# Using FMEA to Determine Your Tin Whisker Mitigation Strategy

In our last post, we discussed techniques to mitigate tin whiskers (TW). To help determine what your tin whisker mitigation strategy should be, consider using failure modes and effects analysis (FMEA). The central metric of FMEA is the risk priority number (RPN). For tin whiskers, the RPN is equal to the product of: (1) the probability of tin whiskers (P); (2) the severity, if a tin whisker exists (S); and (3) how hard it is to detect a tin whisker (D).  In equation form:

RPN = P*S*D

As a first example, consider a consumer product, like a mobile phone with a life of 5 years. With mitigation, on a scale of 1 to 10, P might be 2. For S, we might rate it at a 3, as a failure in the device is unlikely to cause severe harm to anyone. Detection (D) is a problem because the tin whiskers that form later cannot be detected during manufacturing; hence, we would have to rate D as a 10. So, the RPN is: 2*3*10 = 60, which is not too high. Therefore, with P and S at relatively low values, a tin whisker mitigation strategy would likely be successful for any consumer product. It should be pointed out that determining the RPN numbers would almost certainly require supporting data, brainstorming sessions, and a buy-in from the entire product team. The team would also have to determine any appropriate mitigation strategy such as avoiding bright tin coatings on component leads and perhaps using a flash of nickel between the copper and the tin (Figure 1).

Figure 1. In  mission-critical products, coatings may be required. It is almost impossible for a TW to penetrate both layers of coating as shown above.

Now consider a mission-critical product, such as certain types of military equipment. If we assume that the electronics have a service life of 40 years and that a failure could cause bodily harm or death, we could likely end up with a consensus that RPN = 10*10*10 =1000, the highest RPN possible. This situation would demand that special tactics be used to address the tin whisker risk. These tactics were discussed in my paper and presentation given at SMTA Pan Pacific 2019.

Cheers,

Dr. Ron

# Tin Whiskers IV: Mitigation

Folks,

In the last post on tin whiskers, we discussed detection. In this post, we will cover mitigation. Since compressive stresses are a primary cause of tin whiskers, minimizing these stresses will help to mitigate tin whisker formation. There are several approaches to accomplish this compressive stress reduction. The first is to establish a process that produces a matte finish as opposed to a bright tin finish. Experience has shown that a satin or matte tin finish, which has larger grain sizes, has lower internal compressive stresses than a bright tin finish. Studies have shown that avoiding a bright tin finish alone can reduce tin whisker formation by more than a factor of ten. Thicker tin layers will often reduce compressive stresses.

Since a major source of the compressive stresses in tin is due to copper diffusion into the tin, minimizing this diffusion will significantly reduce tin whisker formation. One proven approach to minimizing copper diffusion is to have a flash of nickel between the copper and the tin. Since nickel does not readily diffuse into the tin after initial intermetallic formation, tin whisker formation can be all but eliminated in many cases.

Adding bismuth to the tin, in small amounts, can also reduce tin whisker formation. The bismuth solid solution strengthens the tin. This strengthening will often reduce tin whisker formation.

Another mitigation approach is the use of coatings. Acrylics, epoxies, urethanes, alkali silicate glasses and parylene C have been used. Parylene C appears to be the most promising.

Often a tin whisker will penetrate the coating as seen in Figure 1. However, to be a reliability risk, it must penetrate a second coating.

Figure 1. A tin whisker about to penetrate a polymer coating. Source: Dr. Chris Hunt, NPL.

This situation is almost impossible as the tin whisker is fragile and will bend as it tries to penetrate the second layer of coating. See Figure 2. So, coatings can be a very effective tin whisker mitigation approach.

Figure 2. To be a reliability concern, a tin whisker must penetrate two protective coatings.

The next and last tin whisker post will be on using FMEA (failure modes and effects analysis) to develop a tin whisker reduction strategy.

Cheers,

Dr. Ron

# Tin Whiskers 101: Part III: Detection

Folks,

One of the great challenges of tin whiskers is detecting them. When one considers that their median thickness is in the 3 to 5 micron range (a human hair is about 75 microns,) they can be hard to see with direct lighting. Right angle lighting facilitates visual detection. See Figure 1. In this figure, Panashchenko shows that with direct light (left image), it is impossible to see the tin whisker, however with right angle light the tin whisker jumps out.

Figure 1.* It is not possible to see the tin whisker with direct lighting as in the left image. However, in the right image, right angle lighting makes it easy to see the tin whisker.

