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

Lack of Concern for Tin Pest as a Reliability Issue in Mission Critical Products Still Hard to Understand

Folks,

My recent post on tin whiskers sparked the memory of tin pest in my mind. I have to admit, that with all of the legitimate reliability concerns related to tin whiskers, I am surprised that there has been essentially no parallel concern for the risks of tin pest.

Admittedly, tin pest is much more rare than tin whiskers. Although many complain that we don’t understand tin whiskers, we can create them easily and make the vast majority of them go away. Whereas, it has been shown to be very difficult to create tin pest.

For those who want a refresher on tin pest see this blog posting or my survey paper “Tin Pest: Elusive Threat in Lead-Free Soldering?” Journal of Failure Analysis and Prevention, vol. 10, no. 6, December 2010 , pp. 437-443(7).

Tin pest is a result of an allotropic transformation of tin from its beta phase (white or normal tin) to its alpha phase (gray tin) at temperatures below 13oC. This transformation is accompanied by a change in density from 7.31 g/cm2 to 5.77 g/cm2. The reduction in density requires the tin to expand, thus destroying the structure of the original tin object or solder joint as seen in the figure below.

With tin pest being so rare, why am I concerned with it as a reliability exposure? With billions of solder joints in mission critical circuit boards exposed to cold for many years, it would seem inevitable that some tin pest would form. The effect of the cold is cumulative, it does not get reversed when the weather becomes warm. Applications most at risk would be automobiles, mobile phone towers, and military equipment.

I wouldn’t be surprised that, with typical tin whisker mitigation, that unmitigated tin pest might be more common.

What is the fix? By adding about 0.5% antimony or 2% bismuth to lead-free solder, tin pest can be essentially eliminated. An added blessing would be suppression of tin whisker formation also. However, adding even these small amounts of antimony or bismuth to lead-free solders would require a thorough evaluation. Even these small additions of alloying elements can dramatically change the properties of a solder.

Best Wishes,

Dr. Ron

Revelations at ACI

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

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

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 13.2°C, 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 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.

”The


The photo at left 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, 13/1 [2000] 39-40.

This 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 while 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 counter intuitive. 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.

Since the conversion to grey tin requires expansion, the tin pest will usually nucleate at an edge, corner, or surface. The nucleation can take 10s of months, but once it starts, the conversion can be rapid, causing structural failure within months.

Although tin pest can form at <13.2°C, 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 -30° to -40°C. How much of a worry is tin pest in practice? Probably not too much. Small amounts (0.01 to 0.1%) of some metals, most notably antimony and bismuth, inhibit the formation of tin pest, probably by solid solution strengthening. Because most tin will have such impurities, researchers have actually found it hard to produce tin pest in the lab. A concern, of course, is that these impurities are uncontrolled, leaving open the chance of tin pest showing up in some cold temperature applications. I have written a paper that discusses tin pest in more detail. If you are interested, send me a note and I will send it to you.

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

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.

Tin pest (source: Karlya et al)
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.