Estimated reading time: 7 minutes

Maggie Benson’s Journey: Take Your Assembly Skills to the Next Level

Editor’s note: Indium Corporation’s Ron Lasky continues this series of columns about Maggie Benson, a fictional character who runs Ivy Benson Electronics, to demonstrate continuous improvement and education in SMT assembly. 

Ivy Benson operators Andy Connors, age 20, and Sue March, age 19, have become a little more than friends. Let’s look in on them having pizza after a movie near Ivy Benson Electronics.

“Well, tomorrow is the first SMT class that Chuck Tower will be giving us,” Andy said, and then jokingly, “And the good news—it’s on overtime.”

“I brought the SMT test that Chuck gave,” Sue responded. “Let’s see if we can answer the first five questions together and then check the answers online.”

“I was hoping for a little romance,” Andy said, teasingly.

Sue teased back. “Okay, Romeo, how about this? If we feel confident that we can answer four out of the first five of these questions, I’ll let you hold my hand when we walk to the parking lot.”

Andy groaned a little, but then smirked, “Okay, Juliet, let’s look at the test!”

Sue laid the test on the table and started reading the questions. Question 1 said:

1. How much copper is in SAC305 solder?

1. 3.0 %
2. 0.3%
3. 3.5%
4. 0.5%
5. None of the above

“Let’s see if we can figure out the questions first without looking them up,” Sue suggested.

“Good idea,” Andy said. “We already know that SAC means S = tin (from Sn), A = silver (from Ag), and C = copper (from Cu). I also remember hearing Chuck say that tin is the fundamental metal for solders, and almost all solders start with tin.”

“So what about SAC305? Isn’t silver expensive?” Sue mused. “If so, there probably isn’t that much silver in it or the solder paste would be under lock and key.”

Andy responded, “You know Pete Smith, right? He’s really smart. He once mentioned that there are solder pastes with only silver and no copper, so I’m thinking the amount of copper is likely very small.

They hemmed and hawed as they spiritedly debated on the answer. First, it was 30% silver and 5% copper, but those seemed too high. After a few minutes, they settled on 3.0% silver and 0.5% copper since they felt the numbers would be in order with the letters SAC.

“Okay, let’s use our phones to look up the answers,” Sue suggested.

She entered the search and found a website¹ with the answer.

“We were right!” she said loudly. “This is kind of fun.”

“I agree,” Andy said, laughing. “Let’s look at the next question.

2. The formula for aspect ratio (AR) for a stencil aperture is: d/t, where t is the stencil thickness and d is the line width. Assuming the stencil is 5 mils thick, what is the finest line that can be reliably printed? A rule of thumb is the AR should be 1.5.

1. 5.5 mils
2. 3.5 mils
3. 7.5 mils
4. 10 mils
5. 4.5 mils

“That’s obvious, the answer is C. 7.5 mils,” Sue exclaimed.

“Whoa, wait a minute, how could you get the answer so fast?” Andy asked, feeling a bit dejected.

“Well, look at the formula,” she responded brightly. “If t = 5 mils and AR is 1.5 = d/t, d must be 1.5 times t or 7.5.”

“Yikes, I see it now, but not nearly as fast as you did,” Andy said, letting out an audible groan.

“Even though I didn’t see its value at the time, I was a good student in Mrs. McGillicuddy’s ninth grade algebra class,” Sue shared.

Upon hearing this from Sue, Andy started thinking he needed to “up his game,” and then suggested they look at the next question.

3. Assume that component placement is the “gate” in an SMT assembly process line, which has one chip shooter and one flexible placement machine. The chip shooter takes 60 seconds to place its components while the flexible placer takes 45 seconds. Some chips are being placed by the flexible placer. To improve productivity, what should be done?

1. Nothing, everything is fine.
2. Chips should be taken off the flexible placer and put on the chip shooter as chip shooters are best to place chips.
3. Chips should be removed from the chip shooter and placed on the flexible placer to time balance the line.
4. The chip shooter is so slow, so move all the components to the flexible placer.
5. “That one is obvious, right? The answer is C,” Andy exclaimed.

