Tag Archives: SAC 305

Use an SMT Pre-Test Before Presenting a Soldering Workshop

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Folks,

Let’s see how Patty is doing, it’s been a very, very long time …

Even though Patty and her husband Rob both worked at Ivy University, they seldom drove in together. It was just too difficult to organize their schedules so that it would work out. So, as Patty was driving in to Ivy U, she was listening to the last chapter of Ron Chernow’s biography of U. S. Grant. Her timing was excellent, since she, Rob, Pete, and the Professor were having their monthly book club meeting. Rob, Pete, and the Professor were always recommending books about World War II or the Civil War. Because of this trait, she groaned every time it was the three “boys” turn to suggest the next book. But, she had to admit that she always enjoyed the books much more than she thought she would. She especially liked a book Rob discovered, called A Simple Solder. Patty found this true story, about a young boy in the German army in World War II and how he survived to tell the tale, fascinating. She would never tell Rob, but she read it three times.

When it was Patty’s turn she made sure to avoid those military topics. Recently, she proposed another one of Chernow’s biographies on John D. Rockefeller. She also suggested  iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy–and Completely Unprepared for Adulthood–and What That Means for the Rest of Us by Jean M. Twenge. This book convinced her and Rob to dramatically limit “screen time” for their 9-year-old twin boys.

As she approached her parking spot, the audio book on Grant finished. She was a bit sad, as she had enjoyed this book as much as any. Patty had the impression, from her high school history classes, that Grant led the Union to victory over Robert E. Lee only because he had superior forces, weapons, and supplies. Chernow’s book clearly dispelled that notion. Grant was a great general. In addition, he was an effective and honorable president, if a little too naïve and trusting to avoid numerous scandals among his subordinates.

In a few moments, they met in The Professor’s large office. After they finished their book club chat about Grant’s biography. Patty had a favor to ask.

“Mike Madigan asked me to give a three-day workshop on SMT 101 at one of ACME’s recently acquired facilities. He said he felt the technicians and engineers weren’t very knowledgeable. I’m having trouble deciding at what level to aim the workshop,” Patty began.

“You mean like for beginners, intermediate, or expert?” Pete asked.

“Yes,” Patty responded.

“Well, you should develop it in a logical sense, starting with what soldering is, discuss flux and solder paste, then stencil printing, component placement, reflow, test, etc,” Rob added.

“I agree with Rob’s outline, but you need to find out the current knowledge level of the students,” The Professor suggested.

“I once gave an eight-hour seminar on SMT Defect Modes and How to Fix Them. The workshop was advertised as for SMT engineers and technicians with intermediate experience. At the end of the workshop a person raised his hand and asked an unsettling question,” The Professor continued.

“And the question was?” Pete teased.

“Professor, you have used the word ‘SAC’ many times, what does ‘SAC’ stand for?” The Professor responded.

In unison, Patty, Rob and Pete groaned.

“That’s my concern! At which level do I aim the workshop? If I shoot too low, it might insult people. If I shoot to high it might go over their heads,” Patty responded.

“OK! So, how do I structure the workshop, not knowing the skill level of the students?” Patty asked a little frustrated.

“How about a pre-test?” The Professor suggested.

“OK! But how many questions?” Rob asked.

“It needs to be short, yet comprehensive,” The Professor suggested.

“Seems like a contradiction,” Pete grumbled.

“I think The Professor is right. Look at it this way, let’s say you want to assess if your 14 year old nephew knows much about The Civil War. Ask him three or at most five questions and you can determine if he does,” Patty suggested.

“How about some examples?” Pete asked a bit dubious.

“I’m getting it. How about when was the war fought, who was Robert E. Lee, what is the significance of Appomattox Court House?” Rob chimed in.

“OK, I see you point. If you know two or all three, you probably know a lot, one or less and you don’t know much,” Pete responded.

Patty then suggested, “OK let’s develop a list of ten SMT Pre-Test questions.”

