Tag Archives: Dr. Ron Lasky

Calculating Confidence Intervals on Cpks

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Let’s look in on Patty, it’s been awhile.

Patty was looking forward to sleeping in.  Normally she was up very early, sometimes before 5:30 am, after usually getting to bed too late, so she was looking forward to an alarm set at 7:45 am. The kids were off from school and Rob was taking them skiing, so all agreed a 7:45 am wake up time was reasonable.  Since she had no early meetings, her scheduled 9 am arrival at her Ivy University office was also in the cards.

Patty was sleeping soundly when she heard her seven-year old twin sons shouting, “Mom! Dad! Come quickly.”   At the same time, their two-year old beagle, Duchess, started barking.

Her heart pounding, Patty raced to the racket now being produced by this energetic trio.  As she arrived she saw her sons and Duchess looking out of their back window to see a beautiful female deer eating from their bird feeder, just 30 feet away. The entire family was involved in a bird counting exercise and had noticed, several times, that the bird feeder was “wiped out” overnight. This mystery was now solved.

The entire family agreed that it was hard to be angry at the doe, as deer are such beautiful creatures.

Figure 1.  A Female Deer at the Bird Feeder at Patty’s House

 

It was 6:15 am and it didn’t seem to make sense to go back to bed.  So, Patty stayed up and was off to Ivy U in less than 30 minutes.

Patty had a rather light week as she had guest speakers for her two lectures.  However, she was sitting in for one of the engineering school’s senior professors later in the day.  This fellow prof had asked her to sub for him as he was called to an emergency meeting overseas.  Her topic was manufacturing processes; one with which she felt very comfortable.  But, she had to admit to being a bit nervous sitting in for one of Ivy U’s most famous professors.

As was her usual practice, Patty checked her email first.  After going through the first 5 or 6, she saw an email with the subject header, “Ivy U Professor Wins Prestigious Queen Elizabeth Prize for Engineering.”  As she opened the article, she was stunned as she saw a photo of the professor for whom she was substituting later in the day.  The article went on to explain that this prize was like the “Nobel Prize” for engineering.

As she finished her emails she was relishing the thought of having a less hectic day and week ahead.  Maybe she would even have time to read the Wall Street Journal during a relaxing lunch.  Suddenly, her phone rang, startling her a little.  She picked up the receiver to hear a familiar voice.

“Professor Coleman, this is your most faithful student Mike Madigan,” Madigan cheerfully said.

Madigan was CEO of ACME at large electronics assembly contractor. Patty worked at ACME before becoming a professor at Ivy U. Her husband, Rob, and sidekick, Pete, were also ACME employees, but were now all at Ivy U.  Pete was a research assistant and Rob was just becoming a research professor.  Although they all enjoyed their time at ACME, they were much happier at Ivy U.  All three had a part-time consulting contract with ACME and Madigan was typically their main contact at their former employer.

“Mike! What’s up?” Patty said cheerfully.

“We are evaluating a new solder paste and I’m concerned we might make a mistake if we switch,” Mike responded.

“How so?” Patty asked.

“Well, we agreed that consistency in the transfer efficiency (TE) of the stencil printed deposits was the most important criteria,” Madigan began.

“That sounds reasonable as most of our past work has shown that a consistent TE is a strong determinant of high first-pass yields,” Patty responded.

“Right! But the difference between the pastes is only two percent. The old paste has a Cpk of 0.98 and the new paste 1.00,” Mike went on.

“I sense there is more to the story,” Patty suggested.

“Yeah. The new paste has a poorer response to pause,” Madigan said.

“Yikes!” Patty almost shouted.

Patty had shown, time and time again, that poor response-to-pause in the stencil printing process can hurt productivity and lower profitability considerably.

“My sense is the two percent difference in Cpk, might not be significant,” Mike suggested.

“Mike, I think you are on to something. What printing specs were you using and how many samples did you test?” Patty asked.

“The lower TE spec was 50% and the upper 150%. We tested 1,000 prints,” Madigan answered.

“Let me do some homework and I’ll get back to you,” Patty said.

“One problem. Can you get back by 3 pm today? The new solder paste supplier is coming for a meeting at 4PM and is pressing us,” Mike pleaded.

“OK. Will do,” Patty said, sighing a bit.

“There goes my somewhat relaxing day,” she thought.

It was a good thing she had already prepared her lecture and that it was scheduled for 4:30PM.

For several hours Patty thought and searched through some textbooks on statistical process control.  Finally, she came upon the solution to the problem in Montgomery’s Introduction to Statistical Quality Control.

“Perfect!” she thought.

She did finish early enough that she could read the WSJ over lunch, marveling, as always, that she was the only person her age that enjoyed reading a real newspaper.

She called Madigan at 3 pm.

“Mike, I think I have your answer.  I found a formula to calculate the confidence intervals of Cpks,” Patty started.

“And the answer is?” Madigan asked expectantly.

“The Cpk 95% confidence interval on the new paste is 0.95 to 1.05, however the old paste is 0.93 to 1.03,” Patty began.

“So, even I can sense that they aren’t different,” Mike commented.

“Yes, since the confidence intervals overlap, they are not statistically different,” Patty agreed.

Figure 2. The Confidence Interval of the Cpk on the New Paste is 0.95 to 1.05.

