Category Archives: Dr. Ron

Materials expert Dr. Ron Lasky is a professor of engineering and senior lecturer at Dartmouth, and senior technologist at Indium Corp. He has a Ph.D. in materials science from Cornell University, and is a prolific author and lecturer, having published more than 40 papers. He received the SMTA Founders Award in 2003.

All Wet

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Folks,

I have often pointed out that SAC solder’s poor wetting is both a curse and Godsend.  It is a curse when trying to fill a through-hole in wave soldering, and a Godsend when assembling close lead spacings as shown in the image (below). Indium Corporation colleague and friend, Mike Fenner (image below), pointed out that, when I say “SAC solder doesn’t wet well,” I should be saying “it doesn’t spread well.” His explanation follows:

“SAC is different from SN63, and I think it is helpful to explain the difference by making a subtle differentiation between wetting and spreading.

“The way that solders spread and wet to a surface is a balance of competing forces. We have surface tension acting to make the molten solder shrink into a ball, and wetting forces trying to make it spread across the surface. Wetting is also the action of the solder dissolving into the surface to form an intermetallic. This intermetallic is essence of the solder joint. The balance changes with different alloys, surfaces, and processes.

“Most people are very familiar with the way that tin lead solders behave — and that governs their expectations. The different balance in SAC means the solder tends to spread less for the same wetting and, therefore, can give the impression of a lower quality joint. This lack of spread is usually expressed as ‘poor wetting.’

“I would explain this by saying the active ingredient’ in both solder families is tin. SAC alloys have a ~50% higher concentration of tin than the Sn63 solder alloy. This gives them a higher surface tension which increases the balling (coalescing) force. At the same time, the less dilute tin, in SAC solders, dissolves into a surface faster. So the final SAC joint can have a well formed intermetallic, but not high spread. These relationships will vary with surface finish and, of course, flux chemistry and process conditions come into play, but that’s for another day. Meanwhile I hope this simplified explanation helps.”

Thanks Mike!

Cheers,

Dr Ron

P.S. The solder image is courtesy of Vahid Goudarzi of Motorola.

SnPb vs. Pb-free: Wrong Baseline?

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Folks,

I met Peter Borgesen back in the mid 1980s when he was a research scientist at Cornell working with Professor Che-Yu Li. Later we worked together at Universal Instruments. Currently Peter is a professor a Binghamton University.  All during this time, Peter has been working on materials science-related topics in electronics packaging and assembly, most notably reliability. In addition to his many technical skills, he is a gifted linguist, speaking multiple European languages. Etched in my mind is Peter talking to several European graduate students in their native European languages in the space of five minutes, switching from one to the other effortlessly.

Few people know more about lead-free solder reliability than Peter. So I thought I would get his perspective on my recent post on lead-free field reliability data. His comments follow.

Hi Ron,

I agree that the sky is not falling. Also, we should be talking much more (only?) about life in service. I realize that we don’t know enough about this (and our predictions based on test results are much more off than people want to recognize). The vast majority of practitioners focusing on ‘engineering tests’ are doing worse than wasting time and effort if comparisons of test results do not translate to relative performances in service. There is a lot of ‘sticking heads in the sand’ here.

I am not concerned about the long term life of cell phones, and not very worried about in which respect they do better or worse in service than with SnPb. Intermetallic bonds have generally gotten weaker and more prone to sporadic defects, and cratering is greatly enhanced for the devices Vahid Goudarzi mentions when discussing Motorola field data. I agree those are limited concerns, no sky falling indeed.

What still scares me (in the case of critical applications) or concerns me (in the case of expensive applications) is the naivete with which many seem to think we can learn much about sporadic disasters or long-term reliability of those from consumer electronics experiences.

I am not often interested in comparisons to actual life of SnPb either (any more). We face ever more applications (designs and service conditions) for which we don’t have sufficiently accurate critical experience with SnPb either. The first challenge becomes not to be surprised by effects of long-term aging, combinations of loading, minor differences in pad finish, joint configuration, latent damage, process, …. and their interactions for the specific solder alloy used (!).

While I can’t yet extrapolate test results to life in long-term service (I think we are close, but I really need an extra $1M to prove my hypothesis and turn it into a quantitative model) I can show how current models may easily be off by 2-3 orders of magnitude or more (worse, how they may screw up comparisons of alternatives). It obviously depends on the application whether this really matters (I side with companies who have cut drastically back on testing for many applications).

Keep up the good work.

Peter

I will keep in touch with Peter in the future for updates on his perspective.