“I agree,” said Sue. “Chuck had talked about this from the first day Maggie and John took over the company. ‘The line should be time balanced,’ he would say.”

“It’s sort of hard to understand how someone would not see this,” Andy said. “You will assemble more boards if the line cycle time is at minimum, and it is minimized by time balancing the component placement machines.”

Sue was excited as she explained her answer. “The challenge is to know how many components to move from the chip shooter to the flexible placer. That requires using algebra. I actually think I know how to do that.”

Andy’s heart sank. He wanted to impress Sue but didn’t even know where to begin to solve this problem.

Sensing his apprehension, Sue suggested they had mastered the third question and it was time to move on to the next one.

4. What is a fiducial on a printed wiring board (PWB)?

1. It shows the PWB manufacturer that the etching process is within specification.
2. It is used to align the PWB to the assembly equipment.
3. It is needed as a reference for solder mask.
4. It is the most cost-effective way to minimize PWB warpage.

“I’ve got this one,” Andy said. “The answer is B. I work on the stencil printer most of the time and we need to align the PWB to the printer. When Sam Reynolds was teaching me to do this, he said, ‘Andy, this step is called aligning the PWB fiducials to the printer.’”

“Well, I work mostly on the reflow oven and setting the kits up for the next jobs, so that is a new one to me,” Sue responded as they moved on to the final question.

5. What is closest to SAC305’s melting point?

1. 230°C
2. 210°C
3. 183°C
4. 220°C
5. 200°C

“Since you are the reflow, hotshot, you should know this one,” Andy teased.

“The peak temperature of our reflow ovens is between 240°C and 245°C,” Sue mused.

“So maybe 230°C is the answer?” Andy suggested.

“I’m not sure,” Sue responded. “I remember Chuck Tower commenting that the peak reflow temperature needed to be at least 20°C over the melting point of the solder to assure that it melts. So, I think the best answer is 220°C.”

While she was talking, Andy looked it up on the website.

“Google™ says it is 217°C, so you were right, Ms. Superstar,” Andy teased.

“Wow, five for five, but I think we were a bit lucky,” Sue said.

Andy chuckled and said, “Agreed.”

Sue became a bit pensive, adding, “I wish I could be like Maggie Benson. She is so smart and confident and ‘with it.’”

“Why can’t you be?” Andy asked.

“Don’t tease me,” Sue said. “You’ll hurt my feelings.”

“No, seriously,” he replied. “You’re only 19, she’s in her mid-20s, and you’re smart, too.”

“Why do you say I’m smart?” Sue asked.

“You did that stencil aperture ratio problem in your head, for Pete’s sake,” Andy exclaimed.

“Wow, thanks,” Sue said softly as she beamed a little.

“Look, today is the first day of the rest of our lives,” Andy said, his confidence growing. “Let’s take the offered training at work and go to night school at Tech. In about four years, we will have our AS degree and then maybe we can go to Ivy U and get our BS in four more years.”

Sue blinked at him slightly incredulously and said, “That’s eight years total! Do you know how old I’ll be in eight years? I’ll be 27.”

“How old will you be in eight years if you don’t go to college?” Andy asked with a chuckle.

Sue got the point and gently punched Andy in the arm.

“Okay, Romeo, let’s make a pact to both charge ahead and get our BS in eight years,” Sue declared.

With that, she grabbed Andy’s hand as they walked toward the parking lot.

Okay, some readers will think this vignette is too corny. I disagree. Being a professor at Dartmouth College and working with the wonderful folks at Indium Corporation, I have many opportunities to mentor young engineers and professionals. It is a rewarding calling. I encourage all readers to recognize the Sues and Andys in their lives and be a mentor to them.  Best wishes, Dr. Ron

References

1. “SAC305 May Not Become De-Facto Standard,” by Ron Lasky, Indium Corporation.

This column originally appeared in the May 2022 issue of SMT007 Magazine.

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