Tag Archives: tin whiskers

Tin Whiskers and Toyota: Collision Course?

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New criticism of the reports by the National Highway Traffic Safety Administration and NASA Engineering and Safety Center that led the US Transportation Secretary to publicly absolve Toyota of unintended acceleration problems in its vehicles is breathing new life in what the mainstream media had decided was a closed story.

When the US agencies released their reports in February, Sec. Ray LaHood stated that the findings by the NHTSA and NASA proved Toyota’s electronics were not guilty of causing unintended acceleration. “The verdict is in,” LaHood said. “There is no electronic-based cause for unintended, high-speed acceleration in Toyotas.”

Not so fast, said Safety Research & Strategies, which this week went to press with a report condemning the earlier findings for everything from flawed analysis to conflict of interests.

In the report, SRS claims the tin whiskers found in the vehicle samples provided to NASA did in fact reveal a failure mechanism that was ignored in the NHTSA report, yet that mechanism in accelerator pedal sensor circuits can cause resistive shorts that could lead to acceleration.

The report has become a hot topic among a group of printed circuit board reliability experts, who are pointing to the “extremely small sample size” of vehicles used by NASA to perform its investigations. “There are millions of Toyotas on the road today but NASA was able to look at only a handful,” wrote Bob Landman of HRL Laboratories, on the IPC TechNet Listserv. “Despite the small sample size, they found whiskers.  The Law of Errors tells you what about this fact?  That whiskers are a significant finding.”

Landman noted that in one case, NASA found whiskers in a pedal assembly after a woman who had an incident of sudden acceleration was provided the defective assembly by the dealer that fixed her car. “She learned of the [Department of Transportation] investigation and gave them the assembly, and it found its way to NASA where [researchers] found whiskers shorting the leads of the potentiometer.

Landman also said NASA demonstrated a braking problem under a test track sudden acceleration simulation.  “A NASA driver was strapped in, a NASA passenger had two switches, one to cause sudden acceleration at 45 mph and the other to safely turn off the the sudden acceleration so the vehicle could be brought to a stop.  What happened?  When sudden acceleration was initiated, the throttle was at 100% so there was no vacuum assist and the driver, using both feet on the brake pedal, could not stop the vehicle! It was found that it would take 600 pounds of brake force on the pedal to cause the brake to slow down the vehicle. Clearly, the software does not allow the brake to override the pedal. This is a defective design.”

“Something is rotten in this [NHTSA] report, it seems to me, and SRS found it,” Landman said.

Questions on Tin Whiskers

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Folks,

In a recent post, I shared my perspective on the pluses, minuses and neutral aspects of lead-free solder assembly. In the minus category, I listed tin whiskers. A few people commented that tin whiskers were the biggest concern in lead-free assembly. I have trouble understanding this perspective. I’m not saying these folks are wrong, just that I don’t understand their viewpoint.

First, let me say that I appreciate the concern for tin whiskers in mission critical electronics such as military, aerospace and medical. I am also sympathetic to the fact that, even though these types of electronics are exempt from RoHS, they may have to use RoHS compliant products because non-RoHS compliant products may not be available.

When I discuss the topic of tin whiskers, people will point me to NASA’s tin whisker failures website . However, when one goes to the site, there are only about twenty tin whisker fails referenced, many due to bright tin plate. Bright tin plate should never be used in mission critical electronics as it is virtually assured of producing tin whiskers. In addition, many of the articles referenced do not talk about tin whisker fails. Few if any fails are discussed relevant to RoHS (i.e. almost all fails discussed are prior to July 2006.)

I do not want to minimize the significance of tin whisker fails, some of them cost 100s of millions of dollars (e.g., satellite failures). In addition, there have been a few papers that have discussed the formation of tin whiskers even if mitigation techniques are used. Tin whiskers clearly can cause problems, but do not appear to be common, especially if mitigation techniques are used.

So here is my question, who knows of any verified tin whisker fails when tin whisker mitigation techniques were used? Tin whisker mitigation techniques typically use 2% bismuth or antimony in the tin, assure that the tin has a matte finish and use a nickel strike plating between the copper and the tin to minimize copper diffusion into the tin.

Surely if tin whiskers are a major concern, there should be many fails in the over $3 trillion worth of RoHS compliant electronics manufactured since July 2006.

Cheers,

Dr. Ron

Risk Priority and Tin Whiskers

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Folks,

Tin whiskers continue to generate considerable interest. People often suggest that their risk is great and yet unknowable. RPN may help to clarify the TW risk. What is RPN? It is the risk priority number from failure mode and effect analysis.

As this link tells us:

A failure modes and effects analysis (FMEA), is a procedure in product development and operations management for analysis of potential failure modes within a system for classification by the severity and likelihood of the failures. A successful FMEA activity helps a team to identify potential failure modes based on past experience with similar products or processes, enabling the team to design those failures out of the system with the minimum of effort and resource expenditure, thereby reducing development time and costs. It is widely used in manufacturing industries in various phases of the product life cycle and is now increasingly finding use in the service industry.