Cheers,

Dr. Ron

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

Solder Alloy Density

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Folks,

I have occasionally written on calculating solder alloy density, as there is surprisingly more interest than I thought there would be in this topic. Recently, it occurred to me that it might be beneficial to compare the calculated densities to actual densities of a few alloys to see how accurate the correct formula is (for the derivation of the correct formula see below). The formula assumes “perfect mixing” (i.e., no interactions between the alloy elements). The alloys we investigated were tin-bismuth-silver, tin-silver, tin and tin-bismuth.

To measure the density, I obtained a few alloys from Indium. My student, Evan Zeitchick, determined that a good technique to measure density is to machine the alloy into a rectangular parallelepiped (see photo), weigh it, and calculate its volume from its dimensions.  The results agree with the correct formula to about 1 to 2%. Some people would ask why there is any difference. The reason is that all alloys form different phases, and some form intermetallics. These phases and intermetallics would typically have different densities than that calculated for the alloy. I will have more detail on this work in a future post.

Here is a derivation of the correct density formula:

Many people incorrectly assume that if you have an alloy of x % tin by weight and y % silver, that the density of this alloy would be 0.x*Density tin +0.y*Density silver. This intuitive linear formula is incorrect however, as density has two units (mass and volume).

An easy way to understand the derivation of the correct formula (proposed by Indium  engineer Bob Jarrett) is to consider a 96% tin, 4 % silver example.

Let’s assume I have 1 g of this alloy, 0.96 g is tin and 0.04 g is silver.

The volume of the tin is 0.96 g/7.31g/cc = 0.131327cc

The volume of the silver is 0.04g/10.5g/cc = 0.00381cc

So 1 g of the alloy has a volume of 0.131327 + 0.00381 cc = 0.135137 cc

Hence it’s density is 1g/0.135137cc = 7.39989g/cc

Hence, the general formula is:

1/Da = x/D1 + y/D2 + z/D3

Da = density of final alloy

D1 = density of metal 1, x = mass fraction of metal 1

same for metals 2, 3

The formula continues for more than 3 metals.

I have developed an Excel spreadsheet that calculates density automatically. If anyone wants a copy, send me an email at [email protected]

Cheers,
Dr. Ron

P.S.: Interesting thought: About 165,000 tonnes of gold have been mined throughout history. If all of this gold was gathered into a cube it would only be about 21 meters on a side. At $1,550/oz, its value would be $8.5 trillion, quite a bit less than the almost $15 trillion debt of the US government. Yikes!

Bismuth: Behind the Numbers

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Based on a recent post I published regarding the use of bismuth in solder alloys, John writes:

If Bismuth comes from the production of Pb, and if the use of Pb is being reduced, won’t the availability of Bi be reduced…and the price would increase?”

Just thinking…

Dr. Ron responds:

Lead has been banned from many of its original uses, paints, solders, water pipes, gasoline, etc. However, its increased use in batteries has actually caused lead consumption to rise. The USGS estimates that 88% of lead produced is used for lead-acid batteries.

Many of us in electronics assembly have been focused on the 2006 RoHS lead ban. This may have caused us to believe that lead use in electronics was significant. About 9 million metric tons (MT) of lead are consumed each year, only about 20,000 metric tons were used for solders prior to July 2006, this amount is only about 0.22% of the total. Electronics lead use being so small is likely why the lead industry had little visibility in fighting RoHS. Their important customers were making batteries.

Lead is quite effectively recycled, as about 60% of the 9 million MTs/yr. are from recycling and 40% from mining.

Over 100 million lead-acid auto batteries are sold each year in the US alone. In addition, the use of lead-acid batteries in forklifts, electronic vehicles, and golf carts has increased demand for lead. So, the bottom line is that lead use is expected to grow at about 2% per year.

Considering that we calculated that bismuth use in solders would be at most 5% of total bismuth production, it is unlikely that this use, or lead production reduction, would affect bismuth supplies.

Best Wishes,

Dr. Ron

A Few Questions on Bismuth

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Folks,

A few people asked some questions after a post on bismuth solders. Here they are:

1. The low melting point of these solders is encouraging. What are realistic field use conditions?

Bismuth solders tend to be brittle, so drop shock environments such as mobile phones would not be recommended. However, thermal cycle performance from 0 to 100C is good, so stationary office equipment, televisions, desktop computers, etc., may be good candidates.

2. I am working with your colleagues on an automotive application and I am curious whether you have any idea how this alloy will perform between -40 and 0°C? We have not been reviewing bismuth-containing alloys due to their lower sheer strength, but may need to look at them in the future.

We can find no information on thermal cycle performance at these low temperatures.

