Soldering 101: The Simplicity of Soldering – The Complexity of Solder Paste


Soldering copper to copper with a tin-based solder, such as tin-lead eutectic solder or a common lead-free solder like SAC 305, requires only the liquid solder and copper to form the tin-copper intermetallic bond. This simplicity, with one small catch, was brought home to me by some colleagues at Speedline Technologies. They took a PWB with through-hole components mounted and ran it through a wave-soldering machine without using any flux. The result was comical. The PWB weighed about 10 pounds as it had huge solder ice cycles hanging off of it. Oxides that form on the copper created this mess. Running the board though again with a nitrogen blanket produced a beautifully wave-soldered board that could be ready to ship. So in reality, either a flux or nitrogen, preferably both are needed for successful wave soldering in addition to the solder and copper; however, it is still relatively simple.

Have sympathy for the solder scientists of the late 1970s and early 1980s. SMT was an emerging technology and the world wanted to buy solder paste; however, the only experience many solder scientists had was with wave soldering. In wave soldering, the flux’s main job is to remove the oxides from the PWB pads and components. The solder is in a molten state and its oxidation is not a main concern. In the soldering process, the solder only touches the board for a few seconds and the board only experiences the high temperatures during this brief period.

I imagine some early solder pastes consisted of solder powder with fluxes similar to those used in wave soldering. If so, they probably didn’t work too well. Consider the dramatic differences that solder paste experiences as compared to solder in wave soldering. The “flux” in solder paste has to remove oxides from the PWB pads, component leads, and solder particles, but it also has to protect all of these surfaces from re-oxidation for several minutes while in the reflow oven. To achieve this protection, the “flux” has to contain materials that act as an oxygen barrier. The most common materials used in no-clean solder pastes are rosins/resins. Rosins, or resins which are modified or synthetic rosins, are generally medium to high molecular weight organic compounds of 80-90% abietic acid. They are typically found in coniferous trees. Rosins/resins are tacky in nature, and provide some fluxing activity and oxidation resistance during the reflow process.

The reason I wrote “flux” in the above paragraph is that what most people call the flux in solder paste is a complex combination of materials. These “fluxes” will consist of:

    • Rosins/resins: for oxygen barrier and some fluxing activity
    • Rheological additives: to give the best printing properties, e.g., good response-to-pause, good transfer efficiency, excellent slump resistance, good tack, etc
    • Solvents: to dissolve the other materials
    • Activators: to perform the main fluxing action (removing oxides)

      Figure. Solder pastes are one of the most highly engineered materials.

Modern solder pastes must have good oxygen barrier capability. In most reflow profiles, the solder paste is at temperatures above 150ºC for more than several minutes. During this time, an oxygen barrier is needed to protect both the solder particles and the surfaces of the pads and leads.

A common example of an insufficient solder barrier is the graping defect or its relative, the head-in-pillow defect. If you are experiencing one of these defects, a solder paste with better oxygen barrier properties is bound to help.

Before reflow, the solder paste must print well, possess good response-to-pause, not shear thin, resist cold slump, and have good “tack” to support the components after placement. During reflow, in addition to the oxygen barrier challenge, the solder paste must not exhibit hot slump, should “Avoid the Void,” not create the “head-in-pillow” (HIP) defect, work with all common PWP pad finishes, and produce reliable solder joints in thermal cycling, drop shock, and vibration environments. Whew! What a complex challenge.

As a result I would argue that solder paste is a candidate to be the most highly engineered material in the world… and it certainly is NOT a commodity.

Dr. Ron


Wave Soldering is Here to Stay

Patty was just getting ready to leave her office for a bi-weekly luncheon with the Professor, Pete, and Rob. They had regular meetings like this to discuss new technical topics or to review books. It was Patty’s turn to take the lead in discussing the new book, Rust: The Longest War.

As Patty arrived at the faculty dining room, everyone else was already seated. After ordering, she began the discussion.

“I thought that, overall, the book rated 4 out of 5 stars,” Patty stated.

“It had many interesting stories and brought home that fighting rust is the ‘longest war,’” she went on.

“But shouldn’t the book really be called ‘Corrosion’?” Pete interjected.

