Tag Archives: solder paste

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

No More Cookson

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If you read this announcement about Cookson splitting in two the first question must be, what will this mean for the organization?

My take is, not much. Here’s why:

1. The company will remain public, and the shareholders are the same. (Under the proposal, Cookson shareholders get one share in each of the two new companies.) Had this been an MBO or private equity group, I would expect slash and burn. But the transition as planned should bring much-desired stability to the new organization.
2. The upper management isn’t changing. Had Cookson Performance Materials group CEO Steve Corbett left, I might think differently. But Corbett, who joined Cookson in 1990 and has run Enthone since 2002 and both companies since 2004, is highly responsible for the existing management and operational structure. He knows what he is doing, knows the markets and understands the brands.
3. The debt is manageable. Alent (the new name of the former Cookson Performance Materials) will “get” about one-third of Cookson’s £451 million ($727 million) worth of debt. Given the new company’s sales of £418 million ($675 million) and profitability, it should be able to swallow that meatball.
4. The brands are intact. The Alpha Metals and Enthone brands are well-recognized and respected worldwide. Indeed, after spending some time trying to beef up the somewhat unwieldy Cookson Performance Materials name, the company reversed gears and has been working over the past year to rebuild those individual brand names. Perhaps this was in anticipation of the demerger, but either way, the strategy was well-timed.

In fact, the only casualty I see in all this is the Cookson name, which is, believe it or not, more than 300 years old. One wonders whether the Cookson name was seen as a negative by either of the spinoff companies.

And so goes Cookson. From its founding by Isaac Cookson in 1704 as a collection of metal and glass businesses to its aggregation of a herd of electronics assembly equipment and materials companies in the 1980s and 1990s to the respective divestitures of Speedline, then Polyclad and its Precious Metals business, Cookson has always been in a transition of some sort. It’s hard to believe, though, that this is its final move.

On Stats and Solders

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Folks,

Everyday, we are exposed to the results of surveys and polls. A typical example might be that President Obama is leading Mitt Romney in a poll by 48% to 45%, but the results are not statistically significant. A reasonable question might be, “What does it mean to be statistically significant?”

To determine statistical significance, typically, the statistician will use the criteria that if there is only a 5% or less chance that the conclusion would be wrong, it is considered statistically significant. So, when another poll would state that President Obama leads by 49% to 44% and it is statistically significant, there is, statistically, less than a 5% chance that the conclusion is wrong. The 5% criteria is not cast in concrete. Sometimes 10%, 1%, or even 0.1% might be used. However, tradition has given us 5% as the default value for “statistically significance.” It is also helpful to understand that, the more data points in the sample, the more likely the results will be statistically significant.

But if some data are statistically significant, is it always “practically” significant? As an example, let’s say that you really like chocolate. Your favorite brand is in a taste test and it scores 9.6 out of 10, whereas a new chocolate scores 9.7/10 and the results are statistically significant. On the downside, the new chocolate costs 5 times as much. Is it worth the extra money to convert to the new chocolate? In this case, we have to ask, is the difference practically significant. The answer is, in all likelihood, no. Such a difference as 0.1 point out of 10 is very small, and taste is also subjective. Here, the result might not be practically significant. The subjectivity of a taste test may mean that you either can’t tell the difference or that you still like your favorite chocolate the best.

Let’s consider another less subjective example. Suppose that, in a certain application, solder voiding is a critical concern. So, you measure the voiding of two solder pastes. After collecting hundreds of data points, you find that the average voiding of one solder paste is 8% and that of the other is 7%. Analysis with Mintab software tells you that the difference is statistically significant. But is the difference practically significant? Probably not.
How do you determine practical significance? Typically it would be by experimentation or in some cases by experience. In our example of solder voiding, suppose experiments showed that, as long as the voiding average is below 30%, there will be no concerns. In light of this, engineering may have set a specification that voiding must not be greater than 25% on average. (All this discussion assumes that the spread or standard deviation of the data is not large, but this subject is the topic of another discussion.) In this case, the difference between 7% and 8% voiding may be statistically significant, but not practically significant. A prudent engineer may select the 8% paste if it had other desirable features, such as better response to pause, or resistance to graping, or improved head-in-pillow defect.

Always ask yourself, is the difference both statistical and practical?

The image shows solder joint graping, which is often more of a concern than voiding.