3. I hear that bismuth is rarer than silver. If we start using bismuth in solders, couldn’t that make it very expensive.

An old number from Prismark puts the world solder use at about 50,000 metric tons (MT) per year.  Assume bismuth solders took a 5% market share (I think this would be the highest) that is 2,500 MT of bismuth solder (Bi57Sn42Ag1) or 1,425 MT of bismuth.

Although bismuth’s occurrence in the earth’s crust is 0.009 ppm (silver is 0.075 and gold 0.004 ppm), about 22,000 MT are produced each year.  In comparison, about 2,000 MT of gold, 20,000 MT of silver, 400 MT of indium and 5 MT of rhodium are produced each year.  In comparison to more common metals, total lead production is 8,000,000 MT/year and tin a little less than 700,000 MT.

Realistically, it would seem to me to be unlikely that use of bismuth in solder, at 1,425MT/year out of 22,000 MTs,  would affect the price much, especially if the adaptation rate is more like 1-3%, instead of 5%.

For those interested in how bismuth is produced, this Wikipedia quote may be of interest:

According to the United States Geological Survey, world 2009 mine production of bismuth was 7,300 tonnes, with the major contributions from China (4,500 tonnes), Mexico (1,200 tonnes) and Peru (960 tonnes).[11] World 2008 bismuth refinery production was 15,000 tonnes, of which China produced 78%, Mexico 8% and Belgium 5%.[9]

The difference between world bismuth mine production and refinery production reflects bismuth’s status as a byproduct metal. Bismuth travels in crude lead bullion (which can contain up to 10% bismuth) through several stages of refining, until it is removed by the Kroll-Betterton process or the Betts process. The Kroll-Betterton process uses a pyrometallurgical separation from molten lead of calcium-magnesium-bismuth drosses containing associated metals (silver, gold, zinc, some lead, copper, tellurium, and arsenic), which are removed by various fluxes and treatments to give high-purity bismuth metal (over 99% Bi). The Betts process takes cast anodes of lead bullion and electrolyzes them in a lead fluorosilicate-hydrofluorosilicic acid electrolyte to yield a pure lead cathode and an anode slime containing bismuth. Bismuth will behave similarly with another of its major metals, copper. Thus world bismuth production from refineries is a more complete and reliable statistic.

So I don’t think bismuth supply and price would be affected by its use in solders.

Cheers,

Dr. Ron

By and Bi

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Folks,

When the industry was preparing to transition to lead-free solders almost ten years ago (can it have been that long), tin-bismuth solders were serious candidates. Their low melting point, of about 138C, made these solders interesting candidates to replace tin-lead solder. However, if contaminated with lead, tin-bismuth solders can produce a eutectic phase that melts at 96C. In such situations the resulting solder joint exhibits poor performance in thermal cycle testing. Since early in the transition to lead-free solders it was expected that there would be numerous components and PWBs with lead-based surface finishes, this property made tin-bismuth solders unacceptable.

Another aspect of tin-bismuth solders is that they expand on cooling. This phenomenon can result in fillet lift in through-hole solder joints.

However, as we are now well into 2011, almost no components or PWBs have lead-containing finishes and many portable electronic devices have no through-hole components, so it may be time to reconsider tin-bismuth for some applications.

Some years ago, Hewlett Packard (HP) had performed work to show that adding 1% silver to tin-bismuth solder enabled this alloy to outperform eutectic tin-lead solder in 0 to 100C thermal cycle testing. Even at these low reflow temperatures, HP demonstrated solder joint strength with SAC BGA solder balls that was 65% that of tin-lead solder. Expanding on this work, Indium’s Ed Briggs and Brook Sandy performed stencil printing and reflow experiments consistent with the requirements of current miniaturized components using this 57Bi-42Sn-1Ag solder. All their results were promising. Ed presented a paper at SMTA Toronto that summarized the Hewlett Packard work and reviewed the results of this new work.

Bismuth solders tend to be brittle, so applications experiencing drop shock should be avoided.

So for applications consistent with 0-100C thermal cycling, 57Bi-42Sn-1Ag solder may be something to consider if the high temperature of SAC solder paste is an issue to components or PWBs in a product.

Cheers,

Dr. Ron

No Silver Lining

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Many people have been infatuated by the price of gold in recent months, but the price of silver has also skyrocketed. In 2000 silver was about $3 per troy oz. In the eight years that followed, its price grew to $15/oz. Today it is trading at over $41/oz! This price is almost an all time high, except for the time when the Hunt brothers tried to corner the silver market in 1980. The aberration of their efforts jolted the silver price to just short of $50/oz., but it settled down to $11 or so after the Hunts came under margin call and other pressures.

