Tag Archives: lead-free

Pb-Free Sky is Not Falling

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Folks,

Although a few have suggested that lead-free reliability is an oxymoron, currently most people that have studied the reliability of SAC3XX and SAC105 Pb-free solders would conclude something akin to what Denny Fritz wrote in response to one of my posts:

No one I know will dispute your ranking of SAC better than SnPb solder using the commercial temperature cycle [Dr. Greg] Henshall uses – 0C to 100C. But, harsh environment electronics have to perform to either -40C or -55C, and most use a top end cycling temperature of 125C. IT IS IN THAT WIDE THERMAL CYCLE TESTING THAT SnPb outperforms SAC solders.

It is interesting to consider however, that almost all discussions on lead-free solder reliability are based on lab-based thermal cycling and drop shock testing. What about field results? It occurred to me that I knew someone who might have an answer.

Vahid Goudarzi is a Director of NPI Advanced Manufacturing Technology at Motorola and owns a Six Sigma Black Belt. He was the technical leader in Motorola’s efforts for lead-free and RoHS compliant assembly in its mobile phone products. There are few people I know that are more knowledgeable in electronics assembly than Vahid. Motorola was a very early adopter of lead-free, seeking the advantage of tighter lead spacings that lead-free allows. So, Vahid has been working on lead-free processes since the late 1990s. Motorola has been shipping lead-free mobile phones since 2001. With over 100 million mobile phones in the field since then, Motorola has quite a bit of lead-free field data. I asked Vahid if he could comment on these data. Here is his response:

In general, the reliability of lead-free solder is equal or better than leaded solder except for BGA/CSP/WLCSPs. The high silver content in SAC387 resulted in poor drop performance of these packages as the joints are very brittle. This issue can be addressed by reducing the Ag content of the solder balls.

Being an early adopter, Motorola qualified the near-eutectic SAC387 solder. So, with SAC387 and SAC105 solder balls, Motorola’s field data (for about ten years and over 100 million mobile phones) shows equal or better reliability than leaded solder. While these data do not necessarily support other applications, they are encouraging.

Another encouraging thought is that, since its debut (with RoHS now about to celebrate its fifth anniversary),  about $4 trillion worth of lead-free electronics have been manufactured with no shocking reliability problems.

Although admittedly anecdotal, the IT folks at Dartmouth’s Thayer School of Engineering have purchased over a million dollars in lead-free electronics since RoHS. They have noticed no difference in reliability. This is enough gear, and time, to have the beginnings of statistical confidence. Compare this to the advent of Microsoft’s Vista, it was viewed by these folks as a step backward and they immediately took action to prevent Dartmouth from adopting it. Yet, lead-free adoption went by unnoticed. The biggest reliability problem with PCs is still hard drive failure.

So concerning lead-free field reliability: The sky is not falling!

Best Wishes,

Dr. Ron

Which Surface is Best?

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RoHS has been in effect since, when, 2006? Pretty close to five years now. It’s been around long enough that there’s even talk of follow-on legislation. All of the PCB fabricators have pretty much figured out how to deal with RoHS. There are Pb-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don’t think industry has selected one PCB finish as the “standard” preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It’s inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don’t see quite so much Pb -free HASL, though.

If you’re working with small geometry parts, then you really need to go to immersion silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We’ve seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don’t see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don’t have one preferred finish. It seems confusing, but really it’s not that different than any other product. There certainly isn’t just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

http://blog.screamingcircuits.com/

Pluses and Minuses of Pb-Free Solder

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Folks,

I thought I would take a stab at listing the minuses, pluses, and “it’s a wash” aspects of assembling with lead-free (LF) solder. Here are my first thoughts. Please tell me what I missed or disagree.