 

They chatted for a while and Madigan asked if Patty could join the first 20 minutes of the meeting by teleconference.  It was a bit close to her lecture start time, but she agreed.

Patty had met Madigan’s son at West Point when she visited there to be an evaluator for a workshop two years ago.  She decided to ask how he was doing.

“Mike, how is your son doing at West Point?” she asked.

“Thanks for asking. He is now a Firstie and was in the running for First Captain, but he just missed it.  It’s a good thing he takes after his mom,” Madigan proudly responded.

“Wow! That’s great,” Patty replied.

“I have to admit though, my wife and I are a bit nervous. He has chosen armor as his branch and there is a good chance he will see combat sometime in his career,” Madigan responded with a bit of concern in his voice .

They chatted for a while more and Patty was touched to see so much humanity in Mike Madigan.  He seemed much changed from his gruffness of earlier years.

Cheers,

Dr. Ron

As always, some of this story is based on true events

 

Cpk is Still King in Evaluating an SMT Solder Paste Printing Process

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Folks,

If you think about it, to evaluate any process you typically want to know its precision and accuracy. Look at the dart players in the Figure 1 below. The yellow player has good precision, but his accuracy is off. The green player has such poor precision, it is hard to tell if his accuracy is good. The yellow player will typically be easier to correct, as she just needs to change her aiming point.

Figure 1. The yellow player has greater precision. She only needs to change her aiming point.

 

 

 

 

 

 

 

 

 

 

Recently I was asked to evaluate several solder pastes to determine which printed better. We used transfer efficiency (the volume of the stencil printed solder paste “brick” divided by the stencil aperture volume) as the evaluation metric, expressed in percent. So 100% would be the target. The lower specification limit we choose was 50% and the upper specification at 150%.

Figure 2. Data from Pastes A and B.

 

A good result would be an average of 100% with a “tight” distribution. The “tightness” of the distribution being determined by the standard deviation. Figure 2 shows data from two pastes. Note that Paste A has an average of 100% and a standard deviation of 16.67%, whereas Paste B has an average of 80% and a standard deviation of 30%. Clearly, Paste A is superior to Paste B in both accuracy and precision. But what is the best way to express this difference? Is there one metric that will do it? Cpk is the answer.

Cpk is one metric that is sensitive to both the accuracy and precision. Cpk is defined as:

 

 

 

Where x is the average and S is the standard deviation.

Using these equations, we see that the Cpk of Paste A is 1.0, whereas the Cpk of Paste B is 0.333. Note that Paste B has a significant number of data points (about 17%) outside of the specification limits, however, Paste A has almost no data points out of specification.
So when evaluating most processes, Cpk tells it all!

Cheers,
Dr. Ron

 

Solder Alloy Density Equation: Why What Most People Think is Right is Wrong

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Folks,

It’s hard to believe but I have been blogging for over 10 years. In all of this time, with the hundreds of posts I have made, the most popular topic by far has been calculating density in a metal alloy. One of the reasons for this popularity has been the belief that the density of an alloy is determined by the equation

Eq. 1     

Where x is the mass fraction of metal 1, y the mass fraction of metal two, ? (rho) the respective densities and ?t the total or alloy density. I have shown in the past that the correct formula to calculate the alloy’s density is:

Eq. 2    

This formula is derived below again.

People continue to ask why equation number 1 is not correct, so I have posted an explanation that has been modestly helpful.  I have thought of an example that shows that Eq. 1 cannot be correct and have now derived an equation in the form of Eq. 1 that uses volume fractions instead of mass fractions.  This derivation is also below and the equation is:

Eq. 3       

However, Eq. 3 is not very useful as the volume fraction of each metal is not as readily available as the mass fraction, which is easily measured with a scale.

Now, to give an example that shows that Eq. 1 is unreasonable, let’s consider a thought experiment that will help us conclude that Eq. 1 can be way off. Consider a cubic meter of air in a container 1 meter on a side at room temperature. The cubic meter of air will weigh 1.225 kg. (The fact air weighs this much surprises many people.) Inside the container is 1.225 kg of a fine gold powder. We blow the gold powder into the air and it covers all of the inside with an equal concentration. The powder is so fine that it will remain suspended for a short time. So we will consider this an alloy of gold and air.  The mass fractions x and y are equal at 0.50.  So if Eq. 1 were to hold true the density of the “alloy” would be:

Eq. 4    

Figure 1. The gold dust and air density experiment.

The weight of the 1 cubic meter container would now be 9650.6 kg/m3 * 1 meter3 = 9650.6 kg!  Whereas we know it to be 1.225 kg + 1.225 kg = 2.45 kg. Eq. 2 or 3 will provide the correct answer.

The correct derivations are below:

 

Eq. 5 

Cheers,

Dr. Ron

 

Full Autonomous Autos: Decades Away and In Need of Unprecendented Reliability

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Folks,

Since writing my last blog post, there continues an unending litany of articles about the imminent arrival of the self-driving car. I stand by my position that a fully functional self-driving car is decades away. Let me discuss why.