“I agree. After all, the best story was about the work that was done to refurbish the statue of liberty, and most of that is copper.  By definition, only iron rusts; copper corrodes. We try to be very specific about the differences in our undergraduate materials classes,” Rob chimed in.

“Rob, I remember you telling us that one student wrote a paper that referred to wood corroding,” the Professor said.

At that comment everyone chuckled.

“We can all agree that corrosion is a big challenge to civilization. But, can anyone think of a big downside if iron didn’t rust?” the Professor asked.

Patty, Rob, and Pete looked at each other and then the Professor as they shrugged their shoulders.

“Think biological processes,” the Professor encouraged.

It hit them all at once, but Pete was the first to comment.

Figure 1. Rust: The Longest War

“Blood!” he cried out.

“Precisely! Without ‘rust’ we wouldn’t be here.  Iron’s unique ability to combine with oxygen in the hemoglobin of our blood makes ‘rust’ a requirement for human life,” the Professor explained.

None of them recalled seeing this point in the book.

“So, the conclusion is that rust costs the US over $400 billion per year. But, without it we wouldn’t be here,” Pete summarized as he chuckled.

“Patty, I understand that you had to fill in for Professor Croft as he recovers from a broken leg. The course was Everyday Technology as I recall. How did it work out?” the professor asked.

“Well, first of all, Pete agreed to help. And, it was only for the last two weeks of the term.  The final assignment for the students was to perform a teardown analysis on some electronic product, such as a DVD player, blender, hair dryer, etc.  They had to write a report and give a presentation on their findings.  They worked in teams of 2 or 3,” Patty summarized.

“It’s important to remember that the students that take this course are not engineering or science majors.  The course fulfills a technology requirement for non-technical students.  Most of them had never taken anything apart before,” Pete chimed in.

“Hey! Don’t forget that Patty made me sit in on all of the presentations,” Rob added teasingly.

“So, what were your impressions?” the Professor asked.

“I was impressed by how professional their presentations were and what a thorough job they did,” Pete responded.

Their work was especially impressive considering that almost all of them had never done anything like this this before,” Rob added.

“Anything else?” the Professor asked.

“I was surprised that all of the photos that the students took were taken with a smartphone, even macro shots of small components.  I remember photos from smartphones of 6 or 7 years ago were almost unusable. Those that the students took this semester looked high definition to my eyes,” Patty added.

There was a little more discussion and, finally, the Professor had one last question.

“You all had a chance to see many teardowns. How did it impact your understanding of the state of technology?” the Professor asked.

Patty began, “Pete, Rob, and I discussed this topic quite a bit.  We had to admit that the thing that surprised us the most was that, of the 18 devices that the students analyzed, almost all had a wave soldered PCB with through-hole technology.”

“I agree, we noticed that every power supply board was a through-hole wave soldered board.  I think we only saw a PCB or two that was all SMT.  If the boards weren’t pure through hole, they were mixed technology.  Through-hole and wave soldering are here to stay,” Pete added.

Figure 2. A typical wave-soldered through-hole power supply board.

“We have to consider that most of the devices were lower tech: blenders, toasters, and one hair dryer,” Rob pointed out.

“But, the DVD player struck me the most. It had a mixed technology board in which one side was wave soldered, and a power supply board that was all through hole and wave soldered,” Pete added.

“I think those of us in the electronics assembly field become so enamored with smart phones and other high tech devices that have SMT-only PCBs that we forget that there are billions of lower tech devices that still use wave soldered through-hole boards.  The technology is cheap and it works, so why change?” the Professor summarized.

“So, wave soldering will likely be around for my grandkids!” Patty chuckled.

Pete Rides the Wave

Let’s see how Pete is handling the wave solder crisis.

Pete had to admit that he was surprised by the positive outcome of his meeting with Fred Castle. He had sent Patty a text the day before, after he took the operators to lunch, before meeting Fred. The text was a little negative. So he was eager to send her the good news about the surprises in his two meetings with Fred since then. He was frustrated that he kept on getting her voice mail. Finally she answered.

“Advanced Processes,” Patty speaking.

“Hey, kiddo, it’s your favorite process genius!” Pete responded cheerfully.

