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.

 

BGA Voiding in Electronics Assembly

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.

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.

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.

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

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.

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