I was recently asked about Google’s efforts amide claims of tens of thousands of hours of self-driving.  Wikipedia has the best answer:

As of August 28, 2014, according to Computer World Google’s self-driving cars were in fact unable to use about 99% of US roads.[51] As of the same date, the latest prototype had not been tested in heavy rain or snow due to safety concerns.[52] Because the cars rely primarily on pre-programmed route data, they do not obey temporary traffic lights and, in some situations, revert to a slower “extra cautious” mode in complex unmapped intersections. The vehicle has difficulty identifying when objects, such as trash and light debris, are harmless, causing the vehicle to veer unnecessarily. Additionally, the LIDAR technology cannot spot some potholes or discern when humans, such as a police officer, are signaling the car to stop.[53] Google projects having these issues fixed by 2020.[54]

Ford claims it will have self-driving cars deployed by 2020. However, a quote by Jim McBride, Ford technical lead, sheds some light:

“Q: What are the big technical challenges you are facing?

“A: When you do a program like this, which is specifically aimed at what people like to call ‘level four’ or fully autonomous, there are a large number of scenarios that you have to be able to test for. Part of the challenge is to understand what we don’t know. Think through your entire lifetime of driving experiences and I’m sure there are a few bizarre things that have happened. They don’t happen very frequently but they do.”

Level four is indeed impressive, but it is not full autonomous as described by SAE:

SAE automated vehicle classifications:

Level 0: Automated system has no vehicle control, but may issue warnings.

Level 1: Driver must be ready to take control at any time. Automated system may include features such as Adaptive Cruise Control (ACC), Parking Assistance with automated steering, and Lane Keeping Assistance (LKA) Type II in any combination.

Level 2: The driver is obliged to detect objects and events and respond if the automated system fails to respond properly. The automated system executes accelerating, braking, and steering. The automated system can deactivate immediately upon takeover by the driver.

Level 3: Within known, limited environments (such as freeways), the driver can safely turn their attention away from driving tasks.

Level 4: The automated system can control the vehicle in all but a few environments such as severe weather. The driver must enable the automated system only when it is safe to do so. When enabled, driver attention is not required.

Level 5: Other than setting the destination and starting the system, no human intervention is required. The automatic system can drive to any location where it is legal to drive.”

The difference between level 4 and 5 is enormous.  Just a few days ago I drove a level 2 Volvo SC90. It was a lot of fun. It had autonomous steering and acceleration/breaking. It worked very well, but it needed the lane markers, a not insignificant requirement.

Level 4 could not take you on a trip from my house, in Woodstock, VT, to a meeting in downtown Boston. To start, some of the trip is on roads without lane markers. Let’s also assume that there is construction with hand written signs directing the cars to a detour. There is also a traffic cop who signals you to stop and roll down the window to listen to instructions, a huge pot hole that has a hand-made warning sign is in downtown Boston, etc. None of these challenges would be unusual for a human, but a challenge for Level 4 autonomy.

Ford’s self-driving car has the equivalent of 5 laptop computers.

 

Singapore has implemented what appears to be level 3 vehicles, but there is a human backup and the route is specially selected.

All of this is exciting news.  But getting a vehicle that can handle 99% of human driving tasks with 99.99% reliability (let’s call it Phase I) will be easier that getting the last 1% with 99.99999% reliability (Phase II).  I agree that Phase I may be only years away, but Phase II is decades away.  Without Phase II, the driverless car that has no steering wheel or gas pedal is not achievable.

How does all of this affect us in electronics assembly?   It will be an interesting adventure to work with the auto industry on the extreme reliability required.  My guess is that this reliability need will be a dominant theme in the future.

Note: Probably the best article on this topic was in the June 2016 issue of Scientific American.

Cheers,

Dr. Ron

 

Smartwatches Will Never Be A Dominant Technology

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Folks,

We saw a while ago that the decline in sales of PCs, tablets, and smartphones is easy to understand.  There are two main drivers of this trend:

  1. The market is saturated. In other words, almost everyone that wants one has a device.
  2. These devices have such high capability that upgrading is more often done due to worn out units. It is not driven by the need for the new, but only to incrementally improve existing devices.

It is interesting to consider how much effort has been spent on the reliability of electronic solder joints when the anecdotal experience of most people is that, if a unit fails, it is more often due to some mechanical problem like a worn out keyboard or an audio plug that no longer works.  We are replacing old units, not for electrical fails, but for mechanical wear issues.  It will be interesting if someone starts addressing this need more vigorously.

Even with the market for PCs, tablets, and smartphones stabilizing, the numbers of units sold per year is still large. PCs sell at a rate of about 250 million per year, and tablets at 150 million per year, as discussed in the post mentioned above. Smartphones are truly a phenomenon however, with units per year in the 1.5 billion range. Time will tell, but I wouldn’t be surprised if smartphones will eventually be considered more transformational than PCs.

While such devices are sold in hundreds of millions to billions annually, smartwatch yearly volumes are only in the 10s of millions.  I don’t see this figure moving upward much ever.  Let me explain.

Having used an Apple Watch for over a year now, I think I am qualified to discuss the usefulness of this and similar devices.  First of all, let me state that I like my Apple Watch and use it quite a bit. I like the feature that I can see the outdoor temperature with a flick of my wrist, and I use the fitness tracking app constantly. I used to miss an occasional phone call when my mobile phone was on vibrate. However, with my Apple Watch also vibrating on my wrist, such misses are a thing of the past. In addition, I can pull a Dick Tracy and speak into my watch’s telephone feature if I want.