“Oh, this must be Oscar Patterson!” Patty joked, and they both laughed. Patterson was an annoying chap they had to deal with a few years ago. He topped their list of most annoying people. Pete had almost come to fisticuffs with him.
“How is it going there?” Patty asked.

“Shockingly well. My meetings with Fred Castle were very productive,” Pete answered.

“Well, that is shockingly positive news. But I thought he said, ‘I’ve forgotten more about wave soldering than you’ll ever know,’” Patty responded.

“That’s the first thing he said to me when we shook hands, but he was clearly teasing. He slapped me on the back at the same time and chuckled. He went on to say that he had worked in wave soldering for over 30 years, typically at companies that had processes that were out of control. It was clear that he understood a lot about wave. We talked for 30 minutes about what makes a good wave process. As far as I could tell he was right on in everything he said. I think the operators didn’t pick up on his teasing, by the way,” Pete elaborated.

“What about special cause vs. common cause?” Patty queried.

“He didn’t have a clue,” Pete replied.

Patty was bracing herself. She was concerned that Pete might have insulted Castle.

“And you didn’t tell him he was an idiot?’ Patty teased.

“Patricia! I’m shocked you could even think such a thought,” Pete replied.

Pete went on, “We bonded, and he admitted that he was frustrated with the yield loss increasing. He was studying the situation and spending a lot of time trying to figure out the issues. He said he was having trouble sleeping. He mentioned that, in his last job, he was responsible for the wave processes at 10 locations. He was constantly fighting fires and got good at it. He had never worked at company that performed DoEs and developed optimized processes.”

“I’m dying to know how this situation worked out,” she interrupted.

“Patience, patience,” Pete admonished jokingly. He continued, ”It was clear that Fred likes to learn, so I mentioned that, recently, The Professor had mentioned the importance of understanding the differences between common cause and special cause variation when trouble shooting a process. I suggested that maybe studying these topics might help. So I gave him a few links to The Professor’s posts on common cause and special cause.” (Au:  If you are not familiar with common cause and special cause fails, it will be helpful understanding this story to read The Professor’s post.)

“What happened then?” Patty asked, the impatience in her voice apparent.

“Remember, this is now the end of my first day. I watched the process in the morning, took Molly and Chuck to lunch, and then met with Fred. On the second day I had a morning meeting with the quality director, Pam. Then Castle and I went to lunch,” Pete elaborated.

“And?” Patty asked impatiently.

“Castle was all excited. After studying common cause and special cause all night, he realized that he was seeing common cause fails in his detailed scrutiny of the wave line. By adjusting the process parameters slightly when he found a common cause fail, he was moving away from the optimized process settings that were determined by a DoE, so the failure rate got worse. In his previous job, he was mostly seeing special cause fails, as the processes were not optimized, so he was used to intervening,” Pete explained.

“It seems like he won’t have enough to do now,” Patty commented.

“I suggested he help quality. They are stretched thin and he is a detailed-oriented fellow. He keeps meticulous Pareto charts of the fails,” Pete said.

So, where are things now?’ Patty asked.

“Yesterday and today, first pass yields are at 96%. Fred also started helping quality today. It felt good to help and not offend,” Pete finished.

Patty thanked Pete for the great job he did and complimented him strongly for being successful and making friends at the same time. As she hung up the phone, she saw an email from Pam Olinski in her in box. It was a kind note thanking her and Pete for his help. It recounted much of what Pete had said.

She wistfully looked out her window. She was happy and grateful for all of her success, but, to be truthful, she missed the action of being out on the shop floor solving these types for problems.

She was jolted from her chair when she suddenly remembered it was her turn to take her twin sons to karate lessons. So she packed up quickly to pick them up at her mother-in-law’s, to get them to the gym by 5PM.

Patty and the Professor: The Twiddler


It’s been a while. Let’s look in on Patty…

Patty stared, bleary eyed, at her laptop screen. It was the day after the election. She and Rob were following the election closely as a “statistical thinking” exercise. They had met at a conference with The Professor in late October and agreed that following the election would test their statistical thinking skills. They established beforehand that they would not discuss who they favored, just the data.

