Estimated reading time: 5 minutes

Maggie Benson’s Journey: The Finishing Touch

Let’s check in on Andy Connors and Sue March at Castellanos Electronics after Maggie and John’s decision to buy the company. In our last episode, Sue was working on solutions to minimize solder defects that have been categorized in a Pareto Chart¹, whereas Andy was working on creating a training program to develop some process engineers and implementing an improvement plan developed by Chuck Tower and José Castellanos.

After working so hard to bring Castellanos Electronics up to speed, Sue and Andy decided to go out to dinner by themselves for a change. They had often gone with José or some of the workers but thought they needed some time just for the two of them.

“Let’s go see the new Tom Hanks movie, A Man Called Otto, after dinner,” Sue enthusiastically suggested. “It’s a cute romantic comedy.”.

“Since it’s a romantic comedy, will you let me hold your hand?” Andy teased.

“Yes to the hand holding, but no necking,” Sue teased back, and they both laughed.

“You know the movie probably has that subtitle thing going on again,” Andy grumbled.

“We’ll survive,” Sue joked and punched his arm playfully.

Current movies from the U.S. were often played in English with Spanish subtitles at the theatre near the factory. Whenever they went to see a film, Andy and Sue would both get distracted reading the Spanish and then, if the translations were off, would let out a sigh.

At dinner, they discussed how the work was going at Castellanos Electronics, as they had only a little over a week to wrap things up.

“With José and some of his team, we have implemented the process improvement opportunities that Chuck and José identified. The new solder paste that has excellent response-to-pause has really helped improve productivity,” Andy began.

Sue asked, before taking a bite of her meal, “You were also working on line balancing, right?”

“Yes, surprisingly, their uptime wasn’t too bad, but the lines were almost never time balanced for the component placement machines,” Andy replied. “By time balancing the placement machines, production improved 8%. There was one case where the chip shooter was taking 55 seconds and the flexible placer only 26 seconds, meaning cycle time was 55 seconds. By moving some components off the chip shooter to the flexible placer, each machine needed only 47 seconds—a time savings of about 15%.” He then shifted the conversation to ask Sue about her progress. “How’s it going on the Pareto chart?” (Figure 1).

“We’ve made significant progress on graping, head-in-pillow (HIP), voiding, and insufficients, and we have only tombstoning left to work on. In all cases so far, we have reduced defects by 90% or more,” Sue said excitedly.

“So, what is your plan for tombstoning? I’m interested, since I don’t know much about it,” Andy said.

“Tombstoning occurs in the reflow oven when one side of a passive solder paste deposit melts before the other side and the surface tension of the melted solder pulls the passive upright like a tombstone,” she said. (Figure 2)

“So, we either have to prevent the one side from melting too rapidly or reduce the surface tension forces?” Andy asked.

“Good start, Einstein!” Sue said, laughing. “It helps to have a solder alloy that has a ‘pasty range,’ meaning that it doesn’t completely melt at one temperature. Since we use SAC305, which has a reasonable pasty range, we can’t improve too much in that regard.”

She continued, “Beyond the pasty range, we can try ensuring that the solder paste deposits are the same amount on each side of the passive and that the components are placed squarely with no skewing. I’m going to ask Miguel to check those things out first.”

“So, if addressing those things doesn’t solve tombstoning, we’ll have to reduce the surface tension forces some other way?” Andy asked.

“Yes, we could modify the reflow profile so that the melting is slower, but that might negatively affect graping,” Sue said. “However, we should look into it.”

After thinking a while, she continued, “Modifying the stencil would help a lot, but that would be an added expense and we should only do that if all else fails.” She then showed him an image of a proposed stencil (Figure 3).

“It’s quite easy to see how the stencil would work,” Andy said, excitement building in his voice. “There is almost no paste on the edges of the passive, so there is almost no paste to cause the tombstoning effect.”

They chatted for a few more minutes about Sue’s efforts, then she questioned, “How is the training going on your end?”

“We’re just about finished,” he replied with confidence. “I’ve covered all the material in the Handbook of Electronic Assembly². I think they are well-prepared for the SMTA Certified Process Engineer’s exam³. As a matter of fact, I just gave them three practice quiz questions and they teased me that they were too easy.”

“What were the questions?” Sue asked.

He then shared these three questions:

1. What is the area ratio of a 15 mil-round aperture in a 5-mil-thick stencil?
2. A PWB is presented to the reflow oven every 40 seconds. The PWB is 20 cm long and a 5-cm spacing is desired between the PWBs. What must the belt speed on the reflow oven be to support this 40-second cycle time?
3. The PWB in question 2 must be in the heated tunnel of the oven for four minutes. How long must the heated tunnel be?

“If they think those questions are easy, that’s really promising,” Sue said of the staff members in training. “I think many people would find them challenging.”

Epilogue
Sue and Andy finished all their assignments at Castellanos Electronics. All the workers who took the SMTA Process Engineer Certification exam passed with flying colors. Defects were reduced by 90% overall. The best news was that the team of workers now had the skills to run the factory when José wasn’t there and could also handle new yield, productivity, and quality issues on their own. Since he could now take an occasional vacation, José agreed to stay on as the factory manager, reporting to Maggie and John.

Tune in to the next episode to find out the solutions to Andy’s three problems.

References

1. “Pareto chart,” Wikipedia.org, Sept. 17, 2022.
2. Handbook of Electronic Assembly and A Guide to SMTA Certification, by Ron Lasky et. al, SMTA Publishing.
3. “SMTA: Certification,” SMTA.org.

Download The Printed Circuit Assembler’s Guide to… Solder Defects by Christopher Nash and Dr. Ronald C. Lasky. You can also view other titles in our full I-007eBooks library.

This column originally appeared in the March 2023 issue of SMT007 Magazine.