Unfortunately, the dramatic price increase today, does not appear to be an aberration. Although we may hope that it will soon drop to more historic levels, we may not have reason to expect that it will.

Although not as dramatic, tin and copper have experienced significant prices increases as well. The price of tin has doubled in the last year to $15/pound and copper has increased from about $3/lb to $4.50.  These metals are obviously key ingredients in critical electronic materials such as solder pastes, solder bar, and solder preforms.

In addition, oil, which is used for most organic electronic materials such as PWB resins, flip chip underfill, and epoxy fluxes, has increased to $110/bbl – approaching its all time high of $145/bbl.

All of these price increases have a significant impact on the electronics materials supply chain. Although we are used to price decreases in the cost of our mobile phones and PCs, at this point in time, the price of the materials that go into these devices will be increasing.

As one materials supply chain executive commented at Apex: “It’s not like we can be clever and somehow work around the price increase of silver and these other materials, we have to pass it on to our customer, or go out of business.”


An SPC Rx

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It was Wednesday evening and I had just finished a brief pitch on applications of SPC to a group of 20. I was followed by Jim Hall, who spoke of process mapping using SIPOC.  So did these folks have solder paste under their fingernails, or wave solder flux stains on their shirts, or, perhaps, a solder preform or two stuck in their pant leg cuff?

No — none of these souls would have had any of this type of trace evidence of electronic assembly on their person. You see, they were all medical doctors and students at Harvard’s famed medical school (see image below).  (I hope it is OK that the proud dad shares that my daughter Jessica is a colleague of these folks.) 

Jim and I were presenting to the doctors, because they are interested in process optimization in the healthcare industry. The event was hosted by Dr. Andy Ellner.  He is a professor and doctor at the medical school and is a focal point for these process improvement efforts. I was introduced to him in the summer of 2009 by Dartmouth’s new President Jim Kim.

In November 2009, Jim, our colleague Larry Parah, and I facilitated Andy’s team in dramatically improving the prescription refill process in Brigham and Women’s Hospital Clinic.  Jim and I plan on working with Andy in similar efforts over the next year or two.

The most striking thing that Jim and I left with on Wednesday evening was how profoundly interested these doctors and students were in healthcare process optimization. The Q&A session lasted nearly an hour.

Ah, yes, would that our many colleagues in electronic assembly were as interested in optimizing their processes!

Cheers,

Dr. Ron

Tin Din

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Folks,

Many people responded to my recent post on tin whiskers. A few pointed out that the recent NASA report on the Toyota Unintended Acceleration Issue discussed numerous tin whiskers that were found, one implicated in a failure. The tin whiskers were emanating from tin plating.

We don’t know, however, if tin whisker mitigation techniques were used. In a mission-critical application, such as this, it would appear unwise to use RoHS-compliant electronics, especially since they are not required for automobiles. In other words, autos are exempt from RoHS. Let me be very clear: from a tin whisker perspective, I am uncomfortable with RoHS-compliant tin plating in mission-critical applications. Much more work needs to be done before such tin plating should be used in mission critical applications. In applications where RoHS-compliant electronics cannot be avoided, all tin whisker mitigation techniques should be employed, including conformal coatings.

In addition, in response to my post, a number of people pointed out the difficulty of proving a tin whisker fail and the reluctance of any manufacturer to admit that their products had them.

But my quest remains unfulfilled; the question remains:

“[W]ho knows of any verified tin whisker fails when tin whisker mitigation techniques where 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.”

Restated, here is my point.  Since RoHS, quite a few people take a position something like this: With RoHS-compliant assembly, even the world of non-mission critical electronics is at considerable risk of numerous catastrophic failures, due to tin whiskers, that will cost hundreds of billions of dollars.

I still maintain, that with mitigation techniques, such as recommended by iNEMI, tin whisker control, for non-critical electronics, can be manageable. Non mission critical electronics is about 80% of the $1.5 trillion of the electronics industry.

As I pack up to leave my office today at Thayer Engineering School at Dartmouth, I am across the aisle from the chaps that provide our computers and IT support.  They buy millions of dollars of electronics a year.  In chatting with them they state two things:

1. They have noted no difference in electronics reliability since RoHS implementation, nearly five years ago.
2. On the very rare occasion that they get an electronics failure, it is almost always a hard drive.

Bottom line: Except for hard drives, modern electronics are very reliable for their use life.

I expect my quest will uncover some tin whisker fails, even with mitigation, but the fails will most likely be isolated and not a significant threat to the industry at large.

Cheers,

Dr. Ron

P.S. The image is from Dr. Henning Leidecker of NASA, one of the world’s leading tin whisker experts.