All agreed that Mitt Romney had a greater challenge than President Obama.

As Rob said, “Of the six most populated states, even the Republicans agree that Obama will win California (1), New York (3), Illinois (5), and Pennsylvania (6). Romney is only a shoe-in for Texas (2). Only Florida (4) is a toss up.”

“I thought some analysts were saying that Pennsylvania is in play,” The Professor commented.

“They’re dreaming,” Patty said with conviction. “Pennsylvania has too many big cities; typical Democrat strong holds,” she continued.

“Many pollsters have 255 electoral votes in Obama’s column and only a little over 200 for Romney. It’s hard to see a Romney path to victory. It is statistically unlikely he could win all of the swing states” Rob added.

The Professor beamed as he listened to his protégés intelligently analyze and argue the situation. They all agreed that it was hard to understand why many were referring to it as a close race, although voter turnout could change everything.

As election night went on, Patty felt she could call the election at 8PM EST. However, she was sympathetic that the networks needed a high level of certainty. The major networks were finally calling it at 10PM. When they did, Romney was ahead in the popular vote by about 1 million. Patty chuckled to herself, when a renowned TV anchor commented that it might be a governing challenge to Obama to win the electoral college and not the popular vote. Clearly he had not factored in the fact that, although California was “called” for Obama around 10PM EST, it was called with only a few percent of the votes in. The networks were using exit polls and statistical analysis to make a projection. By the time all of the west coast votes were counted, Obama will comfortably win the popular vote – because of California’s large population. Patty thought this should be obvious to the pundits.

Patty had stayed up until about 11PM to watch the results. It was comforting that her analysis was spot on. However, she was so “wound up” that she couldn’t fall asleep and she was now paying the price.

As her attention shifted back to the email she was writing. Suddenly, she was jarred by a loud, cheerful voice.

“Hey kiddo, pack your bags, looks like we’re on the road again,” Pete said loudly.

As usual Patty thought. “How does Pete always know these things before I do? I’m the boss!”

“What’s the scoop?” Patty asked.

“Remember our facility in Ohio? They are having wave soldering yield and throughput problems,” Pete answered.

“What!” Patty shouted. “We spent a lot of time there six months ago optimizing their wave soldering operation and teaching them the appropriate use of solder preforms. What happened?”

“Not sure,” Pete responded. “I thought we worked really well with their team and developed a good process. It seemed to me it was one of the more productive projects I was involved in in quite awhile.”

“And you didn’t even offend any of the senior managers,” Patty teased.

Pete chuckled but his cheeks did turn a little red. Pete was a terrific process engineer, but he had a little bit of a short fuse, although he was usually right.

“In talking to some of my buddies there, they told me that senior management hired a very senior fellow who is considered an expert in wave. Strangely, things fell apart right after he joined,” Pete explained.

“Well, you are on your own for this one. I’ve got a number of family commitments over the next two weeks,” Patty said with a little sadness in her voice. Patty enjoyed these types of challenges. “As soon as I get the official request, you’ll be on your way,” Patty said. “Oh, and don’t offend anyone,” she teasingly finished.

As Pete left her office, she checked her emails. Sure enough, there was a note from Mike Madigan asking her to intervene in this wave soldering problem.

Two days later Pete was in ACME’s Ohio facility sitting in the office of Pam Olinski, the site’s quality manager.

“Pete, I’m so glad you could come. Three months ago our wave soldering first-pass yield was 95% and our production was about 2,000 boards per day. Yield is now 90% and production is off 15%. Help!” Pam said.

“Tell me about the new guy,” Pete inquired.

“Fred Castle; he has very impressive credentials, but he has been running the wave process like a dictator. He stops the process a lot to adjust the wave machine. I think he will be offended that you are here to audit the process,” Pam finished.

Because of this concern, they agreed that it might be best to have Pete initially view the process from afar. They decided that Pete would be given an operator’s smock and walk around the shop floor for half a day or so.

As Pete arrived on the shop floor, almost immediately he saw Fred stop the wave machine and make some adjustments. While making the adjustments, Fred held a board in his hand — and he looked at occasionally. After the wave machine was running again,

Pete saw that Fred looked carefully at every board. Pete saw one of the wave operators was going on a break. Pete remembered Molly Stark from his visit to optimize the wave process six months ago, so he stopped her and ask if she could join in for lunch.

