Your ‘Common Cause Floor’ will Help Define a Reasonable DPMO Target

Let’s look in on Patty; it has been a very long time …

Patty left her house in Woodstock VT very early on her way to Ivy University. She chuckled at the darkness of the early morning; it reminded her of a book she was reading.  In the book, Gray Girl, Jan Wishart is a young woman in her first year at West Point. The cadets use military time, so, for example, 9:00AM is referred to as 0900 hrs. When it is so early that it is still very dark, the cadets simply call it, “0 dark thirty.”

She had to admit that, even though she occasionally had to leave at “0 dark thirty,” she loved being a professor at Ivy University. She had just finished teaching a statistics class and had submitted the grades – she was ready for the holiday break.  As she drove past the Woodstock Green, she noticed that Christmas ornaments decorated Woodstock’s covered bridge. The entire town was getting ready for Wassail Weekend.

“What a great place to raise a family,” Patty thought.  She, her husband Rob, and their twin 7-year-old sons just loved it there.  It was a very wholesome place for the boys (all three), with many outdoor activities.

She was going in early to meet with The Professor, but, before that, she had to hit the gym for her daily workout.  As she approached the Taftsville Bridge she decided to venture across and take the back road. This route was a mile longer, but crossing the bridge and riding on the back road was more uplifting to the soul.  The back road went along the river and was more picturesque and peaceful than the bustling Vermont Route 4.

The bridge in Taftsville, VT, is a pleasant sight on the way to Ivy University.

Wild turkeys near Taftsville, VT.

After crossing the bridge and driving a few miles, she suddenly had to hit the brakes as a flock of wild turkeys crossed the road – just another reason to like living in Vermont.


Before she knew it, she was in the faculty parking lot.  As with almost all universities, parking was a challenge. But, the sun was just rising on this late November day and the lot was mostly empty – except for Dean Howard’s car.

After her workout and shower, she was in The Professor’s office with her long-term sidekick, Pete.  Her husband Rob would join them soon after getting the boys off to school.  The four of them spoke Spanish and, when together, agreed to converse in this romance language to keep their skill sharp.  If Pete wasn’t there, the three would speak Mandarin Chinese, a language he didn’t know.  No one knew for sure how many languages The Professor spoke, but it was rumored to be about 18.  His parents were missionaries for Wycliffe Bible Translators, so he lived in many countries as a youth.

“Hola a mis amigos, la razón por la que les invité aquí fue a discutir DPMO,” The Professor began.

(The remainder of the text will be in English for our non-Spanish speakers.)

“Gee, I haven’t heard people talk about DPMO in years,” Pete responded.

“Remind us how it is tallied,” The Professor requested.

“Well, in electronics assembly, each lead that is assembled is counted as a possible soldering defect ‘opportunity,’ so you count the end of line defects and divide by the opportunities,” Pete began.

“Don’t forget that you normalize to parts per million,” Patty added.

“That’s where DPMO (defects per million opportunities) comes from,” Rob chimed in as he stuck his head in the door.

“And don’t forget to add one defect opportunity for the component itself,” The Professor added.

“Why the concern for DPMO?” Patty asked.

“One of my clients asked if a DPMO of 20 was good enough.” The Professor answered.

“With continuous improvement, shouldn’t they be striving to improve?” Pete asked.

“Well, to a point. But does anyone have a counter-thought?” The Professor answered, always trying to make a learning experience.

“Well if all special cause defects have been addressed and only common cause variation is left, it may be too expensive to improve significantly,” Patty commented.

Pete opined, ”I remember about 20 years ago, I worked for a large OEM and they were at a DPMO of 20.  They tried to get to 5, but it cost a fortune in engineering expense.  A DPMO of 20 hit their ‘common cause floor.’ It costs much more in engineering expense to try to get below the 20 DPMO than the small amount they would be saving in rework costs.”

“Hitting your ‘Common Cause Floor’ sounds like a new expression that you just created Pete— congrats,” Patty said.

Rob had been busy on his laptop and he suddenly chimed in, “I found an article that suggests that 20 to 50 DPMO is a reasonable goal.”

“Let’s do a shirt-sleeve calculation,” the Professor suggested.

“My client has a DPMO of 20. Each product has about 2500 leads and components. It costs $2 to repair a defective device. And, they make 1 million devices with a value of $100 each and a net profit margin of 5%,” The Professor went on.

“So, 20 DPMO times 2500 equals 50,000 or 5% defects in the 1 million units,” Patty started.

“That means 50,000 reworked devices out of the million manufactured for a cost of $100,000 or 2% of the $5 million net profit,” Rob added.

“Getting the DPMO to much less than 20 will cost millions a year in engineering expense,” Pete stated.

“So, let’s sum it all up,” the Professor suggested. “The ‘Common Cause Floor’ will be different for different manufacturers, but hoping to get a DPMO near 0 will likely be too expensive in engineering costs.”

“And, Pete will become famous for inventing the term, ‘The Common Cause Floor,” Patty joked.

They all ended the meeting with a laugh and a slap on Pete’s back.


Dr. Ron

Let the Data Be Your Driver

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!