In her excellent presentation, “The Art of Metal Whisker Detection: A Practical Guide for Electronics Professionals,” Panashchenko offers these tips for identifying tin whiskers with a stereo optical microscope:

• Use a 3x to 100x stereo microscope
• Have the ability to tilt the sample in 3 axes
• Use a flexible lamp that allows multiple angles of illumination, do not use a ring light
• Use a LED or fiber optic lighting, not incandescent lights which can cause shadowing
• Vary the brightness of the light source

The most important tip is to vary the angle of lighting while varying the magnification. Thus, analyzing a sample should take several minutes, at least. However, even the most thorough inspection may miss some tin whiskers.

In the next post, I will discuss mitigation techniques.

Cheers,

Dr. Ron

*The image is from Lyudmila Panashchenko, “The Art of Metal Whisker Detection: A Practical Guide for Electronics Professionals,” IPC Tin Whisker Conference, April 2012.

# Tin Whiskers 101: Part 2: What Causes Them

Folks,

Continuing our series on tin whiskers. In the last post we discussed what they are. in this post we will discuss what causes them.

Tin whiskers are primarily caused by compressive stresses in tin. The most common cause of the stresses is copper diffusion into the tin as seen in Figure 1a. Such diffusion is common when tin is plated, melted or evaporated on copper. Copper preferentially diffuses into tin exacerbating tin whisker production.

Figure 1. Some causes of tin whiskers

Another cause of tin whiskers can occur when the tin is plated, melted or evaporated on a material that has a lower coefficient of expansion than the tin, such as alloy 42 or ceramic. When temperature increases, the tin is constrained by the lower coefficient of expansion material. This constraint causes compressive stresses in the tin that can result in tin whiskers. See Figure 1b.

Less common causes are corrosion, as seen in Figure 1c and mechanical stresses as seen in Figure 1d.

Since copper diffusion is one of the most likely causes of tin whiskers, this mechanism deserves elaboration. The left image in Figure 2 depicts the mechanism of copper diffusion into tin. The mechanism is so strong that the diffusion of the copper often leaves voids in the copper. Such voids are called Kirkendall voids. The right image in Figure 2 is an x-ray map of copper (green) diffusing into the tin (black).

Figure 2. Copper diffusing into tin.

Clearly, one way to minimize this type of tin whisker growth is to prevent copper diffusing into tin. In a future post, we will discuss this and other tin whisker mitigation techniques.

Cheers,

Dr. Ron

Best Wishes,

# Tin Whiskers 101: What Are They?

Folks,

Tin whiskers are very fine filaments or whiskers of tin that form out of the surface of the tin. See Figure 1. They are the result of stress release in the tin. Tin whiskers are a phenomenon that is surprising when first encountered, as their formation just doesn’t seem intuitive.

Figure 1. Note how thin a tin whisker can be compared to a human hair. The image is from the NASA Tin Whisker Website

They are a concern, as they can cause electrical short circuits or intermittent short circuits as a fusible link. Lead in tin-lead solder greatly suppresses tin whisker growth. Therefore, with the advent of lead-free solders there is a justifiable concern for decreasing reliability due to tin whisker growth in electronics.

Tin whiskers can vary in length and width, as is seen in Figure 2. Note that although only about 10% are as long a 1000 microns (1mm). That length and occurrence rate is such as to cause many reliability concerns.

Figure 2. The length and width of some tin whiskers. The source is also the NASA Tin Whisker Website.

Over the following weeks I plan to post how tin whiskers form and strategies to alleviate them. Most of the information I will post comes from a paper I presented with Annaka Balch at the SMTA PanPac 2019.

NASA has an excellent website that provides much information about tin whiskers and is a source for historic critical failures caused by tin whiskers.

Cheers,

Dr. Ron

# My BBC Interview on Tin Pest

Imagine my excitement when Laurence Knight of the BBC contacted me to see if I was interested in being interviewed on the topic of tin pest, with a secondary discussion on tin whiskers. After a 30-minute phone call, it appeared that I passed muster, as I was asked to come to their studio to have a formal interview. Immediately, visions of visiting London crowded my mind. I haven’t been to London in a while and I would like to see the Tower of London and the London Science Museum again. Suddenly, a dreadful thought sweep over me, they probably have a studio in the US, perhaps Boston. So, in my mind, I quickly settled on a visit to the Boston Science Museum and the Isabella Stewart Gardner Museum to see if there is any news on the fate of the art treasures stolen 25 years ago. Rembrandt’s “The Storm on the Sea of Galilee” was one of the paintings stolen in the 1990 robbery at the Isabella Stewart Gardner Museum in Boston.