Cheers,
Dr. Ron

The Price-Profitability Paradox

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Let’s see how Patty and Pete are making out on their latest adventure….

“Here is the ProfitPro output,” Dave Ferris said as he pointed to a PowerPoint slide on the screen.

Just then, the site general manager, Sally Wilson, and the head of purchasing, Blaine Ellis, arrived.

“Long time no see,” Pete said to Ellis.

Ellis acknowledged Pete, but appeared to be in a foul mood. Everyone settled down and the meeting came to order. Patty was again surprised: Pete always seemed to know everybody.

After introductions, Sally kicked off the meeting.

“As you know, we have a new corporate award program for saving money. Dave is a candidate to win the first award.  But Blaine won’t sign off on it, because his solder paste expenses have, in his word, ‘skyrocketed,’ ” Sally started.

Ellis exclaimed, “My solder paste costs are through the roof. Last year we used 3,000 kilograms and this year we are using 3,100 kilograms and each kilogram costs $10 more. That’s more than $40,000 more. How is this saving money?”

“How has the overall site profitability changed?” Patty asked.

“It’s pretty consistent with what Dave’s PowerPoint slide shows,” Sally answered. “His result is for one of our six lines. We are using the new solder paste on all of the lines now and profitability is up about 8%, or more than $6 million for a year.”

“A lot of the added profit is from cost savings that purchasing has implemented,” Blaine shot in.

“You don’t realize the pressure I am under to reduce the cost of purchased goods. Components, PWBs, connectors, solder paste, flux, packaging, etc., is over 80% of all of our total cost. Corporate has been all over me because of the increase in solder paste cost,” Ellis went on in frustration.

“Part of the increased cost of solder paste is because we ship more product, we actually use less paste per board with the new paste,” Dave responded.

“How so?” asked Sally.

“The old paste had poor response to pause. If we stopped the line for a few minutes, the first one or two prints afterward would be poor because the paste stiffened up. We would have to wipe the paste off those boards and reprint them. This would happen a couple of times per day. The ProfitPro output shows the increased productivity and profitability for the line for which I am responsible. Note that the profits are up $841K!” Dave Ferris went on.

“But my purchasing expenses have gone through the roof!” Blaine Ellis blurted as he stormed out of the room.

Patty, Pete, Dave, and Sally, sat there dumbfounded, looking at each other.

Pete finally spoke up, “Let me go talk to Blaine,” he said as he left the room.

“One of the issues is that Mr. Ellis should not be criticized if a consumable costs more money if it increases profitability. That doesn’t make sense,” Patty said.

“I agree” said Sally. “But much of the pressure comes from ‘Corporate.’”

As Sally was speaking, it occurred to Patty, that, in her new role, she may be able to impact this ineffective corporate policy. As she was mulling over this thought, Pete and Blaine Ellis returned to the room.

Ellis spoke first.

“After discussing the situation with Pete, it occurs to me that young Ferris’ profitability argument may have merit,” Ellis started.

“But Dr. Coleman, I need your help,” Ellis implored.

At this Patty’s ears perked up. She was not used to being called by her last name nor was she aware that she had a Ph.D.!

“I think I know what you need,” Patty responded. “We need to change the corporate criteria for evaluating the effectiveness of purchasing, to include situations like this. I’m quite sure I can do it,” Patty finished cheerfully.

The meeting concluded with all agreeing that Dave Ferris should be given the corporate award and Patty reaffirming her commitment to change the corporate policy.

In several hours, Patty and Pete were on an airplane heading home.

“OK, out with it,” Patty teased Pete.

“What?” was Pete’s sheepish reply.

“How did you know Blaine?” Patty asked.

“Remember, when I told you that I tried out for Olympic volleyball years ago?” Pete responded.

“Yes, ” Patty replied.

“So did Blaine. I’m not sure which one of us was more humbled by the experience,” Pete chuckled.

Cheers,

Dr. Ron

In Search of a Problem to Solve

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It has been a while, let’s look in on Patty …

Patty had to admit that she was very fortunate. She had yet to turn 30 and she was a Senior Vice President at ACME.  There was even a small article about her in Fortune magazine. But she had to admit that, at some level, she was bored. She missed the action of being out on the line and solving problems.

With these thoughts she headed toward the lunch room. She had avoided eating lunch with the execs and still ate lunch with the young engineers that were her age. No one thought it strange. Pete was occasionally the old-timer in the group, as he was approaching 45 years old.

