Tag Archives: defect rates

Saving a House, Losing a Mansion

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Patty and The Professor agreed to work with two of the local process engineers to develop a plan of attack to try and find the lost productivity.

Patty spoke first. “It’s tempting to look just at the new solder paste, but this approach wouldn’t be thorough.”

The Professor and the two process engineers, Joe and Ann, agreed. So they went ahead and developed a thorough productivity assessment plan, including uptime and line balancing measurements and evaluating changeover and assist times. Ann pointed out that one of the five lines was still using the old paste. All agreed that this situation was good news as they would have a new paste to old paste comparison. It was already lunch time and everyone was hungry, so off they went to a local Outback. While riding in the car, Patty’s cellphone rang. It was Rob.

“Hey Patty,” Rob cheerfully started. “Guess what I shot last night at the Golf Club of New England — a four under par 68! The pro told me it was the best round this year at the course from the back tees.”

“Rob, that’s great!” Patty cheerfully responded. Truth be told, she was really happy for Rob. He was the No. 2 golfer on the men’s team at Tech a few years ago as a senior. She was a junior then and was the best women golfer in Tech’s history. The few times they played then, she beat him. Ever since her dinner date, after their success at AJAX, they had been a couple. At the time she had been thinking of breaking up with Jason and Rob’s invite to dinner was all the catalyst that she needed. In the past year or so, Jason would just watch sports on TV and drink beer. He didn’t have a fitness program or a real plan for his life. Rob was so much different. He worked out, mostly to improve his golf game and he was getting a master’s degree part time.

After they started dating, Rob and Patty played golf together with some other guy friends from Tech. She usually shot the low score, but the three other guys were longer off the tee than she was. Her superior iron play and short game made the difference.

At lunch this working foursome talked about the audit they were about to perform.

“There is one comical thing we should tell you before we start,” Joe said with a twinkle in his eye. “I’ts about the ‘Saving a House Program.’ ”

At that, Ann started laughing and inadvertently started choking on her “sweet tea.” Patty was about to perform the Heimlich maneuver when Ann revived.

With Ann still red in the face and laughing, The Professor requested, “Yes, please tell us.”

Joe chimed in, “So that Ann doesn’t choke to death, let me take a stab at it. The new cheaper solder paste has not been very popular and has generated many complaints. The new COO, Fred, decided he had to do something. He estimated that the new paste saves $100,000 a year on all five lines; that’s about what a modest house costs locally. So he tells all of the complainers that using the new paste saves enough money in a year to buy a new house. He even found a house for sale on the internet for $100,000 and had posters of it made with the saying: ‘Saving Enough for a House.’ It worked; people stopped complaining.”

“Joe, can you tell us what some of the complaints were about the solder paste?” asked The Professor.

“Well, for one thing, it is stiff coming out of the tubes or jars, we have to knead it or it won’t print,” Joe responded.

“Hmm,” both Patty and The Professor mused.

“Also, if we stop a line for a few minutes the paste stiffens up and we have to perform some dummy prints to kneed it,” chimed in Ann. “Sometimes even after this, the first print has to be discarded due to poor hole fill. It wastes time and solder paste.”

“Don’t forget the smell,” Joe teased.

At that, Ann just about spit up her sweet tea.

“The new paste literally stinks,” Joe added. “Fortunately, the vendor added some perfume recently.”

“What about reliability of the finished product?” The Professor asked evenly.

“That’s what is surprising. It’s as good as the old paste.” Ann replied. “We performed some tests and asked around, the reliability is very good.”

“A pleasant surprise indeed,” The Professor said.

The little group finished lunch and headed back to get to work on the audit. Ann and Patty and Joe and The Professor formed teams and went off to the factory. They performed detailed analysis of changeover times, assist times, line balancing, uptime, etc., on the four lines using the new solder paste and the one line using the old solder paste.

As Patty approached one of the lines she saw a cheerful looking gent about 45 years old replenishing the solder on one of the stencil printers. Ann introduced her to Wilbur and asked if it was OK for Patty to ask him some questions.

“Darlin,” he said to Ann in his backwoods drawl, “Anything you gorgeous gals want to ask me is jus fine.”

“How does replenishing the new paste compare to the old paste?” Patty asked.

“Well, it takes a lot longer, stirring the paste and all, but to “Save a House” I’m willing to put up with it, sighed Wilbur.

After a day-and-a-half of work, the team reassembled. The Professor suggested that Patty lead the discussion. Many calculations and comparisons were performed, finally after several hours they were ready to meet with Fred Perkins and Jane Wilson. Patty agreed to speak.

Patty, addressed the small gathering. She presented the approach they used to collect data, their analysis techniques and the fact that they had reached a consensus. The evidence, she said, is persuasive that:
1. The site productivity is down about 8%, which will reduce profits about 12%.
2. The main culprit appears to be the new solder paste.

At this Fred slammed his fist on the disk. His face a bright crimson, he shouted at Patty, “Liar, you corporate types are all alike! You come here from your Ivory Tower and tell us how to assemble a product. You have never had to meet a payroll and make a profit in your life. I’ve been out on the line. It only takes two or three minutes longer per changeover with the new paste and replenish times are even less.”