Even this meager plan was soon dashed, when Laurence informed me that they could likely use a PBS studio in Vermont or New Hampshire. I actually ended up in a radio studio at Dartmouth. Somehow by using an ISDN telephone line and recording at Dartmouth and in the UK they can achieve acceptable fidelity.

From the outset, I wanted my message to be:

1. The reliability concerns for tin whiskers are well founded, however there are many mitigation and design techniques that can reduce tin whisker risk.
2. Tin pest is much rarer than tin whiskers, however there appears to be little effort in mitigating tin pest at all. This lack of attention may cause some tin pest failures in cold environments.
3. Interestingly, the mitigation for tin pest (2% bismuth or 0.5% antimony) also dramatically suppresses tin whiskers.

So, on January 20, I was interviewed in the Dartmouth radio station (alas only ¼ of a mile from my office) for 20 minutes. The broadcast occurred on January 29. I didn’t know that my interview was part of a much larger story on tin metal. The 20 minutes I spoke was pared down to just a few. I let you decided if the trimmed version got the message across I intended. I am speaking about 35% through the audio file.

Cheers,

Dr Ron

# IPC Tin Whisker Conference Sheds New Light

Folks,

I just returned from SMTA/INEMI’s Medical Conference in Milpitas (near San Jose/San Fransisco), where I spoke on tin whiskers. I then quickly traveled to Costa Mesa (400 miles south, near Los Angeles), to IPC’s Tin Whiskers/Reliability Conference, where I spoke on Weibull Analysis. Both shows where attended by 80-plus people.

Most noteworthy, at the IPC Tin Whiskers Meeting, was Raytheon’s Dave Pinsky’s presentation, titled: “Tin Whiskers at a Large Defense OEM: Past. Present, Future.” In addition to discussing claims that some vendors have of producing whiskerless tin coatings and other topics, most helpful was his mention of a report by the Government Electronics and Information Technology Association (GEIA) on Tin Whisker Risk Mitigation. This standard, GEiA-STD-0005-2 discusses mitigation details thoroughly. Not only has the knowledge of tin whiskers increased, there appears to be more confidence in coatings for mitigation. However, in mission-critical applications, a well thought out, multiple mitigation approach to tin whiskers is still needed. The image below, taken by NASA, shows a tin whisker next to a human hair. Tin whiskers are very thin indeed!

Cheers,

Dr. Ron

Some time ago I wrote a post, “Questions on Tin Whiskers.” Reader Michael responds below. He makes some good points.

Dr. Ron, I’m responding to your blog regarding tin whiskers. I actually have a failure analysis report I did a couple of years ago in which failure of our product was due to this issue and occurred on a part that came into RoHS compliance only 3 months prior.

I’m not sure that your question of identifying whisker issues in product that proper steps have been taken to mitigate the problem is a constructive one. The fact is that many of the component manufacturers from overseas jumped into compliance without any thought or regard to this issue thereby flooding the industry with components such as plagued my company. We have not had this issue since we’ve specified an alternate finish.

These whiskers are so delicate that most problems disappear when the technician starts to work on the failed unit and the problem never re-appears so it is written off as an anomaly, loose/bad connection and not investigated any further. It was only my own curiosity as to the number of “no problem found” failures of our keypads we had suddenly encountered that caused me to dig deeper and when I looked into the connector I was amazed at the crystal city staring back at me. I couldn’t believe what I was seeing after all of these years.

After seeing this problem first hand I became, and am, quite convinced that there were and are people who will be losing life, limb, and property because this forced compliance with its risk was not given proper worldwide attention.

A popular topic on my blog is solder density calculations. Rhonda writes

I am a precious metals recycler and would very much appreciate your verifying the validity of an equation that approximates the Karat Value of various alloys of gold based on S.G. which I will call density or “D,” and the Karat Value is “K.” The equation is seems to hold relatively true even when the exact composition of the alloy is unknown, although the percent of error obviously will increase as density decreases. I would also appreciate not only verification but also more specific information on percent of error for densities below about 14 or 15 g/cc. Here is the equation:

K = 0.0089D^3 – 0.550D^2 + 12.5299D – 77.06

Thank you so much for whatever assistance you can provide.

These types of equations can only work for one alloying metal with the gold.  This one is only for copper.  It is also calibrated in Rhonda’s favor as it reads the karat level about 10% low.   I was able to determine this by using the Excel Solder Density worksheet that I developed. If the alloy was gold and lead, a 50% by weight gold (12 karat) would show as 15.7 karat with this equation and Rhonda would lose her shirt.

In response to my blog post on copper as the precursor to civilization, Harvey writes about pollution from early mining operation.