As she sat at lunch with her friends, Patty also had to admit that she was jealous of all of the group’s talk about solving technical problems. She was now responsible for corporate strategies and seldom got her “hands dirty.” So she missed the technical challenges on the shop floor.

After lunch she stopped Pete.

“Hey, Pete, could you stop by my office?” Patty asked.

“Kiddo, for you anything … even that,” he answered and they both chuckled.

As Pete sat down in Patty’s office, she asked him, “How do you like your new job?”

“What’s not to like? Twice as much money and working with you!” Pete answered.

“But don’t you miss … ,” Patty stopped and struggled to gain her composure.

Peter helped her, “Working on the shop floor solving process problems?”

“Yes, so much so that I could almost cry,” Pete finished.

They were silent for awhile.

Then Pete suggested, “Why don’t I see if I can find us a problem.”

Patty smiled. Pete was always well connected.

A few days passed and Patty had just about forgotten about their meeting. There was a knock on her door and Pete stuck his head in.

“Hey kiddo, we have an assignment,” Pete shouted cheerfully.

Patty perked right up.

“What’s the scoop?” she asked.

“You know the new program that rewards cost savings?” Pete asked.

“Sure, I think it is a great idea,” Patty responded.

“There is a conflict in our plant in Santa Clara. Management wants to give a $10,000 reward and the senior purchase manager is blocking it,” Pete elaborated.

“Why?’ Patty asked.

“The engineer deserving of the reward purchased a solder paste that improved uptime,” Pete said.

“Sounds great, what is the issue?” Patty asked. “Let me guess. The better solder paste costs more?” she asked.

“Yep!” Pete responded, “One penny per gram.”

“Mike Madigan wants someone to negotiate the situation. Why not us?” Pete asked.

Patty quickly sent Mike an email offering to help. He gave her the go ahead shortly thereafter.

In a matter of days the arrangements were made and Patty and Pete were on a jet from Boston’s Logan Airport to San Jose, California.

Their flight had taken off and they were enjoying a snack, when Pete commented, “Let’s hope we don’t find someone there like the guy who wanted to assemble the boards without the boards,” Pete chuckled.

At this comment, Patty almost choked on her sparkling water. About four years ago, when Patty was just starting out, they were working on a critical project. The manager in charge wanted the boards to be assembled on a certain date.  Unfortunately, the PWBs did not arrive on time, even though all other components, connectors, and the other hardware where ready. The manager, in frustration, came out to the line on the scheduled start date and was furious that the boards were not being assembled.

The manager asked the lead engineer, “Why aren’t the boards being assembled?”

The lead engineer responded, “The PWBs did not arrive from the vendor.”

To this the manager responded, “Aren’t you going to assemble them anyway?” (See note below.*)

This was their favorite story about the occasional comedy in electronics assembly.

It seemed like no time at all and Patty and Pete were sitting in the conference room that had been reserved for the meeting. They introduced themselves to a young engineer who was sitting in the room waiting for the meeting to start. His name was Dave Ferris.

“So Dave, you are the cause of this meeting, eh?” Pete teased.

“I guess so. I can’t believe how hard it is to sell productivity here. The amount of time the new solder paste saves enables us to produce 1,000 more units per year on each line. And these boards are super expensive, with high margins. Admittedly the solder paste costs $0.01 more per gram, but the additional profit is over $800,000 per year for each of our three lines,” Dave Ferris explained.

“How did you perform the calculations,” Patty asked.

“I went to a workshop run by this quirky, cheerful guy everyone calls ‘The Professor.’ He was amazing,” Ferris replied.

Pete and Patty both chuckled.

“We know The Professor well,” they chimed in unison.

“We assume you used ProfitPro for the calculations?” Pete asked.

“Yes,” Dave responded with a surprise in his voice that they would know about such things.

Will Patty and Pete save the day?  Will Dave get his award?  Stay tuned to see.

Cheers,

Dr. Ron

*As hard as it is to believe, the story about building the boards without the PWBs is true.  Thanks to ITM.

Talking Cleaning with Mike Bixenman

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Folks,

There is a lot of interest in cleaning PCBs assembled with no-clean solder pastes. Recently I discussed the topic with my good friend Mike Bixenman of Kyzen.

