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


Thermal Mass

I mentioned thermal mass in a recent post and was thinking over my oatmeal that its a subject deserving of more attention. That’s more attention to thermal mass, not to oatmeal. Although oatmeal is a pretty healthy food, so it probably deserves more attention then it gets these days.

When most people think of thermal issues, the considerations tend to be around operating conditions. Will the part generate too much heat? Will there be enough airflow or is there enough surrounding material for adequate conduction cooling? All of those are pretty important — especially with the obvious like fast processors and big honking power components. But there are a lot of thermal issues related to manufacturing that have to be considered as well.

Reflow soldering is supposed to gradually and evenly warm the PCB and parts. Then, the temperature will spike up just high enough and long enough to melt all of the solder before dropping again. This is were thermal mass trickery comes in to play.

If you have a tiny passive part and one pad has a lot more copper than the other — it can even be a problem, even if it’s an innerlayer with more copper under one pad then the other. That extra thermal mass can delay the solder melt on that side of the part slightly. That delay in melting can cause the surface tension on the side that did melt to pop the part up like a tombstone. Placing a very large component too close to one side of a very small part can also cause the same problem.

If you have really tiny parts, give your layout (inner and outer layers both) and placement a scan to make sure you haven’t inadvertently created a heat sink on one pad and not the other.

Duane Benson
Quick. Call Wilford Brimley

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.

What to do with Zig-Zagging TC Readings?

What’s wrong with this picture?

Well if you have ever used Kapton tape to attach a thermocouple, you have certainly seen your share of profiles like this!

So what, it is a perfectly good profile, right?  Yes, but no.  I had a customer who was using KIC’s Navigation (auto prediction) to help create a better “deep in-spec” profile.  The only problem, they were trying to optimize on a TC reading that was bouncing literally all over their PCB.   Navigator is an awesome tool, but it can only work with what you feed it.  If you feed it garbage, it will give you garbage.  In their case, it was trying to find them a new solution where literally every time the board was run the bouncing TC that was attached (or I should say was not very well attached) with Kapton was giving false readings.    Navigator would give a different solution based on what the TC was reading at that given time.   It is like try to put post-it notes on the ocean.

Solution is very simple, eliminate the TC reading from your profile.  You can easily do this with the profile you just ran.  Look what happens, you go from a far out-of-spec profile of 126% PWI to a far in-spec profile of48% PWI.

So you saved your hard work this time, but you are after all one thermocouple reading short.  You added that TC to your profile for a reason.  Next go around, do yourself a favor and use a better material for attachment, such a conductive double side aluminum tape, which by the way, a recent study from RIT proves it a superior attachment method aside from sticking to your PCB much better.