RPN is an important part of FMEA. It is the product of three numbers that range from 1 to 10. The first number is the severity (S) of a possible fail. A “10” would be given if the failure injured someone, “7” would be assigned if the failure caused a high degree of customer dissatisfaction, whereas a “2” would be given if the failure has only minor negative effects.

The second number is occurrence (O) of a fail. The highest rating is a “10,” which would be a failure every day (reminds me of Windows ME!) or one fail in 3 events, whereas a “7” would be a failure every month or one in 100 events. A “2” is a six sigma fail rate.

The last number is detection (D) of a potential fail. A”10” would suggest that the detection of a potential fail is either not performed or not possible. A “7” is a manual detection approach that may not be reliable, whereas a “2” is 100% effective potential failure inspection.

So obviously a product with a RPN of 10 x 10 x 10 = 1000 is a disaster, its failure is dangerous, frequent and incapable of being detected beforehand. Industry rules of thumb suggest that and RPN of 200 needs to be addressed and an RPN of 75 is usually considered acceptable.

Let’s look at a “ball park” RPN for tin whiskers. We will assume the application is a critical IC in a PC. Let’s assume that a severity rating of “S” of 8 (failure renders the unit unfit for use) is reasonable. TW are hard to inspect for future fails, so detection, “D,” could be as high as a 10. At this point we are at 8 times 10 equals 80 for both. A bad start.

Occurrence (“O”) for TW failure modes is dramatically different. When trying to assess the occurrence of TW fails, one is often directed to NASA’s web page . Many reference this web site that lists a little more than a score of TW fails. What escapes me is that people don’t seem to appreciate the rarity of less than 100 fails in decades of data collection. Surely TW fails are not common. I could find no report of a failure of a RoHS compliant product anywhere on the internet. So it would be hard to rate “O” any higher than a “2.” I suspect that the reason few TW fails have apparently occurred is due to TW mitigation techniques that are widely practiced.

I would expect that “modern” process defects like the head-in-pillow or graping defects could have a much higher RPN than TW, if assembled without proper process controls and materials. However, there is little need to worry about these defects either, if you use the right solder paste and practice some assembly process precautions.

Sans Data, Toyota ‘Whiskers’ Claims Razor Thin

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After my recent post on the fact that no data link tin whiskers to the Toyota sudden acceleration issues, there continue to be more posts saying things like “Tin Whiskers Implicated in Unintended Acceleration Problems.”

Many of these posts link back to the earlier TechEye post. The basis for all of the posts is a paper written by EurIng Keith Armstrong titled, “Toyota ‘Sticking Pedals’ Recall is a Smokescreen,” and subtitled, “Their sudden unintended acceleration problem is caused by electronics either due to EMI, lead-free soldering or software ‘bugs.’ ” It does not appear that Armstrong’s paper was sponsored or refereed.

Since it appears that this entire wave of reporting implicating tin whiskers, in this important issue, emanates from Armstrong’s paper, it is helpful to quote his entire comments on tin whiskers

9.0 Lead-free soldering:
In recent years, various countries and trade blocs (including the European Union) have banned the use of lead on electrical solder, on the basis that lead going into landfill when electrical and electronic products are disposed of is bad for the environment, and hence for people.

But many accuse them of being shortsighted -– lead has been added to solder in quite large amounts for many decades because it made the other main constituent, tin, behave much better, considerably improving reliability.

Now that lead has been removed from solder, which is now mainly tin (with a little silver and copper added) all sorts of new possibilities arise for short-circuits and open-circuits, and intermittent shorts and opens, mainly on printed circuit boards (PCBs) and mainly associated with small-footprint integrated circuits (ICs), especially ball-grid arrays (BGAs).

Its really just another cause of intermittent or fixed short-or-open circuits in electronic PCBs and modules – but one that would not have been any problem until a few years ago, and so could have caught Toyota by surprise.

John R Barnes has created a monumentally huge library of references to the problems of lead-free soldering, especially tin whiskering, see www.dbicorporation.com/rohsbib.htm. Prepare to be totally overwhelmed!

Removing lead from solder has the following effects:

9.1 Tin whiskers
These will grow out of soldered joints and can contact other conductors, causing short-circuits between PCB copper traces and the pins of connectors. They are often no longer than 0.5mm (about 1/50th of an inch) but can grow to 1mm (about 1/24th of an inch) or longer, especially in damp conditions.
Even at 1/50th of an inch they can short between the pins on a modern integrated circuit (IC). And the process of removing the PCB for inspection can brush them off, so you never find them.
And if you didn’t accidentally brush them off, they are so thin they are very hard to see – you need a powerful microscope. They are as fine as the finest spider-web threads, yet can carry sufficient current to short-out the electronics. You won’t see them unless you are looking for them.
Being so thin, they can wave around in the breeze and/or due to shocks, vibration and acceleration, causing intermittent short-circuits.