Dr. Ron (DR): Mike, many of the best performing lead-free and lead containing solder pastes today are no-cleans. They have been designed to solve assembly problems like graping and the head-in-pillow defect. For the vast majority of applications, the small amount of residue left by a no-clean is not a problem. However, some assemblers want the performance of no-cleans, but need to clean the no-clean residue as they have extreme reliability or cosmetic requirements. Are there cleaning solutions for these situations?

Mike Bixenman (MB): Absolutely!

DR: Can you tell use a little bit about these cleaning solutions?
MB: Several factors come into consideration when engineering electronics assembly cleaning agents. Design factors include the soil make-up, heat exposure, Z-axis clearance under bottom termination components, material compatibility, and cleaning equipment. Typical process goals require that all flux be removed in one cleaning cycle, shiny solder joints (no chemical attack to the alloy), fast production speed, no material effect to labels and other materials of construction, long chemistry bath life, and low operating concentrations.

Cleaning solutions vary depending on the cleaning equipment. For solvent systems, a solvent cleaning agent is needed – with properties that allow for non-flammability, constant boiling mixture, and being environmentally-friendly to workers and the environment. For solvent cleaning agents that are rinsed with water, the cleaning agent requires a solvent mixture that can be rinsed with water while matching up to the soil and cleaning equipment. For aqueous cleaning agents, the cleaning agent is engineered with properties that provide solvency for the soil, polarity for inducing a dipole and/ or to oxidize and reduce the soil, low surface tension to reduce the wetting angle, buffers to stabilize pH, defoaming to reduce the tendency to foam at high pressures, and inhibitors to widen the passivation range on metallic alloys.

The most critical property is the nature of the soil. As soldering temperatures rise and the time exposed to higher temperatures increase, solder paste material supplies must improve the oxygen barrier and prevent flux burn out. This requires higher molecular weight compositions that may change the nature of the soil and the cleaning solution needed to remove the soil. Other factors such as processing conditions and how these conditions can change the soil’s cleaning properties must be considered. For example, excessive exposure to heat may polymerize the flux residue rending the soil uncleanable. To better understand and plan for these factors, solubility testing and matching the cleaning agent to the soil assist formulators in designing cleaning agents that are effective on a wide range of soldering material residues.

DR: What type of equipment is typically needed?
MB: Two key factors must be matched to clean:
1: Potential energy of the cleaning agent for the soil and
2: Kinetic energy of cleaning machine for delivering the cleaning agent to the soil necessary to create a flow channel needed to rapidly displace the soil.

The cleaning machine requires energy to deliver the cleaning fluid across a distance and create enough force to deflect fluids under the Z-axis. The capillary attraction for moving the cleaning fluid into an out of tight gaps is created by fluid flow, spray impingement pressure and surface tension effects. When cleaning under tight standoffs, cleaning agents that wet (form small droplets) improves capillary action, penetration and wetting of the residue. The solubility rate is dependent on the soil, temperature effects and concentration of the cleaning agent needed to dissolve the soil. Hard soils clean at a slower rate and remove the soil in a concentric (tunneling effect) manner. Soft soils clean at a fast rate and remove the soil in a channeling (multiple tunnels) effect.

The Z-Axis gap height has a direct correlation to the energy required to penetrate and remove the soil under components, time required to clean the soil and wash temperature. The irony is that lower Z-axis gaps increase capillary action of the flux for underfilling the bottom side of the component. When this occurs, flux residue dams up and closes any flow channels under the component. Research findings indicate that high pressure coherent spray jets are needed since energy drop is less and defective energy is higher. The wash time needed to clean under a 1 to 2 mil gap as compared to a 4 to 6 mil gap can range from 4t o 8 times longer. Higher wash temperatures increase the softening effect and aid in penetrating and removing the soil. The net effect is that, as components decrease in size, the Z-Axis gap height reduces and the cleaning factors needed to clean the soil increase. These effects favor spray-in-air cleaning equipment over immersion cleaning equipment.

DR: How are the results of cleaning assessed, so that we know that the boards are truly clean?