But these features are not enough.  First of all, I must have an iPhone for the Apple Watch to work, but, more importantly, the small size of the watch’s face makes it difficult to use by tapping. Remember calculator watches? It is simply easier to just get out my iPhone 6S to perform various tasks. This is an important aspect of the interaction of humans and electronics; human size factors dictate that a certain minimum size exists for a device to be useful. For most folks, this size is that of an iPhone 6S,® or others might need an iPhone6S Plus, or the largest Samsung Galaxy or equivalent smartphone from other manufacturers.

Considering its diminutive size, I don’t see the smart watch being a dominant technology unless someone invents a projection screen device as envisioned in the Cicret.  And please remember the Cicret is only a concept, not a working device.

Cheers,

Dr. Ron

PCs, Tablets, and Mobile Phones are not Dying (and Will Continue to Present Voiding Challenges)

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Folks,

Looks like Patty and Rob are on another adventure.  Let’s look in ….

Patty had been driving the same 2001 Saab station wagon since college. It had been a great car, but, with almost 200,000 miles on it and its outdated safety features, perhaps it was time for a change. Both her and Rob’s parents had been bugging them about getting a new, safer vehicle for a while. Finally, for her birthday, both sets of parents chipped in to give her a significant down payment on a new car.  They even suggested which specific car she should get. It was a car with one of the best safety records, not an insignificant concern for doting grandparents.  The manufacturer has a goal of no deaths in its automobiles by 2020.

As Patty and Rob went shopping, they were overwhelmed by the features that 2016 autos have. Detections of cars in the “blind spot,” warnings when the car leaves the lane, warnings and prevention from backing in to something, reading the speed limit signs, pairing to smartphones, the internet, and on and on.

“Patty, these aren’t cars; they are computers that you can drive,” Rob commented.

“Actually this car has 13 computers,” the salesperson chuckled.

“What is the soonest we can take the car home?” Rob asked, expecting it to be 3 or 4 days.

“You can take it home in an hour,” the salesperson affirmed.

In an hour, Patty and Rob were driving home in their new car, amazed at its capabilities as a “computer on wheels.”

“Rob, look at this. As we pass the speed limit sign, the speed limit is shown on the speedometer,” Patty exclaimed in amazement.

They stopped in their driveway and played with the car’s features for 30 minutes, streaming music from their smartphones, connecting to the internet, and changing many modes on the dashboard display.  It was more fun than their first time playing with a tablet.

Figure 1.  Patty and Rob’s new car has 13 computers

Two days later, it was Monday and Patty, Rob, and Pete had been asked to see the Professor for a brainstorming session.  Recently, as Patty’s career had skyrocketed, she had been working with the Professor less and less.  The trio agreed to meet in Patty’s office so they could head over to the Professor’s office together.

“Hey, this is just like old times!” Pete exclaimed.

“I agree,” added Patty, “I miss some of the adventures we used to have.”

The professor welcomed them in.

“I hope all of you had a chance to review the material on the many links that I sent you,” the Professor began.

They all murmured that they had.

They reason I asked you to come is that I am going to be interviewed on national television, The topic is, ‘The Death of PC, Tablets, and Smartphones.’ I thought you all might be able to help me prepare.

They all though in unison, “Us help the Professor prepare?!”

“What are your thoughts on the ‘Death of the PC,’” the Professor asked his humble mentees.

“One of the links you sent has shows PC sales declining,” Rob said.

Figure 2. PC sales peaked around 2011 and have been declining since then.

“But, do you think it portends the end of PCs?” the Professor asked.

“This is something I have thought about ever since you sent us the links.  I think the ‘death of the PC’ people are missing some key points,” Pete replied.

“Such as?” the Professor encouraged.

“When I was a teenage we got an IBM PC XT. It had a 10MB hard drive. We replaced it in three years,” Pete began.

“Why did you replace it?” Patty asked.

“It didn’t have enough memory or processor speed for the new games.  The new PC had a 200MB hard drive. We kept that one for about 3 more years and the cycle repeated,” Pete answered.

“And what about today?” the Professor asked.

“My parents have a six-year-old computer. They recently complained they needed to upgrade it because the audio plug is worn out, some keys on the keyboard are intermittent, and it doesn’t have enough USB ports. No problem with the memory; it has 6GB of RAM and a 250GB hard drive,” Pete answered.

“So, it did not run out of memory or computer speed?” the Professor asked.

Patty interrupted, “I remember the Professor and I talking about ‘the constancy of memory metrics’. The argument was that a photo is about 1MB, a song 5MB and a movie about 5,000MB.  These metrics are approximately constant. Initially, the size of these metrics overwhelmed early computers, but now these memory metrics are small compared to the capability of current technology. The impact was that early computers had to be changed often, because people wanted to store more photos, songs, etc., but now, with computers having 1TB of memory, getting a new computer for this reason is not so compelling.”

“Maybe with the exception of some new video games, but admittedly this is a small part of the market,” Rob added.

“Well, is the PC market dying then?” the Professor prompted.

“No way!” Pete jumped in. All of us use our PCs for hours each day.  Am I the only one longing for my PC when I answer an email from my smartphone?” Pete asked.

They all chuckled.