The iNEMI organization has published guidelines (www.inemi.org) on how to ensure that tin whiskers don’t grow too long, but I don’t know to what extent these are followed by suppliers of electronics to the car industry in general, or Toyota in particular.

(Photo courtesy NASA)

Note that, in this paper, there are no data or any evidence regarding tin whiskers discussed from investigating any of the vehicles in question. All of this paper is opinion. In addition, the title of Armstrong’s paper leaves no room for any other cause: it has to be electronics or software. This position is very strong indeed for having no supporting data.

More recently Bob Landman added these comments to the tin whisker discussion: “The increased use of electronics in automobiles when mixed with RoHS can make for a deadly cocktail. We don’t know what the causative agent [in regard to the Toyota recalls] was, but I have heard recently of new autos showing up at dealers that will not start. That cause has been linked to tin whiskers.”

Bob heard this. There is no report and no data. Until Bob gives us a reference for some analysis and data, his comments are little more than hearsay. I searched the web in vain to find information related to Bob’s quote. In addition this comment is a little surprising, tin whiskers are usually associated with a certain amount of aging, hence not usually found in new products.

That tin whiskers exist and cause failures is irrefutable. NASA has an excellent website related to tin whiskers and failures caused by them. However, the total number of tin whisker fails reported is less than 100. Many other types of electronic failure modes would appear to be much more common.

My purpose of writing this post is not to suggest that tin whiskers are not a concern in lead-free electronics. However, it is a fundamental principle in engineering and science to only make pronouncements on how something failed, when they can be supported with data. No data support implicating tin whiskers in the Toyota incidents. It is also troubling how readily many people referenced the work of Armstrong without apparently reading what he said and checking his sources and lack of data.

Cheers,
Dr. Ron

Tin Pest: A Forgotten Concern in Pb-Free Assembly?

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If tin pest were a living thing it might complain, “I can’t get no respect.” Reason: Tin whiskers get so much attention, while tin pest is forgotten.

Although my feeling is that tin whiskers are a greater concern, the number of recorded fails related to tin whiskers is less than 100. Compare this to the number of hard drive fails — about 100 million! With that in mind, let’s learn a little about tin pest.

Tin is a metal that is allotropic, meaning that it has different crystal structures under varying conditions of temperature and pressure. Tin has two allotropic forms. “Normal” or white beta tin has a stable tetragonal crystal structure with a density of 7.31g/cm3. Upon cooling below about 13.2ºC, beta tin turns extremely slowly into alpha tin. “Gray” or alpha tin has a cubic structure and a density of only 5.77g/cm3. Alpha tin is also a semiconductor, not a metal. The expansion of tin from white to gray causes most tin objects, afflicted with tin pest, to crumble.

The macro conversion of white to gray tin takes on the order of 18 months. The photo — likely the most famous modern photograph of tin pest — shows the phenomenon quite clearly.

This photo is titled “The Formation of Beta-Tin into Alpha-Tin in Sn-0.5Cu at T <10ºC" and is referenced from a paper by Y. Karlya, C. Gagg and W.J. Plumbridge, "Tin Pest in Lead-Free Solders," in Soldering and Surface Mount Technology, vol. 13 no. 1. 2000, 39-40.

The tin pest phenomenon has been known for centuries and there are many interesting, probably apocryphal, stories about tin pest. Perhaps the most famous is of the tin buttons on Napoleon’s soldiers’ coats disintegrating on their retreat from Moscow. Since tin pest looks like the tin has become diseased, many in the middle-ages attributed it to Satan as many tin organ pipes in Northern European churches fell victim to the effect.

Initially, tin pest was called “tin disease” or “tin plague.” I believe that the name “tin pest” came from the German translation for the word “plague” (i.e., in German plague is “pest”).

To most people with a little knowledge of materials, the conversion of beta to alpha tin at colder temperatures seems counterintuitive. Usually materials shrink at colder temperatures, not expand. Although it appears that the mechanism is not completely understood, it is likely due to gray alpha tin having lower entropy than white beta tin. With the removal of heat at the lower temperatures a lower entropy state would likely be more stable.

Because the conversion to gray tin requires expansion, the tin pest will usually nucleate at an edge, corner or surface. The nucleation can take scores of months, but once it starts, the conversion can be rapid, causing structural failure within months. The effect is also cumulative, so warming the sample will stop the growth, but it will continue once the sample is cold again.

Although tin pest can form at <13.2ºC, most researchers believe that the kinetics are very sluggish at this temperature. There seems to be general agreement in the literature that the maximum rate of tin pest formation occurs at -30º to -40ºC. What is the real risk of tin pest in Pb-free electronics? Not great. Modern researchers have had trouble reproducing it, even in the lab. The reason for this is likely that test samples contain small amounts of metal "contaminates" (<0.1%), such as bismuth, antimony, lead and a few other metals. These trace metals solid solution strengthen the solder and inhibit the expansion needed to form tin pest. Unfortunately, copper and silver (the typical Pb-free metals added to tin), do not appear tin inhibit tin pest growth.