The Importance of Oxygen Barrier in Solder Pastes

Folks,

Pity 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 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 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 in the reflow oven. To achieve this protection, the “flux” has to contain materials that act as oxygen barriers. The most common oxygen barrier 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, they provide some fluxing activity, and provide the critical oxidation resistance during the reflow process.

The reason I wrote “flux” in quotation marks in the above paragraph is that what most people call the flux in solder paste is actually a complex combination of materials. These “fluxes” 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).

Because of these complexities, and the material’s multi-functionality, they are sometimes referred to as, “flux-vehicles.”

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

The graping defect. A common example of cases where the solder barrier was insufficient is seen in 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.

Cheers,

Dr. Ron

The ESD Habbit, or An Unexpected Shock

Excitement is building here. In a little over two weeks from today, The Hobbit movie will be released to theaters. I’m sure everyone reading here knows the story, but in case you don’t I’ll spoil it for you.

It’s a story about Biblio who is, according to Spock, the bravest little hobbit of them all (google that if you don’t get the reference. You’ll be glad you did). Biblio is minding his own open source robotics business when the Wizard of Menlo Park (in CA, not NJ) invites 12 MCU designers over for a meal and discussion about the merits of hardware peripherals vs. bit-banged peripherals. The MCU guys convince Biblio to go with them to The Lonely Mountain Chip Fab and help them kill a terrible ESD Spike problem. Actually, it’s the Wizard that convinces the MCU guys that Biblio could help. The next day the MCU folks left early and Biblio ran out to catch up with them without even an ESD smock.

The ESD problem came from the North because it’s more humid up North and that tends to dissipate ESD. Our Terrible Spike didn’t like the idea of being dissipated without having first destroyed a few gold interconnect wires. The MCU guys need those gold interconnects to remain intact, so they brought a secret encryption key and enlisted help from the technician Biblio.

First though, they had to get past the TO-92 packaged parts that wanted to squash them into jelly or tacky flux. Fortunately, despite the bumbling of technician Biblio, the Wizard bought solder with no-clean flux which made the TO-92 parts stop moving once applied. After the TO-92 parts stopped working in daylight, they made a brief stop to inspect the last Homely Chip Fab in the Silicon Valley and see where the light sensitivity came from.

Passing over the Siskiyou Mountains on the way to Oregon and The Lonely Mountain Chip Fab, it started raining so they went into a cave and ate porc for dinner. Biblio ate so much that he fell asleep in the corner behind a chair where no one could see him and his buttons popped off. The missing buttons didn’t bother him too much because those ones had a de-bounce problem anyway. Luckily, the weren’t Grayhill switches or they would have hates Biblio forever, even if he used an achient gold Tolkien-ring network to bypass more porcs.

Biblio wasn’t the most skilled technician and he caught his pine cones on fire while trying to solder new switches into place, but the wizard was able to re-layout the board using Eagle CAD and an FPGA that could take many forms and would satisfactorily control the machinery and bears at the local honey production facility. But the FPGA brought them all into the murky world of Verilog and VHDL. That would have been fine except that the search engine spiders hadn’t crawled the eleven Wikipedia pages they needed to properly map out the clock routing.

The MCU guys got hungry and wouldn’t wait for Biblio to come back with pi so they rushed in causing so much in rush current that the lights went out with a snap. After eleven clock cycles in his new hall-effect switch, Biblio knew that the de-bounce problem would be gone except when he plugged the barrel jack into his Apple computer. But with no static guards to wine too, he had no choice but to use the Apple barrel jacks to get power to MCUs and switch open the flip flop made from a streaming-transistor logic gate.

Annoyingly, they split the story in two and the movie will end at this point. We’ll have to wait another year to see if Silicon Oakensubstrate is robust enough to kill the terrible ESD spike and pass final QC.

Duane Benson
One oven to reflow them all

http://blog.screamingcircuits.com/

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.

Cheers,

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.