MB: The first level that we judge cleaning performance by is the visual presence of the residue post cleaning. Most cleaning processes have no problem with removing surface residue from the assembly. The issue is the residue under the bottom side of the component. This complicates the issue since the residue under a specific component is where most failures occur. These site-specific failures may reduce the confidence in existing IPC standards that correlate anion and cation ionic residues over the entire board surface area. So, when designing the cleaning process, we use test cards with bottom termination components and judge cleaning performance by the level of flux residue remaining under those components. To achieve this value, all components are removed and the surface area of the residue under components is graded and statistically analyzed.
Let me finish by adding that highly dense interconnects assembled onto circuit boards is advancing at a rapid pace. Traditional SMT component spacing between conductors was larger. No-clean post soldering residues posed minimal risks to reliability. The information age has spoiled us in expecting higher functionality in smaller spaces. As assembles reduce in size and increase the levels of functionality, cleaning becomes more important. I hope that the cleaning factors discussed in this interview provide insight into cleaning process design considerations that may be of help.

DR: Mike, thanks. Who should folks contact if they would like more information on cleaning boards assembled with no-clean solder pastes.
MB: Thanks for letting me share with your readers. I would be glad to help anyone with the cleaning challenges they face. Contact me at [email protected].

Cheers,
Dr. Ron

Patty on Call

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Let’s see how Patty is doing with the latest crisis …

Upon hearing Claire Perkins inform her that Rob was in the hospital, Patty froze and her face looked ashen. She quickly recovered and got her cell phone out to call Rob’s mother.

“Mom, what has happened to Rob?” Patty said, her voice quavering a little.

“He hurt his back at the gym, he can hardly walk. He collapsed under a heavy barbell. His head was injured too. He was unconscious for five minutes. I’m almost at the hospital now,” Rob’s mother, Hilde Gunther replied.

“I’ll see you there,” Patty said.

Both Sam and Mike insisted that someone take her to the hospital, but Patty refused.

Patty looked at her watch, it was 9AM. Rob was working a “swing shift” for six weeks and didn’t have to go into work until 10AM, so he went to the gym from 7:30 to 9AM most days. Patty had been teasing Rob that his workouts were getting too vigorous. She knew he was trying to snatch over 250 pounds as he was in a friendly competition with one of his friends, Fred, to see who would be the first to accomplish this significant feat. She wondered if this goal led to his accident.

The drive seemed to take forever, but soon she was at his emergency room bed. Rob was awake but his face was black and blue.  Patty didn’t notice her mother-in-law, as she quickly ran to Rob’s side.

“Rob, what happened?” Patty cried.

“The good news is, I snatched 250!” he chuckled, which caused him to grimace in pain. “It was 260 pounds that was my downfall, I collapsed under the weight,” Rob went on.

“How bad are your injures?” Patty asked, a little frustrated with Rob’s levity.

“My back hurts so much, I can hardly walk, my face just looks bad. I’m going for an MRI in a few minutes, they’re worried I might have a slipped disk,” Rob answered, becoming much more serious.

Just then an MRI tech came.

“Well Mr. Gunther, we are going to squeeze you in, so I need to put you ‘On Deck’ for an MRI that opens up. Realistically, it could be two or three hours,” the tech commented.

Both Patty and his mother kissed Rob on the part of his head that wasn’t black and blue as he left. After Rob was taken away, Patty chatted with her mother-in-law for about 30 minutes.

Even though to some people it would seem strange, Patty had a way of compartmentalizing things, she knew she could not help Rob, except to pray for him which she had already done. So, she decided to do some work on her laptop. Fortunately the hospital had WiFi.

Patty had some unfinished business from what she learned on her trip investigating NMAC/I/O. She wrote an email to the GMs of the sites using that cheaper solder paste that had the response to pause problems or that required kneading before being used, suggesting that they change to one of two corporate-approved pastes that didn’t have these issues. She also wrote a note to the people that were using a full wavesoldering process for a PWB that had only two through-hole components, saying solder preforms should be used with the reflow process.

As Patty finished the emails, she observed the activities of the MRI section of the hospital where she was waiting. It occurred to her that this was a process, just like assembling electronics. Instead of stencil printers and component placement machines, there was an MRI machine. There were techs that ran the MRI machines, just as there were operators on an SMT line. The nurses were like the process engineers, and there were some medical doctors that were like mangers and execs at her company. Instead of producing electronics, the MRI section was producing MRI scans. There was little difference.

Patty got curious and she decided to ask the scheduling assistant a few questions.

“Excuse me, my husband is getting an MRI and I have a few questions,” she asked Sara Carter the assistant.

“Sure,” Sarah said, “go ahead.”

