To Minimize BTC Voiding, Start with the Right Solder Paste

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.

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.

 

Calculating Optimal Solder Paste Printing Aperture Parameters

Folks,

A while ago, I developed an Excel-based spreadsheet, StencilCoach, that calculates optimal stencil aperture parameters for several common SMT solder paste printing applications. These applications include standard apertures, passive apertures, pin-in-paste (PiP) apertures, and preforms with pin-in-paste (PiP+) apertures. These algorithms are now online at http://software.indium.com/. Over the next several posts, I will review the use of this software tool.

Let’s first look at standard apertures. After logging into to the website, click on “Stencil Coach” and then “Standard Apertures.” The page gives the definitions for “Aspect Ratio” for a rectangular aperture and “Area Ratio” for circular and square apertures. The aspect ratio, which is defined as the width of the rectangular aperture divided by the stencil thickness, should be greater than 1.5. Whereas the area ratio, for circular or square apertures, is given as the area of the opening divided by the area of the sidewalls. This formula simplifies to D/4t, where D is the diameter of a circular aperture or the width of the square aperture. The area ratio should be greater than 0.66. These recommendations are not standards, but are good rules of thumb.

Let’s consider a situation where a PWB has rectangular apertures with a pitch of 35 mils and circular and square apertures with pitches of 40 mils each. The stencil thickness is 6 mils. See if you can develop the pad and aperture sizes and reproduce the figure below. Hopefully this tool will help you design your stencils.

Cheers,

Dr. Ron

PS: If you need a hand, feel free to contact me at rlasky@ indium com.

The Limits of Mixing: A Chocolate Chip Example

Folks,

We tend to think of mixing as something that can completely even out those things being mixed. As an example, let’s assume you are making chocolate chip cookies and would like to have 10 chocolate chips in each large cookie. You make enough batter for 100 cookies and then mix in 1,000 chocolate chips. After mixing for a long time you put 100 dollops of the batter on the baking pan and bake up the cookies. Upon inspecting the cookies, to your dismay, you find that you have only 13 cookies with 10 chocolate chips. More than 40 cookies have 30 percent more or 30 percent less than 10 chips. Worse yet, 3 cookies have 4 or less chocolate chips and 7 have 16 or more. See the graph below. You decide that you did not mix them enough, so you make another batch and mix for 4 hours.  The results are the same.

Statistics tells us why the above scenario is so.  In a case like this one, the number of chips in a cookie is described by the Poisson distribution. The mean will be 10 chips, since we are using the Poisson distribution, the standard deviation will be the square root of the mean or 100.5=3.16, or about 3 chips. One way to ensure a more even distribution of chocolate would be to divide each chip into 10, so we would have 10,000 smaller chips in a batch. On average each cookie would now have 100 chips and the standard deviation would be 10. Plus and minus one standard deviation is about two thirds of the data, so two thirds of the cookies would have +/- 10% of the desired amount of chocolate, a much better result. If we divided the chips into even smaller sizes, we would further tighten the distribution.

How does any of this relate to solder preforms or solder paste? In the new world of lead-free solder pastes, where it is common to have 3 or 4 alloying elements, some in very small concentrations, it can be difficult to control the concentration of the alloying elements throughout a sample of the alloy. The limits of mixing are just part of several processes that are required to assure that a modern lead-free solder has a consistent formulation. These are some of the topics you should discuss with your solder supplier to ensure consistency in any solder alloy you purchase. Asking to see assay analysis of a solder alloy is often a good idea, too.

Cheers,

Dr. Ron

 

Best Wishes,

Dr. Ron

Weibull Part III

Folks,

Our discussion of Weibull Analysis continues…. Let’s say you have worked hard and assembled some SMT lead-free PCBs for thermal cycle testing. You used the best lead-free solder paste and some lead-free solder preforms as you assembled several through-hole components with the pin-in paste process.  You were a little concerned with the assembly process as the board was thermally and physically massive and the reflow process needed to be a bit above the recommended temperature and time.

The results of the thermal cycle testing are shown in Figure 1 below. You dutifully report the characteristic life (or scale) as 2,387 cycles and the first fail at 300 cycles. You were quite disappointed, as in the past similar, but slightly smaller boards, had a slightly higher scale, but more importantly, the first fail was about 1,000 cycles. Anyway, you write up your report and file it away.

Figure 1. A Weibull plot of the thermal cycle data.

Hold on! The data are screaming at you the something is going on. Look at the same data in Figure 2. Note two distinct lines shown in green. These two separate lines suggest very strongly that there are multiple failure modes. The line furthest to the right is likely the typical failure mode observed in the past. The line to the left is a new early failure mode. It could be due to something like oxidized pads or some other phenomena not seen when testing similar but smaller boards. Root cause failure analysis should be performed to try and understand to new failure mode.

Figure 2. A Weibull plot of the thermal cycle data with multiple failure modes noted.

Now for a human interest note: One of the rewarding aspects of being a professor at Dartmouth is the outstanding nature of many of the students. They are not just good academically, but often are talented artistically, athletically, etc. This point was brought home to me recently.  In a class I teach, ENGS 1: The Technology of Everyday Things, we were recently discussing the conservation of angular momentum (CoAM). One of the most striking ways to demonstrate CoAM is an ice skater’s spin. I went on the internet and could not find a good video of a spin. I then remembered that one of my former students, Julia Zaskorski, was on Dartmouth’s figure skating team. I asked her if she had a video she could share. It appears here. She is a materials science and physics major. Who knows, maybe we will see her at APEX or SMTAI in a few years.

Here is a little bio in her own words:

My name is Julia Zaskorski, and I’m a junior from Wellesley College taking part in the 12 College Exchange Program at Dartmouth. At Wellesley I am majoring in physics with the intent to pursue mechanical engineering. Despite Wellesley’s relationship with nearby MIT, Wellesley does not have its own engineering program, so I sought out the more self-contained curriculum and atmosphere at the Thayer School of Engineering.  In addition to the draw of the Thayer School, the Dartmouth Figure Skating team was also a hugely motivating factor for my exchange, as Wellesley does not have a team, let alone a rink.  I have known the coach of the Dartmouth team for several years now, and to finally see my name on the roster for the team is a dream come true.  The engineers, as well as the winter activities here in Hanover, pulled my heart to Dartmouth long before I’d ever set foot on campus. 

Cheers,

Dr .Ron

 

Can Your Mortality be Modeled with Weibull Distribution?

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

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

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

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

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

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 mikeb@kyzen.com.

Cheers,
Dr. Ron