At these comments Jane rolled her eyes and glared at Fred. It was clear she wasn’t intimidated by him.
Patty shot back, “Fred you are correct; let’s look at the numbers. We measured your average uptime at about 25%, which is quite good. That means the lines are running two hours in an eight hour shift or 120 minutes. Eight percent of 120 minutes is about ten minutes a day. A typical line has two changeovers a day each requiring 2 extra minutes and 6 solder paste replenishments ,taking an extra 1 minute each with the new paste. This totals 10 minutes, hence cuts production by 8%.”

Fred screamed back, “This is mathematical gobblygook. I saved the company $100,000 a year.” At this he stormed out of the room.

The remaining folks stared at each other. Finally Jane broke the silence, “It never occurred to me how precious a few minutes here and there can affect profit. With the new paste, we will lose about 12% of our total profit of $10 million, or $1.2 million per year. It appears that while Fred was ‘saving a house,’ we were ‘losing a mansion.’ ”

Epilogue: Three weeks later Fred was “promoted” to corporate compliance officer. Jane became the new site CEO/COO. The old solder paste was reinstated a day after Fred left. A few of the old-timers kept some of the “Saving a House” posters for future reminiscing.

Let the Data Be Your Driver

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I was recently asked to give a presentation and audit an assembly line regarding minimizing “tombstoning” of passives at a major electronics assembler. As my presentation brought out, tombstoning can be caused by many factors: the reflow profile, the solder metal composition (for lead-free applications, SAC 387 tends to tombstone more than SAC 305), off-center placement, nitrogen reflow atmosphere, buried vias, etc. After two hours of talking, I walked the line that “had a problem with tombstoning.”

As I started asking, it became clear that no one knew the magnitude of the problem.

“How many passives are on each board?” I asked. No one knew.

“How many DPMO (defects per million opportunities) for tombstones have you had recently?” Also unknown.

As people scurried to get the data, it dawned on us that tombstoning might not be as big an issue as was thought. It was more of a local legend.

Finally, we got some data. Each board had about 1000 passives, and the company had produced 100 boards with a total of two tombstones in the past two hours. Tombstones were the only defect. Hmmmmm, two bad boards out of 100 = 98% first-pass yield, not bad! From a DPMO perspective, they had two defects per 200,000 (two defect opportunities per passive) opportunities or 10 DPMO, which is beyond world-class. This level of DPMO would be very difficult to improve on without massive engineering investment. It is “in the noise” and it is likely caused by “common cause” variation.

I then asked how much money it costs to repair a tombstone; as expected, no one knew. My guess was less than $2. This situation is the rare case where yields are so good, it may not pay to make engineering investment to improve them.

This isn’t the point of the story, however. In a case like this, the response — whatever it is — must be data driven. Only with the proper failure rate data, plotted in a Pareto chart, and a complete understanding of all costs, can the appropriate action plan be developed.

Always be data driven!

The Obvious and Not So Obvious About Yields

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It was Charles Talbert’s first major assignment after graduating from Tech top in his Industrial Engineering class. He was excited and didn’t want to blow it, but how hard could it be? All he had to do was select the contract manufacturer with the best yields. His company, Excalibur, has rapidly become a leader in designing premier laptops and mobile phones. Excalibur’s exciting and highly functional designs have made it the envy of the industry and a great place to work. So Charles wanted to add value by helping Excalibur find the best EMS firm. To make his job even easier, senior management performed preliminary screening, limiting the candidates to two: ACME and AJAX. Charles visited both and found they both had excellent quality systems in place including an effective continuous improvement program founded on statistical process control. It looks like it would come down to the yield numbers.

ACME argued that it was clearly the best choice as it had superior yield in both laptop and mobile phone manufacturing. AJAX argued that, while that was true, AJAX’s overall yield beat ACME’s 96.6 to 95.4% (table). How is this possible? And which vendor would you choose?

Laptops
No. Built Yield (%)
ACME 90,000 95
AJAX 10,000 93

Mobile Phones

No. Built Yield (%)
ACME 10,000 99
AJAX 90,000 97

Overall Yield (%)
ACME 95.4
AJAX96.6

When Six Is Really 4.5

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Folks,

In teaching Six Sigma workshops at Dartmouth, we ensure that everyone understands that “Six Sigma,” as presented in the industry, is in fact mathematically 4.5 sigma. So when folks say Six Sigma is 3.4 defects per million (dpm), they are in fact not referring to plus-and-minus six standard deviations from the mean (even though they may not know it), as 3.4 dpm is only 4.5 sigma.

The true six sigma defect rate is 2 defects per billion. The figure shows this error.

Where does this confusion come from? When Six Sigma was developed, it was defined as a Cp of 2 and a Cpk of 1.5. These process capability indices are where the confusion lies. A Cpk of 1.5 permits a shifting of the process mean of 1.5 sigma, hence the true statistical measure of Cpk = 1.5 is 4.5 sigma (or 3.4 ppm). True statistical six sigma (Cpk = 2) is elusive indeed at 2 dpb!

Cheers,
Dr. Ron