“So, it seems that we are concluding that, today, the performance requirements for PCs, mostly laptops, have leveled off and upgrades are needed less frequently. These upgrades are often driven by mechanical failures such as connectors and keyboards, not necessarily the need for more memory or faster processor speed.  It is natural then to expect sales of PCs to level off and even go down some as, in addition to these points, the market has reached saturation.  Everyone who needs a PC has one,” the Professor summed up.

“Yeah, and the 238.5 million sold last year is not really small potatoes,” Rob added.

“What about tablets? Are they going away?” the Professor asked with a mischievous smile.

“Again, the data show a downward trend, but I’m not a believer that they are going away either,” Pete commented.

Figure 3.  Tablet sales are declining.

“I think a similar thing is happening here,” Patty mused. “Tablets are so powerful that there just isn’t an incentive to purchase one frequently. We have an iPad II that we bought in 2011 that we still use, although it doesn’t run some of the newer games.”

“And they sure are popular with our boys. We have to limit the time they spend on them,” Rob added.

“What about people using large smartphones instead of tablets?” Patty asked.

“That has definitely cut into tablet sales. Some of the new smartphones are so big that they are almost comical.  They are as big as some of the mini tablets,” Pete opined.

“Professor, I thought one of the links you sent was fascinating: 4.6 billion mobile phone users in a world of 7.3 billion people!” Rob exclaimed.

“I have a friend who works in humanitarian engineering in third world countries. He tells me that people in some places he visits, will go without food to have a cellphone. In the past, communicating with relatives 60 miles away was a one week commitment of time, because of the primitive transportation. Now, they can do it instantly,” the Professor shared.

“What about the fact that there are as many mobile phones as people on the earth,” Pete exclaimed.

“I guess some people have more than one,” Rob suggested.

“So are mobile phones dying?” the Professor asked.

“I think it is the same argument. When I was starting out at ACME, I had a mobile phone that could take photos, but the quality was really poor. By 2010 the photo quality was good, today it is excellent. I hardly ever take a camera with me, my smartphone photos are excellent,” Patty said.

“So, I’m guessing you don’t need to get a new smartphone as often because the technology has now stabilized, and improvements are only incremental?” the Professor asked.

“Precisely,” Patty responded.

“I think we agree; PCs, tablets, and mobile phones are here to stay, but their sales will be flat or slightly down due to market saturation and technology maturity.”

“Here, here,” Pete chuckled.

“Where do you see electronics growing?” the Professor asked.

Patty and Rob then shared their exciting experience in buying a new car and all of the electronics it has.

Pete then chimed in, “Don’t forget the internet of things (IoT).  I think this is the future of electronics growth, but it is not one device.  The number of devices is innumerable – and growing! And I think it will help electronics grow even faster than in the past.”

They discussed IoT for quite a while and then Rob had a thought.

“Bottom terminated components and especially QFNs will be with us for a long time as they are in all of these devices.  So the work we did for Mike Madigan on voiding should have a lasting impact,” Rob posited.

“Patty, you need to do something about Rob. He’s becoming too serious,” Pete teased.

Everyone laughed at that and got up to leave after what they all felt was a fruitful meeting.

Best wishes,

Ron

Using Solder Preforms to Reduce Voiding in BTCs

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Folks,

Let’s see how Patty and the team are doing on their presentation on voiding for Mike Madigan …

Patty was kind of down. Like millions of others, she and Rob watched, in horror, as Jordan Spieth had his meltdown at the 2016 Masters Golf Tournament. Some newscasters considered it the biggest meltdown in golf history, but Patty considered Rory McIlroy’s 2011 and especially Greg Norman’s 1996 meltdowns to be worse. She felt the NY Daily News did the best job of comparing the five worst Masters meltdowns. She agreed that Spieth would surely recover, certainly better than Ken Venturi in his famous collapse in the 1956 Masters. She was surprised that so many newscasters often seemed to not put history in as strong a perspective as it deserved.

As she sat in her office, she was reminded that she needed to finish her part of the presentation that Mike Madigan needed on minimizing voiding. Her topic was, “Using Solder Preforms to Minimize Voiding.” To her, voiding appeared to be the hottest issue in electronics assembly.  Especially voiding under bottom-terminated components, or BTCs. Rob and Pete were coming by in a few minutes to review her progress. Just as she finished, they were at her door.

“Hey, Professor! What’s the scoop on using solder preforms to minimize voiding?” Pete asked, clearly teasing by calling her “Professor.”

They all chuckled a bit and Rob added, “Yes, Professor. Let’s hear it.”

Patty began, “Remember a few years ago the standard approach to using preforms, to minimize voiding under BTCs, was to use a flux-coated solder preform and place it on the thermal pad on the PWB after printing a minimum amount of solder paste?”

“Sure! A great paper was written on it, by some of the folks at Indium Corporation,” Rob said.

Then Pete added, “I gather there is a new approach?”

“Well, think about the motivation to find another technique,” Patty replied.

“A specialized preform needed to be made, it needed flux coating and placing it was a bit of a challenge,” she continued.

“So, what’s the new technique?” Rob asked.

