that create inhalation issues.
OSHA's consideration of inhaled zinc. One must do considerable research to discover the documents referenced in the writing of that article. In fact, in a cursory search, the OSHA site has no specific listing for Zinc, but rather Zinc Oxide. For any business concerned with safe and legal operations in America, the issue requires referencing the OSHA classification of whatever the worker may be exposed to in the workplace. There is clear documentation available from OSHA that states inhalation of airborne Zinc particles (Specifically NOT Zinc Fumes nor Zinc Oxide) is considered to be much more than a respiratory irritant.

The following is taken directly from OSHA documentation referencing the hazards of inhaling zinc particles:

NAME: Zinc
IMIS: Z100
CAS: 7440-66-6
DESCRIPTION: Solid.

HEALTH EFFECTS: Irritation-Eye, Nose, Throat, Skin---Marked Respiratory Effects---Acute lung damage/edema Chronic(Cumulative) Toxicity-Suspect Carcinogen or mutagen

I trust this will clear up any questions as to whether statements made in that document are not in line with the OSHA listing.

I am not a metallurgist, nor a doctor. The document in question elsewhere on this site drew attention to the POTENTIAL hazards of inhaling zinc particles in order for our client to determine how his crew should proceed in the operation. Throwing caution to the wind is not prudent in dealing with large scale contaminations of any kind.

VRM

Valan Martini
- Atlanta, Georgia

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2005

"It is not for any individual to make a claim as to hazards of any material."

Sorry, but I couldn't disagree any more strongly, Valan.

It may not be safe to rely on an individual's claims -- but they're the starting point of most progress. Rachel Carson's was the seed that started the whole environmental awareness movement … while the bureaucrats were busy carpet bombing our suburbs with toxic insecticides and exterminating whole species like the Dutch Elm through shortsighted city planning. Carson, an individual, made claims and informed the world how recklessly we were acting under the direction of government bureaucracies.

Facts are facts irrespective of pronouncements. Various compounds are surely not actually carcinogenic in the USA but not in the EU, or vice versa, based on the differing pronouncements from the bureaucrats of the varying countries. Nickel is forbidden in jewelry in the EU, while our bureaucracies refuse to do anything at all about the heartbreak of nickel itch that the jewelry industry is inflicting on so many young women. Silence is not the answer!

From my own experience of 40 years designing electroplating plants, while OSHA was wishy-washying about whether hexavalent chrome was carcinogenic, it was terribly difficult to get chrome plating fumes treated with respect because some architects felt it "was not for any individual to say" that they were hazardous. If someone feels that something is hazardous, they should speak up; and if they feel that, in a world of limited resources and countless ever-present risks, zinc whiskers aren't a significant inhalation hazard for most of us, they should say that too.

Sorry, I can't find mention of your article on this site, Valan, beyond possibly "One article I recently read". If your referenced report is available, please provide a link; and if you can tell us what thread you are referring to on this site, I'd appreciate it. Thanks!

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

P.S.: This discussion of the shape of asbestos particles brings to mind Melanie's "Psychotherapy" :-)

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In silver and copper, the phenomenon has been known for about 425 years now, and for about sixty years in tin, zinc, and cadmium. Check out Scientific American [affil link to magazine on Amazonaffil links], 203:1, 1960, p. 64 -- although the focus is on whisker growth from vapor, basal growth is mentioned. Metal whiskers are discussed in the Handbook edition of R. Holm's 'Electric Contacts' ⇦[this on on Amazonaffil links].

Small amounts of lead, antimony, and a few other elements impede basal growth whisker formation -- probably by blocking material transport along grain boundaries.

Interest in metal whiskers is episodic, waxing when another costly failure is attributed to them.

Norman Helmold
- Greenbelt, Maryland
2005

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September 13, 2010

Cadmium Whiskers were noted by Bell Labs at least as far back as the 1940's and tin lead (i.e. solder) was touted as a readily available and well proven surface finish in industrial electrical widgets where appearance is not a factor. Cad was adopted because of its cheapness and effectiveness from an electrical point of view (Cd is fairly conductive, CdO is more conductive than many other cheap metal oxides). Solder coatings while more costly than Cadmium were still cheaper by far than silver and better electrically than Zn. Zn became popular in electrical applications where protecting gross structural elements from corrosion was of greater importance than good surface electrical conductivity. As time and technology progressed whiskers from Zn and Cd became more and more of a threat to the shrinking power levels of electronic signals and the shrinking spaces between the conductors that carry them.
As environmental awareness/political correctness (take your pick) started to pressure against the use of lead-containing finishes even inside electrical and electronic devices, pure tin was touted as the perfect substitute by the good folks who know little or nothing about electronics and metallurgy. Cheapskates in industry have always tended to want to use pure tin rather than tin/lead.
Turned out to be a real mistake to use pure tin with sensitive signals and tight conductor spacings though. It has done a lot of damage both to trivial consumer products that would probably be tossed anyway after a year or two as well as space satellites, medical devices, and military systems and transportation safety infrastructure.
Tin whiskers are a real threat, and while certain coatings can help mitigate the risk, nothing other than encapsulation in a truly hard material like epoxy will stop them. Such encapsulants render a system non-repairable and cause reliability issues of their own (cooling, residual stresses, CTE mismatches, etc).
Other metals that grow whiskers include indium, silver, lead, and gold (I have heard of but never seen this last one myself). None of these form whiskers that are a threat to reliability except inside of microcircuitry where dimensions are measured in nanometers rather than mils or millimeters. Ditto for whiskers from alloys like tin/lead solder. [Exception: silver is reported by some to form incredible whiskers in the presence of high concentrations of H2S, so look out if selecting a plating for use in electrical equipment around some petroleum processing equipment and chem labs (plating baths perhaps?) - generally the smell or the occasional operator keeling over dead will be a good clue.]
The exact mechanism of whisker growth is poorly understood, but it is generally regarded as mechanism whereby stressed plating layers relieve compressive stresses. Tin is the most studied and best understood for the most part. So plating should always be the second to last operation if whiskers are a concern, after all bending, cutting, drilling, etc. A stress relief anneal (at or slightly above reflow temperature should be last. Tin should not be used over metals with which it forms intermetallic phases that are less dense that the tin and metal that went into them (e.g. copper) but may be safer on surface where the intermetallics tend to be higher density (e.g. nickel). temperature cycling can be a stress inducer (especially with tin it seems). The presence of moisture is a factor (a poorly understood one).
Sound complex? Sorry! But there is a lot of info out there if you are willing to dig for it. I highly recommend nepp.nasa.gov/whisker/ as a great resource that is fairly specific to high reliability electronics but with links to stuff of more general interest.

Jonathan Kanner
- Goleta California USA