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parity
Gordan, having done some research in this area myself, I saw that part of this ...
Haldor
"Both the RoHS directive and the tin-whisker issue present challenges, but they ...
Understanding and mitigating tin whiskers
John O’Boyle, Maxim Integrated Products
11/15/2011 9:44 PM EST
“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.
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.
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.
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Kristin Lewotsky
11/16/2011 2:55 PM EST
What do you consider the best solution? Have you run into problems with tin whiskers in the past?
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zeeglen
11/16/2011 5:19 PM EST
Good article.
Yes, a power supply with mains input of 480VAC had two fuseholders spaced side by side, the expandable tin-only end clamps ended up about 6 mm apart (1/4") with the fuses installed. After a few years they started randomly shorting the mains ahead of the fuses.
Under an optical microscope on used but good units we could easily see several tin whiskers in various stages of formation reaching out to each other across the gap. Of course at some point in their growth a slight voltage spike would initiate an arc and form a high-current conductive plasma that would open the mains panel breakers on the 480 volt bus. Not good.
I seem to recall that the whiskers only grew inside the gap that spaced apart the 480VAC conductors, never did see any whiskers on the outer sides. Maybe the electric field within the gap was contributing to the whisker growth.
The temporary solution was to squeeze a gob of RTV into the gap and hope for the best. This was a few years back, do not know if this cured the problem. If it had been up to me I would have chosen an epoxy instead, but who knows, maybe those sharp whiskers might forced their way through hard epoxy. For a permanent solution would have inserted a plastic insulator into the gap.
I've also seen random shorts through epoxy-glass FR4 pcb material in higher voltage areas such as switching supplies with spikes up to 70 volts, mainly in areas around vias. Don't know if this was tin whiskers forcing their way through the FR4 or what caused it, the damage was destructive and permanent.
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Nol
11/18/2011 3:08 AM EST
What you describe in the last paragraph seems to be Conductive Anodic Filament (CAF) in PCB. Check a link below for more details: http://www.parkelectro.com/parkelectro/images/CAF%20Article.pdf
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zeeglen
11/18/2011 3:36 PM EST
Very interesting and informative. Thanks for the link.
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Gordon Davy
11/16/2011 5:21 PM EST
Mr. O'Boyle has written a superb article on the subject of tin whiskers, and documented it well, too. He states that a conformal coating "mitigates," not "eliminates," the risk of a whisker-caused short circuit.
The original purpose of conformal coatings is to prevent not whisker shorts but dendritic growth shorts. The coating covers a flat board and its lands and solder connections with an insulating layer that prevents the formation of a continuous layer of water between adjacent areas of metals with different electrical potentials, thereby preventing dendritic growth. (Among the many differences between dendrites and whiskers is that the latter grow from component terminations. They grow away from any surface, and if long enough can cause a short circuit.)
Getting a thickness on component terminations anywhere close to the Table 2 values, using _any_ of the commercially available conformal coatings has proved to be impracticable. (This impracticability can be seen in cross section by measuring the coating thickness on _any_ termination of _any_ component removed from _any_ assembly that has had a conformal coating thickness applied to meet the J-STD-001 requirements.)
To be concluded in next posting.
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Gordon Davy
11/16/2011 5:21 PM EST
The coating is so thin on terminations because it is applied as a liquid. Liquids do not coat the sharp edges so characteristic of terminations. The best that can be hoped for is that the whiskers that penetrate the coating from below (i.e., from the tin termination finish) do not penetrate the coating on adjacent terminations.
A new way of coating the tin, with a metal cap layer instead of a polymer, was presented in a paper at the most recent CALCE Tin Whiskers Symposium (available at http://www.ldfcoatings.com/slides/Day1-13-Davy.pdf). The most practical metal appears to be nickel, and it can be deposited exclusively on metal surfaces by what is known as "electroless" metal deposition. Whiskers penetrate some metals in days, but a nickel cap only about a hundred atoms thick (35 nm) has been found to be virtually impenetrable. The paper argues that a nickel cap thicker than this does not simply _retard_ penetration, but truly prevents it - permanently.
Gordon Davy, Ph.D.
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parity
1/18/2012 8:44 PM EST
Gordan, having done some research in this area myself, I saw that part of this problem was derived from mfg'rs trying to keep costs low. A nickel under plate between the copper lead frame and pure tin plate would essentially stop the intermixing of tin-Cu intermettalics which drive the formation of tin whiskers.
An over plate of electroless ni is another added step that producers do not want. Why? Their installed plating systems were designed for one pass tin lead alloy plating - not a two step plating process. It may work, but getting producers to adopt it is another question.
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RWatkins
11/17/2011 10:18 AM EST
Most of the manufacturers are using pure tin coatings for RoHS components. However, those of us assembling these parts are using Tin/Silver/Copper alloy solders to maintain RoHS status. The main impetus behind Tin/Silver/Copper alloys as we have been told is to avoid the beta-tin to alpha-tin transformation in cold-exposed applications. This paper addresses pure tin. What about these Tin/Silver/Copper alloys and whisker phenomena?
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J-TX
11/17/2011 10:37 AM EST
Conformal coatings seen to have the greatest impact in tin whisker growth mitigation, with Parylenes applied at 1 to 1.5 mil showing great ability to inhibit their growth.
Parylenes are solvent and catalyst-free thin organic coatings that offer solutions to many existing challenges of the electronic industry.
Parylenes truly conform to the parts due to their molecular level deposition characteristics.
Contact SCS Coatings for more details.
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rogerrobie68
11/18/2011 2:10 PM EST
I would blame these little buggers on a problem board or two...
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Magnus_RF
11/19/2011 6:42 PM EST
Excellent article and references.
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Haldor
11/28/2011 8:55 AM EST
"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."
Funny he would end the article with this rosy summary, when the bulk of the article is describing how there is really very little that can be done to avoid reliability problems with lead free circuit boards.
Exactly why is everyone still falling in line with this dumb, PC idea? Talk about diminishing returns. 80% of lead is in vehicle batteries which are total exempt, but we must stamp out the 0.03% usage of lead in ICs. What a joke.
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