“Well, I chatted with Tim Jensen. Although the original technique is still used, a preferred technique using 0201- or 0402-sized solder preforms has been developed.  The preforms are purposely placed off center so that the BTC is at an angle.  This angle allows the solder paste volatiles to escape.  Since these preforms are a standard size, and not flux-coated, they will typically be less expensive and easier to handle in the assembly process,” Patty elaborated.

“How well do they work?” Pete asked.

“They work quite well. Look at these data,” Patty replied. (see Figure 1).

Figure 1. Preforms of either 0201 or 0402 reduce voiding by up to 50%.  Note that the standard deviation is also tighter by using preforms.

“Looks like the 0402 preforms do a little better than 0201s,” Rob commented.

“Yeah! And using two of them instead of one seems to help a little,” Pete added.

“It’s also striking how the preforms tighten the data up. Look at how much the standard deviation is reduced by using them,” Rob added.

The trio spent the next several hours collating all their PowerPoint slides into one 45-minute presentation. Patty then scheduled a meeting with Mike Madigan to review the entire presentation.

Epilogue: Patty, Rob and Pete reviewed the presentation with Mike Madigan using WebEx.  Mike implemented the recommendations after reviewing them with his critical customers.  By using the best solder paste, making minor modifications to the SMT processes, and using solder preforms where appropriate, ACME was able to reduce voiding to less than 10% in all products and less than 5% in most.

Cheers,

Dr. Ron

 

 

To Minimize BTC Voiding, Start with the Right Solder Paste

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Let’s see what’s up with Patty ….

Patty was just dropped off at O’Hare airport after finishing a 3 day workshop on Lean Six Sigma statistics, design of experiments, and statistical process control. Interestingly, the students were lawyers. In recent years more and more service-based organizations were adopting lean Six Sigma and it was a long time since Patty had taught such a workshop to engineers. She noted that although the lawyer’s math skills were a bit rusty, they were very good listeners and picked up the math behind lean Six Sigma topics very quickly.

After paying the cab driver, she entered the terminal and went to see an agent. She was early enough to get an early flight home, so she had called the people at the online ticket agency during the cab ride. They said the change fee would be over $300, she felt that was just too much to pay. She was delighted to see that it was only $75 at the terminal.

She looked at her paper boarding pass and saw that she had more than two hours, just enough time for a relaxed lunch at Wolfgang Puck while she read USA Today. Patty was the only person her age that she knew who enjoyed reading a paper newspaper, she guessed that she picked the habit up from her dad.

The two hours went by quickly and she was standing in line waiting to board the flight to Boston’s Logan Airport. She had now been at Ivy U for a few years and traveled much less than when she worked at ACME. She had forgotten how stressful and unpleasant traveling was. As she stood in line, the man in front of her put his smartphone on the scanner and the scanner could not read the QC code. He and the agent fumbled for a while before they got it to work. This was another place where, in her opinion, paper was still king.

Patty got on board and settled into her middle row seat. She groaned a little bit at how uncomfortable and cramped it was. Patty was reminded of what her dad used to say in situations like this; “I know it is a bit uncomfortable, but just think what the 49ers went through to get to California,” he would tease.

After takeoff, she turned on her laptop. She absolutely had to send some emails, so she signed on to the onboard WiFi. She got sticker shock when she saw that it cost $18.95!  Even though Ivy U would pay for it, the high price galled her.

After she finished the emails, a wave of fatigue swept over her and she needed a break.  She chuckled to herself when she thought of a recent event. She had taken two of her best teaching assistants (TAs) to lunch and the conversation somehow came to discussing people who hid Jews from the Nazi’s in World War II. Patty mentioned to her two young protégés about an excellent book and movie she read and saw as a teenager, The Hiding Place. The story is about Corrie Ten Boom and her family and how they hid, and hence saved, many Jews from the Nazis in Holland during WWII. Although the movie was made before she was born, it was shown at Patty’s church every few years, for the new sets of youngsters who came along. Patty mentioned to her two superstar TAs that the film was produced by Billy Graham’s organization.

“Who is Billy Graham?” they both asked in unison.

Patty struggled to keep her composure as she explained who he was. How could they not know this?  She decided to examine the situation a bit further.

“OK, you two. Who was Mickey Mantle?” Patty asked.

The youngster’s both looked at each other.

“We have no clue,” they chuckled.

Patty though she would try a few more, “Nikita Khrushchev?”

Nothing.

“Roy Orbison?”

Nothing.

Patty started humming a few bars of Orbison’s most popular song.

“Oh, Pretty Woman,” the boys said in unison.

Patty thought to herself, “Each of these young lads are the best student in every class that they take and yet they don’t know these ‘celebrities’?”

The next day Patty arrived at her office early to meet with Rob and Pete to discuss how the presentations that they were making for Mike Madigan on voiding were coming. Patty had arrived so late the night before, that Rob was already asleep. She did not see him in the morning as it was her turn to get the boys ready for school and he was off early to get in his 90 minutes of exercising. So, they had no chance to discuss the progress of the presentation.

“Pete, your presentation of BGA voiding is terrific. How is my hubby doing on BTC voiding?” she chuckled as she looked at Rob.

“I feel like I’m going to get yelled at ’cause I didn’t do my homework,” Rob said sheepishly.

“Yikes! We only have a few days,” Patty responded. “And I have yet to do my part on using solder preforms to minimize voiding,” she went on.

