Estimated reading time: 4 minutes

Maggie Benson’s Journey: Breaking Down the Math

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

Andy Connors and Sue March were heading to their favorite pizza parlor before pre-calculus, their first class at Valley Tech.

“I really enjoyed dinner at your house on Sunday,” Andy said. “Your parents were so welcoming to me. I’m touched.”

“Well, I’ve never had a boyfriend until now and they’re happy for me,” Sue replied.

“I, I, I’m your boyfriend?” Andy stuttered.

“Unless, you don’t want to be,” Sue responded.

Andy knew instinctively that maybe words were not needed. He stood up, went to Sue, pulled her up, and gave her a long hug. An observant customer might have noticed a tear forming in his eye.

“Okay, Romeo, let’s discuss what we expect in tonight’s class,” Sue teased.

“Well, the syllabus says we will review algebra and exponents. I’m comfortable with algebra, but not so much with exponents,” Andy said with a sigh.

“Let’s try one,” Sue suggested. “If X³ + X⁵ = 64, what is X?”

“Well,” Andy said, “as you taught me a few days ago, the exponents are added, so it is X⁸ = 64. So, what times itself 8 times equals 64? Is it 2?” He knew he was right. After a few more examples, which Andy got right, they left for class.

Later, both Sue and Andy felt the first class was easy and Sue suggested they get some ice cream then discuss what they learned in Chuck Tower’s stencil printing class.

“I thought the stencil design discussion was quite straightforward,” Sue began.

Andy’s heart sunk a bit. Everything was straightforward to Sue. But this was one of his strengths.

“Well, for one thing, the ‘Five Ball Rule’ was clear,” Andy began. “At least five ‘balls’ of the solder paste to fit within the width of the stencil aperture is easy to understand.”


“I found out we typically use solder pastes with Type 3 or Type 4 solder powder¹ so we can determine the range of solder ball sizes and the largest solder ball from the Solder Powder Type Chart,” Sue commented.

“And the aspect ratio, the width of the aperture divided by the height, being greater than 1.5, is now clear to me, but the area ratio...” Andy said.

“I agree, it’s a little more complex. It is the area of the aperture opening divided by the area of the sidewalls. To make the solder paste stick to the printed wiring board pads more than the side walls of the aperture, the ratio must be greater than 0.66,” Sue explained.

“You know, I think I finally get it,” Andy said.

“But can we derive what the area ratio is?” challenged Sue.

Andy proceeded to draw a circular aperture. “Well, we know the area of the opening is pi times the radius squared,” he said.

“What about the area of the side walls?” Sue teased. Andy paused and seemed stumped.

“Well, what is the perimeter of the circle?” Sue teased.

“Two pi times the radius?” Andy said with uncertainty.

“Yes, so then the area of the side walls would be?” Sue questioned.

“Wait, now I see it—two pi times the radius times the stencil thickness!” Andy said triumphantly.

“Isn’t it interesting that the area ratio is the same for square apertures?” Sue commented.

“And I actually understand how it is derived,” Andy said, beaming.

As they were chatting, Chuck and Tanya Brooks came to their table. “Andy, Sue, it’s great to see you here,” Chuck said. Andy, Sue, Chuck, and Tanya shook hands, and Sue and Tanya hugged each other.

“This place has the best ice cream around,” Chuck opined.

All agreed and small talk ensued for a few moments. Finally, all sensed Chuck wanted to say something.

“Sue and Andy, you two are the superstars in our SMTA certification prep class. We need some more instructors. How about you two joining our instructor team?” he asked.

“Sure,” Andy said, “we would love to!”

Some more small talk ensued, and eventually the group broke up and headed to the doors of the shop. As Sue and Andy walked to Andy’s car, Sue looked extremely upset. “What’s the matter?” Andy asked.

“I’m afraid to speak to a group; I don’t think I can be an instructor,” Sue replied.

Andy thought for a while and then gave her a hug. “How about this? I will speak and conduct the class and you will answer questions that you are comfortable answering,” Andy suggested.

The relief on Sue’s face was palpable. “Thanks, Andy, maybe approaching it this way will make me feel more confident speaking in front of a group,” she said with an obvious sign of relief.

Will Andy ever catch up to Sue intellectually? How will Sue handle helping teach the class?

Stay tuned to find out.

All figures are used with permission of the copyright owners, Jim Hall, Phil Zarrow, and Ron Lasky.

Reference

1. “Does Solder Paste’s ‘Five Ball Rule’ Remain Valid in SMT Today?” by Ron Lasky, Indium Corporation.

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