After about 20 minutes of back and forth, our team of four converged on these 10 questions.

SMT Pre-Test

  1. What does the letter “S” in SAC stand for?
  2. How much silver is in SAC 305?
  3. PWBs are coming off of the final component placement machine at a rate of one every 20 seconds. The PWBs are 20cm long and should be placed with at least 4cm of space between them. What must the reflow oven belt speed be to accommodate this cycle time?
  4. The starting temperature is 25°C. It needs to be 145°C in one minute. What heating rate is needed, in °C/s, to achieve this temperature?
  5. About how much does silver cost per troy oz.? (+/- 30%)
  6. Which is a closest to typical stencil thickness?
    • 5 microns
    • 20 mils
    • 5 mils
    • 20 microns
  7. Which is closest to a typical lead spacing for a plastic quad flat pack (PQFP?)
    • 0.1mm
    • 0.1 mil
    • 0.4mm
    • 0.4 mils
  8. Which has finer solder particles, a Type 3 or 4 solder paste?
  9. What does OSP stand for?
  10. Place an arrow at the eutectic point of the tin-lead phase diagram below.

Would you like to try the pre-test? The answers have to be what you know without looking anything up. Send me your answers at [email protected]. The first person to get 100% will get an item of memorabilia signed by Patty, Rob, Pete, and The Professor.

Cheers,

Dr. Ron

How Far Can We Go to Replace Lead?

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The end is nigh for lead in solder, as our columnist Tim O’Neill writes this month in CIRCUITS ASSEMBLY.

Rules governing use of the materials — Directive 2015/863, aka RoHS 3 — are coming online and will be in full force by 2019.

Suppliers have until July 22, 2019 to meet the stricter provisions, which includes no more than 0.1% lead in medical devices, which are joining consumer, industrial and other electronics products on the effectively banned list.

The question Tim poses is, What comes next? Already, the future of commonplace unleaded alloys such as SAC is being questioned. As Tim writes, “It is even feasible SAC 305 will be dislodged by a new de facto alloy that better serves the needs of the market.”

A Norwegian scientist believes he may have the answer. As noted in Phys.org this week, Dr. Henrik Soensteby of the University of Oslo is working on an alternative alloy that contains nothing but common — and essentially benign — elements. In conjuring up his alloy, Soensteby is mixing sodium, potassium and oxygen with niobium, a very strong metal typically used in steel. While niobium dust is reported to cause eye and skin irritation, it reportedly is nontoxic, at least in the volumes used.

It’s not so clear yet how much niobium would be needed. Brazil is the biggest supplier of niobium, producing more than 85% of it each year. Other sources include Zaire, Russia, Nigeria and Canada. World production is relatively light: around 25,000 tonnes per year. Some scientists believe there are ample supplies still in the ground. There’d better be: Some 5 million tonnes a year of lead ores are mined each year, although obviously not all that goes into electronics.

Soensteby is optimistic he can use atomic layer deposition (ALD), a vapor phase method that uses gas at controlled temperatures to stimulate a reaction with the substrate; the output is thin films. It is an emerging technology in semiconductor manufacturing. There are many, many questions, of course. First and foremost, does the alloy actually, you know, work? Also, ALD typically involves higher temperatures than are used in electronics assembly: Would it work with today’s packaging? Will other technologies such as 3D printing or Joe Fjelstad’s solderless Occam process supplant the need for solder in any form?

Still, materials science is the most exciting area of electronics today. We may make fun of folks who walk around with smartphones seemingly permanently tethered to their ears, but we also have them to thank.

 

Register now for PCB West the Silicon Valley’s largest PCB industry trade show: pcbwest.com! Now with full-day electronics assembly tutorials!

 

Alloy Melting

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Folks,

Richard asks:

Dear Dr. Ron,

Recently we had a solderability problem with tin-finished component leads and SAC305 solder paste.  One of our engineers claimed that the problem was that the tin finish melts at too high a temperature (Tm= 232°C) for the SAC305 solder paste (Tm = 219°C) to melt it.