Minuses

1.    Pb-Free requires higher reflow temperatures
The Tm for LF solders, in the 217-229C range, has created numerous challenges:

a.      PWB warpage and damage

b.      Component damage

c.      New defect modes such as graping and head-in-pillow defects (although concurrent reduction in solder paste deposit sizes for 0201 and 01005 passives and 0.3 mm CSPs also exacerbate these defects)

d.      Defects related to increased oxidation

e.      Increases in voiding

f.       Increases in tombstoning

2.      The higher cost of LF solder, mostly for wave soldering

a.      It’s not just the silver, tin is much more expensive than lead

3.      Poorer wetting of LF solders, creating the most significant challenges in wave soldering

4.      More rapid copper pad dissolution on PWBs in wave soldering

5.      LF solder attack of wave solder machine components

6.      LF reliability in harsh thermal cycle testing appears poorer than SnPb solders

7.      Tin Whiskers

It’s a Wash

1.      Short-term reliability in consumer product-type environments

2.      Protection of the environment if discarded products are improperly disposed of

a.      Lead in electronics has never been shown to cause a problem in land fills

3.      Since July 2006, about $3 trillion of products have been manufactured with LF solder, with no “the sky is falling”-type of problems

Pluses

1.      LF solder’s poor wetting enables finer lead spacings (see photo courtesy of Motorola)

a.      It may be argued that some modern electronic products (e.g., smartphones) could not be made with SnPb solder

2.      It is safer to recycle LF solders, especially if performed in a non-controlled environment
OK — your turn. Please comment.

Best Wishes,

Dr. Ron

Some Consensus on SAC

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Back in November, I posted comments on lead-free availability. In this post, I mentioned that I chaired a session at SMTAI on Alternate Alloys. At this session, Greg Henshall presented a paper on the  Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies.  In addition, Richard Coyle presented an overview of the work of three companies titled “The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package.” Both projects evaluated Pb-free thermal cycle reliability as a function of silver content and compared the results to SnPb reliability.

Both papers concluded that, as far as 0oC to 100 oC thermal cycle reliability is concerned, in their experiments

SnPb < SAC105 < SAC305 < SAC405

Coyle’s presentation summed it up best: “Each of the SAC alloys outperformed the SnPb eutectic alloy in every test, including the long, 60 min. dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb.”

To be clear, it was two papers by two different groups coming to the same conclusion. It would probably be a stretch to say that the conclusions of either group were “almost unique”.

Denny Fritz responded to this blog post with this point: “No one I know will dispute your ranking of SAC better than SnPb solder using the commercial temperature cycle Henshall uses – 0C to 100C. But, harsh environment electronics have to perform to either -40C or -55C, and most use a top end cycling temperature of 125C. IT IS IN THAT WIDE THERMAL CYCLE TESTING THAT SnPb outperforms SAC solders.”

Denny’s point is well- taken. I believe it can be said that SAC alloys have demonstrated acceptable reliability in commercial, non harsh environments (i.e., mobile phones, PCs, consumer electronics, etc.). However, it cannot be said that acceptable reliability for SAC has been established for military (RoHS exempt) and harsh (i.e., automobile engine compartment) environments.

A short time ago, Werner Engelmaier wrote an article on this topic (Global SMT, vol. 11, no. 1, January 2011, pp. 38-40), referring to my post he said: “Of course, ‘Dr. Ron’ selectively picks data agreeing with the point of view he held from the inception of the Pb-ban under RoHS on a plot with an expanded x-axis overemphasizing the differences and supporting a solder joint reliability ranking of SnPb < SAC105 < SAC305 < SAC405.”

Ouch! My motives were not quite so nefarious, I chaired a session and wanted to share the conclusions.

However, Werner makes good points in his article, data exist disagreeing with this reliability ranking and he suggests some good points on how to conduct reliability tests so that comparisons can be made between data sets.

In reading some of his other articles, I was delighted to find that we actually agree on the state of lead-free reliability in thermal cycle testing. Here is a statement of his circa 2008 (Global SMT, vol 8., no. 8, August 2008, pp. 46-48.): “It has been 2 years since the infamous ban of Pb-solders under RoHS. What have we learned? For solder joints, no dramatic differences in reliability are apparent. The data bases for LF-solders have grown, the favored LF-solders might be shifting, and no reliability model exists as of yet. Nevertheless, progress has been made.”