“I’m only teasing. I have quite a bit of info,” Rob said.

“We have been out of the mainstream for a while and one thing is for sure, voiding is the number one issue among assemblers today.  So many people are assembling QFNs and are struggling with voiding. Voiding with some solder pastes can be over 50% of the area,” Rob went on.

“Wow! With 50% voids, think of how poorly the heat is being transfer away for the BTCs,” she looked at Rob and chuckled. “Remember, ‘BTC’ not ‘QFN,’ Patty went on.

“Yes ma’am,” Rob jokingly replied.

“Can you imagine the effect on reliability and field issues with so little heat being removed? The ICs inside the BTCs must be frying” Pete added.

“Voiding at this level has got to be really costly,” Patty mused.

“One of the things that really helped me was that I found quite a few experiments on voiding,” Rob added.

“What were some of the key points?” Pete asked.

“Well, as you might expect, the solder paste is typically the most critical part of the process. Some pastes have voiding lower than 10% with others above 50%,” Rob replied.

“What about the process?” Patty asked.

“Well, the reflow profile can be very important, as is controlling the PWBs and components. But, with the best pastes, it has been found that you can control the voiding content even if you can’t change the reflow profile and the PWBs and components have some issues,” Rob responded.

“Look at the x-rays of poor and good voiding between two pastes,” Rob said.

“What a difference,” Patty and Pete said in unison.

“What about the stencil design and venting?” Pete asked.

“Chris said that stencil design for venting is not as critical as once thought, although a window pane design is usually used,” Rob replied.

Figure 1.  The window pane design for the stencil is used to permit venting.

“So it sounds like starting with the best solder paste solves 90% of the problem and adjusting the process, say with the right reflow profile, helps refine the result,” Patty summed up.

With this Rob went off to put the finishing touches on his PowerPoint® slides for his part of the presentation, while Patty started working on her part of the presentation on using solder preforms to reduce voiding.

Two weeks later.

Patty’s mom and dad came for a visit on a Sunday. Her mom had graciously offered to bring a complete Sunday dinner. Patty, Rob and the boys were grateful for the delicious meal. As they began to eat, Patty shared the story of her best students not knowing Billy Graham, et al.

“But, what was even more surprising was that I ended up asking 10 or 20 more students and only one had ever heard of any of these four ‘famous’ people,” Patty sighed.

“It’s your age,” Patty’s mom replied.

Thirty years old was not that far in the rear view mirror for Patty and she really didn’t consider herself old.

“These youngsters were born in the late 1990s, a generation after these people were prominent,” her mom went on.

“Mom’s right.  Do you know Billy Sunday, Ty Cobb, Glenn Miller, and Trotsky?”  her dad asked.

“Who?” Patty asked.  And then she chuckled, getting the point.

After a brief pause, she said, “I do know who Trotsky was; tell me about the others.”

Cheers,

Dr. Ron

As always, this story is based on true events.

 

BGA Voiding in Electronics Assembly

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Patty had to admit that the last few weeks were exciting.  Her talk to US Army Rangers and Navy Seals on critical thinking went really well.  Now, the local newspaper was asking her to comment on political polling in the current presidential primaries.  Patty was just finishing her response to the paper before a meeting with Pete to discuss the voiding presentation that they were working on for Mike Madigan.  Her response follows:

Dear Editor:

My favorite candidate was trailing in the polls by only 1% in my state, but on primary day he lost by 5%.  Why isn’t polling more accurate?

Sincerely,

Disappointed

 

Dear Disappointed,

Pity the pollsters. They have to predict what will happen by sampling a manageable number of people, say 1,000. This situation creates several challenges. The first is that their sample should represent the population as a whole. This challenge is not easy. They need to assure that the 1,000 people represent the population of the entire state. If they get an inappropriate number of old, young, wealthy, lower income, educated, less educated, etc., in these 1,000 people then their prediction will be off. As an example, let’s say that 45% of a state’s residents have a bachelor’s degree or higher, yet their sample has 60% with a bachelor’s degree or more. This difference will likely make their sample non-representative of the population as a whole and will skew the results.

Let’s go back to your candidate, whom we will call candidate A. It ends up that candidate A was supported by only 47.5% of the total population and his opponent, candidate B, by 52.5%, giving the difference of 5% that you mentioned. Let’s assume that the pollsters establish a good sample of 1,000 people that is very close to representing the state as a whole. It is unreasonable to expect that the 1,000 people polled would exactly have 47.5% or 475 supporting candidate A, due to statistical variation.  To show the likelihood of a number different than 475, we have to use the binomial distribution as seen in  Figure 1 below. Note that there is about a 10% (0.1085 in the figure) chance that a population of 1,000 will have 495 or greater supporting candidate A. This uncertainty, added to the difficulty of establishing a perfect sample, makes polling error of 5% or so not uncommon.

Figure 1. Note that, even though 475/1,000 is the most likely, if the larger population has 47.5% supporting candidate A, there is a 10% chance a sample of 1,000 could have 495 or greater favoring candidate A.

 

Just as Patty finished her response, Pete came to her office door.

“Hey kiddo! Can we go over my thoughts on the voiding in BGA balls section on voiding for Mike Madigan?” Pete asked cheerfully.

“Sure. What do you have so far?” Patty asked.