The morning passed quickly, and Pete was off to lunch with Molly. As Pete had suggested, Molly brought another operator, Chuck Petrus to lunch. Pete insisted on treating, so Molly and Chuck left their brown bags behind.

In total, Fred stopped the line four times during the almost 4 hours of Pete’s observations. Each time he made adjustments on the wave machine.

After exchanging pleasantries Pete asked, “Why was that fellow stopping the wave line so often?”

Molly got quite animated and answered, “That’s Fred Castle, the supposed wave genius. He stops the line every time there is a defect and adjusts the wave machine parameters. A number of us complained to him that he shouldn’t make adjustments on the machine that with just one fail. That’s what you taught us.”

“What did he say?” Pete asked.

“ ‘I’ve forgotten more about wave soldering than you will ever know.’ No one has said a word since,” Chuck responded.

“You and Patty taught us about special cause and common cause variation. I don’t think Fred understands that,” Molly commented.

“He’s also a knob twiddler,” Chuck added.

Does Fred know the difference between common and special cause variation? Is that the root of the yield and throughput problems? What is a knob twiddler? Stay tuned to find out.

The End of Cleaning

Patty was eating her lunch at her desk while reading Golf Digest. She had been training with weights, performing stretches and getting lessons from a long drive pro in Grantham, NH. It was at 90-minute commute, but it was worth it. The sophisticated machines that the golf center there indicated that her average drive was up from 250 to 268 yards. Still way short of Rob’s 294, but she was making progress!

Patty was kind of depressed as she read; the Tiger Woods scandal had her in a funk. Her feelings were summed up by another person, who suggested they were in a state of mourning. Her mind was drifting when she was startled by Pete’s knock at the door.

“Hey kiddo, pack your bags looks like another trip,” Pete cheerfully announced.

Pete, how is it that you always know what’s going on before I do? I’m supposed to be the manager,” she teased.

“Somes got it, somes ain’t,” he quickly shot back.

They both chuckled. Patty and Pete made a good team. He was well-connected and knew what was going on. Through Patty and the Professor, Pete was encouraged to go to night school to get his degree and was always trying to learn things from both of them. Through Pete’s “knowing the ropes,” he was a wise counselor to Patty on the realities of getting things done. They both helped each other immensely and they both knew it.

“So, what’s the scoop?” Patty asked.

“Well, remember our parent company bought out a privately held company in Virginia a month or so ago, right?” Pete responded.

“Sure,” said Patty.

“Senior management went on a tour and concluded that the place is a mess. They are going to ask us to perform an audit and develop an improvement action plan,” Pete went on. “Bring your golf clubs; the courses are open down there.”

Patty went into her office and checked her email. Sure enough, there was a dispatch from the site GM telling her to see him about a trip to the new facility in Virginia. Patty went to see him and received very broad instructions.

“That place looks like a pig sty in a swamp. Go do your magic and give us a plan to fix it up. Oh, and by the way, the former owner has stayed on as the site’s GM. Ahh … ahh, he is a little rough around the edges … thought I’d give you a heads up,” the GM shared.

The trip was a breeze and Patty did bring her clubs. She and Pete decided to wean themselves from The Professor on this one. As they arrived they met John Davis, the operations manager. John seemed pleasant but serious, and a little subdued, almost like a puppy that had been kicked too many times. He also didn’t make eye contact when he talked.

As they walked out into the shop floor, Patty was aghast. Not only was the floor disorganized but it was filthy. As she walked toward one of the reflow ovens it almost looked like tinsel was hanging from the ceiling. She couldn’t figure out what it was.

“John, what is that hanging from the ceiling above the reflow oven?” Patty inquired.

John was taken aback, as if he never noticed the hanging material.

Patty pointed and said again, “This stuff.”

“You know, I never paid much attention. I’m not sure what it is,” he finished.

As they approached the reflow oven, they could see sticky material hanging from the ceiling like stalactites.