Also interesting, early copper mining and processing led to the first examples of human induced environmental damage. There are documented sites in the Alps where copper processing by prehistoric peoples has left areas treeless to this day, due to heavy metal contamination.

Mining and smelting were very tough businesses in ancient days.  In addition to pollution, many workers died from toxic fumes.

Dr. Ron

# Tin Bells Going Off

You may remember that more than a year ago there was much speculation that tin whiskers may be behind the Toyota unintended acceleration problem. At the time I spoke out because there was no data to support the speculation. Now there are data, as Mike Pecht and his CALCE Team at the University of Maryland have found numerous tin whiskers in the Toyota brake assemblies of concern.

Although the tin whiskers were not implicated in any failure, their presence is cause for alarm, and action should be taken to address this issue. Tin whiskers should not be found in mission critical devices. Pecht’s team has an algorithm that calculates the risk from tin whiskers that are discovered. The risk is 140 per 1 million — not high, but with a million or so Toyotas on the road, clearly this is cause for alarm.

As you may know, I live in Woodstock, Vermont.  Many friends have asked how we are doing after hurricane Irene. Personally, my wife and I escaped with no damage to our house and only a bit of inconvenience (no water for 5 days). The town of Woodstock suffered considerable damage, but was, on the whole, fortunate. Some of the neighboring towns had all roads in and out washed away. Route 4 between Woodstock and Rutland has numerous sections destroyed. The flooding was declared by the governor to be the worst disaster in Vermont history. The photo is from the Valley News. It shows a wooden pedestrian bridge built to carry supplies into Bethel, VT by foot. There is no passable road, even for ATVs.

Best,

Dr. Ron

# Tin Liability: Careless Whiskers and Toyota Acceleration

A failure mode is reemerging that has been responsible for the loss of billions of dollars worth of satellites, missiles and other equipment — the culprit is the electrically conductive entities known as ‘tin whiskers’.  Now one research group says that tin whiskers may be responsible for the sudden acceleration in Toyota Camry models from the year 2002 and possibly beyond.

Earlier this year we reported that the US Department of Transportation (DOT) said that Toyota’s problem was not in electronics.

Now, University of Maryland’s Center for Advanced Life Cycle Engineering or CALCE researchers have found the potential for tin whiskers in the electronic control module or ECM.  Circuits Assembly broke the story, quoting the CALCE report as follows:

“The ECM contains surface mount electronic devices connected with tin-lead solder to a multilayer PCB. … Interconnect terminals of the perimeter leaded devices were found to be plated with tin. In addition, tin plating was found on terminal pins of the edge connections. As previously discussed, tin-finished leads can grow tin whiskers which can lead to unintended electrical shorts.”

“We know whiskers can form on tin finished terminals,” said Michael Osterman, senior research scientist and director of the CALCE Electronic Products and System Consortium. said.  “In this case, Toyota has tin plating in a rather sensitive area, where the system relies on changes in resistance to provide a signal for acceleration.”

The studied pedals furthermore have been shown to cause shorts known to spur sudden unintended acceleration.

The odds of tin whiskers: 140/million. Someone known to this blogger recently drove a 2010 Camry and noticed subtle but perceptible decelerations that were not led by the driver. Was it tin whiskering?  Hard to say, even CALCE’s study figures that the whiskers would only form in 140 cars per million, which is statistically very significant but als makes it statistically unlikely that my friend’s only Camry experience would be on the wrong side of those odds.

It’s also worth noting that the whisker syndrome is probably not limited to Toyotas.  Nonetheless, the spotlight has fallen where it has fallen, and tin whiskers pose a serious problem in that warrants attention.

Tin whiskers. Tin whiskers develop — or may develop — on any product type that uses lead-free pure tin coatings.  Thus, in greener, lead-free products, tin whiskers can pose a major safety, reliability and potential liability threats to all makers and users of high reliability electronics and associated hardware. The CALCE brain trust concluded that existing approaches are not sufficient to control tin whiskering in high-reliability systems such as automobile electrical systems.

US Secretary of Transportation said Toyota is “all clear” in February. The official blog of the US Secretary of Transportation on February 8, 2011 stated:

NASA engineers pored over more than 280,000 lines of software code looking for potential flaws that could initiate an unintended acceleration incident. Alongside NHTSA, they bombarded vehicles with electromagnetic radiation to see whether it could make electronics systems cause the cars they control to gain speed.

And today, their verdict is in. There is no electronic cause behind dangerous unintended acceleration incidents in Toyotas.

We will continue to follow this story.

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