“I’m focusing on the importance of the reflow profile.  Have you seen this graph,” Pete began.

Figure 2. The hot soak profile produces the fewest voids in CSP and BGA balls.

“Wow! That really shows the benefit of a hot soak profile over a cool soak profile. But, I am most surprised at how much benefit a hot soak profile has over a ramp-to-peak profile (RTP),” Patty commented.

“Isn’t the timing of the higher temperatures important, too?” Patty asked.

“My next point precisely. Look at this graph,” Pete said enthusiastically.

Figure 3.  The combination of the reflow profile and flux characteristics that produces outgassing before the solder becomes liquid (the red curve) will minimize voiding.

“The process engineer needs to assure that most of the flux is volatilized before the solder melts, as in the red curve, not as in the black curve where almost all of the flux is outgassing during the melting it the solder (Tm). This situation is assured by the correct combination of flux and reflow profile,” Pete said confidently.

“Anything else, Professor Pete?” Patty asked.

“It is really helpful to work with your solder paste supplier to obtain the red curve. They should be able to tell you what type of reflow profile and solder paste will most likely provide this kind of result,” Pete finished with a chuckle.

And he added drolly. “Right … Professor Pete.”

“Rob’s working on voiding on thermal pads for BTCs right?” Patty asked.

“Yep. He said he will be ready in two days,” Pete answered.

What will Robs plan be for minimizing voiding with BTCs?  Will Patty be happy with it?  Stayed tuned for the details.

Best Wishes,

Dr. Ron

Voiding: A Critical Issue in Electronics Assembly

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Folks,

Looks like Patty and the team have a new assignment from Mike Madigan. Let’s look in ….

Patty had just waved to her twin boys as they got on the school bus when her mobile phone rang.  The voice was unfamiliar.

“Professor Coleman, this is Mel Ott.  I’m doing some classroom trading for a bunch of Navy Seals and Army Rangers at a location about an hour from Ivy U. I wondered if you could conduct a workshop on critical thinking for these folks?” Ott asked.

Before Patty knew it, she had agreed to do it. As she drove to the university, she kept on thinking,

“Me teaching Army Rangers and Navy Seals!?”

However, a few moments later, an outline for a workshop was forming in her mind. One topic would be: Which animal is implicated in more human fatalities in the US each year?

  1. Bears
  2. Mountain Lions
  3. Horses
  4. Deer
  5. Sharks

A few moments later, Patty was in the Engineering building complex and was rounding the corner to her office. She saw Pete and Rob waiting outside the door.

“By the looks on your faces, I can tell that we must have another assignment from Mike Madigan,” she said with a chuckle.

“This one is a little strange, even for him,” Pete began.

“Yeah! Look at this note he sent us,” Rob chimed in.

Ever since the three of them left ACME to join the ranks of Ivy University, ACME CEO Mike Madigan continued to use their services. They were paid a fair consulting fee, which all agreed more than paid for Christmas presents and vacations. In addition, Madigan convinced the board of directors at ACME to generously contribute to Ivy U’s general fund. In the three times Patty had met Ivy U’s president, he pointed this out to her with his appreciation. So, the bottom line was that the three of them were quite responsive to Mike’s requests.

Together they read his note:

“Team, our biggest customer is concerned with voiding. They claim it to be their number one concern. Since the three of you left, we have drifted a bit in keeping on top of these things. I am away for a week in Eastern Europe and my wife and daughter are joining me after that for a bit of a vacation in Slovakia. My wife’s heritage is from there, steeped in the traditions of the Rusyn peoples. So, she wants to visit the hometown of her great grandfather. Bottom line is that I will be gone for more than two weeks, without reliable Internet access, so I will be out of touch.

I need you to prepare a presentation on voiding that I (with you) will give to the customer’s president the day I get back. The presentation should have recommended actions. The pitch is at 2PM, 20 days from today. Come at 11AM and join us for lunch.”  Mike

“It’s just like Mike to give us an assignment with no details and we can’t ask him any questions and he schedules the meeting without asking if we are available,” grumbled Pete.

“I, for one, think it is great he is going on vacation,” Patty said brightly.

“Good point,” Rob added.  “I can’t recall him ever taking time off.”

“Well, what is our plan?” Patty asked.

“I’m almost certain that they are not interested in champagne voids, Rob pointed out.

“I agree, since they are mostly associated with immersion sliver finish while ACME’s customers mostly use OSP finish,” Pete added.

“I think the big issue today is voiding in quad flat pack no leads (QFN) thermal pads, BGA voiding is sort of passé,” Rob suggested.

A BGA void image, taken by CALCE.

“Oops! We are supposed to call them bottom-terminated components (BTCs), right?” Patty asked.

“OK. You’re right on that one,” Rob chuckled.

“So, let’s focus on BTC thermal pad voids. But, I think, for completeness, we should cover BGA voids, too,” Patty said.

“Pete, if you can cover BGA voids Rob and I will pull together something on BTC voids.  Let’s put it on our calendars to meet one week from today to review our material,” Patty sort of commanded.

“Yes, ma’am,” Pete and Rob said in unison.

Patty was about ready to get annoyed, but they all burst into laughter as they got up to leave her office.  Even though Patty was now a prof, she still had a lot of manager in her!

Cheers,

Dr. Ron