“About how much does an MRI scan cost?” Patty asked.

“It varies depending on the extent of the scans needed, but $3,000 is a good estimate,” Sarah responded.

Patty asked more questions and learned that there were 5 MRI units and she assessed the headcount and floorspace needed to support the MRI unit. She also found out that each of the 5 MRI units averaged 9 scans per day. It then occurred to her that she could use ProfitPro to estimate the cost of a typical MRI scan. Under The Professor’s tutelage she has gotten quite good at estimating burden labor rates, etc, which would be needed for the calculation. She got her laptop out and using ProftiPro, in a few minutes estimated that the hospital’s cost of an MRI scan should be only $390!

“Why does it cost our insurance $3,000?” she thought.

It then occurred to her that her good friend from her days at Tech, Emily Chen, was a radiology resident at the hospital. She decided to send her a note and, in addition to telling her about Rob, ask about the MRI scan cost.

After sending the email, she asked her mother-in-law if she would like to get a cup of coffee. In a short time, they were heading to the hospital cafeteria. Before they left, they found out that Rob was just starting his 45-minute MRI scan.

Fifteen minutes later they returned, and Patty was surprised that she had already received an answer from Emily.

“Patty, I’m so sorry to hear about Rob. You probably won’t hear the official news on his MRI until tomorrow, but I will take a look at it and call you later today. BTW, my boyfriend works in the finance department here. I’ll find out about the cost. But, your numbers sound way off.”

Twenty minutes later Rob was finished. His doctor had given him some pain killers and muscle relaxers, so Rob was a little more comfortable, but the doctor wanted Rob to stay overnight for observation. Rob soon fell asleep from the medication. Patty decided to stay with Rob and by 4PM, she asked her mother-in-law if she could pick the boys up from day care.

At 4:30 PM another email arrived from Emily.

“Patty, good news. I looked at Rob’s MRI scan and it looks fine. He probably just severely strained a muscle. He’ll be as good as new in a month or so” Emily’s note began. Emily’s note went on, ”My boyfriend looked up the cost for the hospital to run an MRI scan. You were close, it costs $410. Neither of us can believe it. Where does the extra $2600 go?”

Dr. Ron note: I have done some investigations into MRI scan costs. As surprising as it sounds, these numbers are about right, the base cost for a hospital to perform an MRI scan is in the $400 range, but they have to charge $3,000 to break even. Considering that many hospitals are non profits and are losing money adds to the confusion.  At this point, I don’t claim to understand the cost structure of running a hospital, but one would think that one of the most critical questions in the current healthcare cost crisis in the United States, would be to understand why $3,000 must be charged for a $400 procedure to break even.  

 

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

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

Shear Thinning

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Solder pastes are a very complex “fluid” of high viscosity. Their behavior, when experiencing shear stresses, is “non-Newtonian,” meaning that their viscosity is not constant as the shear stress varies. The viscosity of solder pastes is high when there is little or no shear stress and low when shear stresses are high such as when the paste is forced through a stencil aperture. This property is called thixotropy. The solder paste being thixotropic is ideal, as it enables the stencil printed “brick” of solder paste to retain its shape after it is printed, yet the low viscosity, when stressed, allows good filling of the stencil aperture.

Many might assume that this relatively complex phenomenon is the end of the story. However, there is at least one other well-known property of solder pastes during printing that is important: response to pause. A solder paste with a poor response to pause will stiffen when permitted to idle for as few as 15 minutes. When this occurs, the first print likely will have insufficient solder paste for effective assembly. Hence, response to pause is a critical variable to measure when evaluating a potential solder paste.

Another important solder paste property has only recently become well known: shear thinning. My Indium Corp. colleague Tim Jensen was one of the first to point this out. Shear thinning is a property of some solder pastes in which the viscosity becomes lower and lower as the paste is repeatedly printed (see figure below). The x axis is number of prints, the y axis is the viscosity. It is normal for the viscosity to go down during the print, but the viscosity should recover as the “good paste” does, not have a downward trend as the “bad paste.” The resulting drop in viscosity that the bad paste exhibits will often result in too much paste being printed and potentially lead to defects such as shorts or solder balls. Unfortunately, if not tested for, shear thinning might first be observed after a paste has been implemented on the line.

If you are interested in a method to test a paste for its resistance to shear thinning, send me a note and I will send a test protocol.