Best Wishes,

Dr. Ron

Put Brakes on EMI ‘Conclusions’

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I know and respect the team at Circuitnet, but it seems like they made a pretty serious goof the other today.  Their top story  had headlines stating, “Tin Whiskers Behind Toyota Recall.”  The link to this story takes us an article with the title “Electronic Tin Whiskers may be behind Toyota recalls.”

So we start with a headline telling us that tin whiskers are behind the recall and when we go to the main article we see that tin whiskers may be behind the recalls.  The person that the article is quoting is Keith Armstrong an EMI (electromagnetic interference) expert.  In this article Armstrong states that EMI may be the culprit in Toyota brake malfunction.

From what I see in the article, Armstrong has no data, and has not looked at a failed Toyota brake system.  He is just arguing that EMI may be the culprit.  Who knows?

Armstrong is then quoted as saying that tin whiskers, in the lead-free solder, may be to blame for the recalls and he then references work by John Barnes.  Barnes’ exhaustive summary has nothing to say about tin whiskers in Toyota braking systems, just a bit about tin whiskers in general in the over 1,000 pages about lead-free issues.  Armstrong is then quoted as saying that the tin whisker problem, “has caused serious problems in the computer industry previously.”  The article at this link is dated Nov. 12, 2002, and is simply a call for papers on tin whiskers at a conference.  Strong suggestions for having no data!

I don’t want to minimize the concern for tin whiskers, but the headline in Circuitnet and the article it links to have nothing factual to do with tin whiskers in the Toyota recall situation.  Given the seriousness of this situation, this misleading reporting is troubling indeed.

Cheers,
Dr. Ron

P.S.: One commenter to the main article points out that Toyota uses leaded solder in the brake electronics.  I don’t know if this is true, but given the RoHS exemption that auto electronics has for lead, it would not surprise me.

In summary: Double yikes!!

Toyota Recall Has Pb-Free Critics in Overdrive

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Are Toyota’s gas pedal failures caused by a breakdown in the electronics system? And if so, are the much-publicized recalls tied to a lead-free problem?

That’s been the hot topic on the TechNet email forum for over a week now. The mainstream media, of course, has gotten hold of the issue too, and is running with it like a Camry with a stuck gas pedal.

Here’s a list of some articles to date:

  • MSNBC is considering the likelihood of an issue with the electronics sensors.
  • The Los Angeles Times notes that the electronic throttle system uses sensors, microprocessors and electric motors, rather than a traditional link such as a steel cable.
  • AOL Autos and Autoblog look at a recalled pedal and discuss how possible sources of the problems.

Bob Landman, a reliability expert and a Life Senior Member of IEEE, has been vocal that the connection between lead-free solder and tin whiskers is both real and potentially deadly.  He asserts “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 regards to the Toyota recalls] was, but I have heard recently of brand new autos showing up at dealers that will not start.  That cause has been linked to tin whiskers.”

We do not yet have enough information to determine whether tin whiskers or even lead-free solders are to blame. One would hope Toyota would come clean about the true cause, if indeed it can be determined, so that the industry at large can learn from their mistakes.

UPDATE: Toyota today stated the cause was not electronic in nature.

The Pb-Free Bookworm

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How many books did you read last year? John R. Barnes claims 300 himself, and that’s just cover-to-cover. (In his free time, he devoured some 800 magazine articles.)

After seeing his own opus, I believe him. The former Lexmark engineer’s tome, Robust Electronic Design Reference Book, checks in at about 1,500 pages, including 122 pages of references. (Writing it took him 4,200 hours, says Barnes, who apparently documents pretty much everything.)

None of that it is the point of this post, however. Rather, I want to call attention to Barnes’ “other” effort — the documenting of all the available references to lead-free electronics. He has painstakingly cataloged and alphabetized the list, with links available here. This is a must bookmark for anyone involved in lead-free manufacturing, and the industry owes Barnes a standing ovation for compiling it.

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

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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.
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?

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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.