The trio studied it and suddenly Pete exclaimed, “It’s flux dripping from the ceiling with cobwebs hanging on it.”

“How is this possible? Aren’t the flux condensation and cob webs cleaned up during routine cleaning?” asked Patty incredulously.

“We don’t ever clean,” sighed John.

Patty and Pete were speechless.

“How can you never clean?”, asked a stunned Pete.

Our GM, Oscar Patterson, says cleaning is a waste of money.

Neither Patty or Pete know what to say.

Patty and Pete, accompanied by John, continued their audit over the next few days. They were pleasantly surprised to see that uptime was a respectable 25%. They got to know John a little and, on the third day of their visit, were surprised to see that he was more dour than the past two days.

“John, what’s up?” asked Pete.

“It’s hard not to be discouraged,” said John.

“How so?”, Inquired Patty.

“Well, Mr. Patterson went to SMTAI and heard a paper in which the speaker said that it has never been shown that nitrogen in the reflow process improves quality and reliability. In addition, he heard that nitrogen makes tombstoning worse,” replied John forlornly.

“Well that’s true, in surface mount assembly.” replied Pete and Patty in unison.

“But Mr. Patterson turned the nitrogen off on our wave soldering machines. He didn’t even tell me,” moaned John.

“Yikes!” exclaimed Patty.

“Looks like the Big O struck again,” Pete chimed in.

Patty was going to respond to Pete’s comment, but she thought she would wait until they were alone.

Patty then commented, “A good solder paste should not need nitrogen in reflow, but nitrogen almost always helps in wave soldering. Did Mr. Patterson perform any experiments to show that acceptable yields could be obtained without using nitrogen in the wave machines?”

“No,” replied John, “he just canceled the blanket PO for nitrogen and was beaming when he announced at a staff meeting that by not using nitrogen we save $10,000 a month.”

“Didn’t anyone ‘push’ back?” Patty asked.

“You don’t know Mr. Patterson, Patty,” John replied.

“You mean the Big O,” Pete interjected.

Patty glared at Pete.

Patty then asked, “It’s almost quitting time. Have you got a few scrap boards and a cylinder of nitrogen?”

“Yes, I think so,” said John, “We certainly have scrap boards and I have a few cylinders of nitrogen we use for other purposes.”

Patty then suggested that they perform an experiment. Fortunately, Patty and Pete now bring a camera and video camera with them, so Pete was assigned to video the proceedings. Patty ran a few scrap boards through the wave soldering machine with no flux or nitrogen. The boards looked hilarious when they came out of the wave, they had huge stalactites on the bottom. The 4” x 6” boards must have had a pound or two of solder on the bottom. Pete was laughing so hard that he couldn’t hold the camera steady. Even John cracked a smile.

“Kiddo,” where did you learn that?” Pete asked. “I’ve been around a long time and this board takes the cake.”

“I took some training from the folks at Speedline and we did this in a class,” answered Patty. “But wait, the best is yet to come.”

She then asked John to turn on the nitrogen and they ran the same board through, still with no flux. The board came out of the wave looking fine. John inspected it.

“It looks like it could be shipped,” John said with amazement.

“Absolutely amazing,” added Pete.

“That’s why nitrogen is important in wave soldering,” Patty summed up.

“It is possible for a robust flux to get excellent yields in wave without nitrogen, but a DoE should be performed to verify this hypothesis,” Patty added.

As they left the building for the day, Patty admonished Pete. “I told you to behave. What is this ‘The Big O’?”

“I spent a little time getting to know the operators. Everyone calls Oscar Peterson the “Big O” behind his back. He is 6’4, 380 pounds. The word is he is a real dictator; it’s his way or the highway. He is also a miser, always looking to cut costs. The operators clean the restrooms and make the place as look good as they can by cleaning during their breaks.” Pete summed up.

Will Patty and Pete confront the Big O? How will it go? Is nitrogen in the wave really that important?

How are the plans for Patty’s wedding going?

Stay tuned for the latest.


Dr. Ron

Note: It may be hard to believe, but I witnessed both the end of all cleaning at a facility and the end of using nitrogen in wave soldering at the same assembly facility of a large (>$10 billion) company. So, this story is based on fact.