My understanding is that certainly above 232°C both will melt and form a good solder joint, but even if the temperature was less than 232°C, say 225°C, the tin would melt. Can you explain this phenomenon?

Richard,

Thanks for this question, which can be interpreted two ways. The first would be that, in a reflow oven at temperatures above the melting point of both metals, the one with higher melting temperature prevents the metal with a lower melting temperature from melting it. This is not true, since both metals would come near to the temperature of the air in the reflow oven and melt.

The other perspective would be that the temperature in the reflow oven is above the melting temperature of SAC 305, but below that of tin. So, how can the tin melt?  To consider this situation let’s say the oven is at 228°C. Will the tin on the lead or pad finish melt? The answer is yes. But, let’s try to understand the phenomenon with gold and tin first.

Metals that have extreme melting point differences often dissolve in each other. As you stated, tin melts at 232°C, whereas gold melts at 1064°C.

This phase diagram can be found here.

Figure 1. The gold tin phase diagram

To make a gold-tin solder, all one has to do is have a bath of tin at some moderate temperature, say 350°C. Insert the gold and the gold will melt and flow into the molten tin. This is true even though the gold melts at 1064°C. This effect can be shown experimentally. A similar phenomenon exists with gold and mercury. Mercury reacts with gold at ambient temperatures. The phenomenon can be used to extract tiny gold particles from soil and is commonly used today in artisanal gold mining. Unfortunately this use of mercury is often toxic to the miners and pollutes the environment.

Considering electronics assembly solders again, let’s assume that some liquid tin-lead solder is heated to 200°C. See Figure 2a. As seen in this figure, a ball of tin at 25°C is held above the molten tin-lead solder. The ball of tin is immersed into the molten tin-lead solder in Figure 2b. The tin-lead solder forms a meniscus around the solid tin. Even at room temperature the tin atoms are vibrating, and as a result, some of these atoms on the tin ball will end up flowing into the tin-lead solder. This action will leave a vacancy in the tin ball that may be filled by a lead atom from the tin-lead solder. In the vicinity of the newly arrived lead atom, the melting temperature of this micro spot of tin-lead alloy will be lowered as tin-lead solder has a melting temperature below that of tin. This process will continue until all of the tin will intermix with the tin-lead solder and flow into it as seen in Figures 2c through 2f.

Figure 2a Figure 2b Figure 2c Figure 2d

 

Figure 2e

 

Figure 2f

Cheers,

Dr. Ron

Can Your Mortality be Modeled with Weibull Distribution?

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Folks,

In the last posting we saw how Weibull analysis helped us to determine that SACM lead-free solder (SAC 105 with about 0.1% manganese) has comparable (actually better) thermal cycle performance versus SAC 305 solder.  Software like Minitab will give us even more detailed information about the performance of the solder joints in stress testing as we see in Figure 1.

In addition to the Weibull plot, we also have the Probability Density Function (PDF), the Survival Function and the Hazard Function. The PDF tells us when it is most likely that a test board will fail in a test population, as shown by the inserted red line. We see that it is a little less than 2,000 cycles. The Survival Function shows the percent of surviving test boards. We observe that the expected life (the 50% point) is quite close to the maximum of the PDF. The Hazard Function tells us the rate at which the test boards are dropping out.  It increases with time, but there are few boars left so the PDF drops down at the end of the test, even though the fallout rate is the highest.

It is interesting (and perhaps appropriate in the wake of Halloween) to consider if human mortality follows a Weibull distribution. I used some data for the Centers for Disease Control that are a little over 10 years old for males in the US.  So, the mean life expectancy is a little low at 72 years. (I was a little lazy: the old data were a little easier to work with than new data, some conversions are needed to make it work.) The data appear in Figure 2.

As you can see, just like a solder joint, your life expectancy can be modeled quite well by the Weibull distribution.

Cheers,

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