First-Pass Yield, Continued


Let’s see how Patty is doing with her latest challenge …

Patty had decided to call The Professor and see what advice he had to offer in preparation for her visit to the facility in Sherbrooke, Quebec  that the senior management  of her company wanted to buy. She was having trouble understanding how it was possible to have 99.5% yield, great uptime, and balanced lines and still have poor profitability.  After a short discussion, The Professor seemed like he was ready to sum the situation up.

“Patty, I think you will find that the poor profitability is the result of high rework costs,” he said.

“But, Professor, how can that be when the first-pass yield is 99.5%? There is almost nothing to rework,” Patty replied.

The Professor chuckled, “Keep an open mind,” he advised.

Then he continued, “Don’t worry, you will figure it out in a heartbeat”.

Patty wished she could be so confident. As she was about to say good-bye, she mentioned to him her observations of so many teens being glued to their smartphones during her recent Williamsburg vacation. She also shared her concern for her two sons growing up in this over connected world.

“Patty, the main thing your sons have going for them is that they have you and Rob as parents. You will help steer them in the right direction, I’m sure. Remember to lighten up a little, after all they are only 5 years old,” The Professor chuckled.

As he was about to say good-bye, he thought of something else to share with Patty.

“Say Patty, you remember that, here at Ivy University, we have information sessions with high school students that are interesting in coming to our engineering school, right?” he asked.

Patty thought for a moment and remembered how impressive that was. It was the only university she knew of in which professors would meet with high school students and their families to discuss the benefits of an Ivy University engineering education.

“Sure, Professor, it’s a great thing Ivy U does,” Patty answered.

“Because of this program I have spoken to hundreds of high school students, I have also given presentations to high school students in larger groups. Give me a few moments with a high school student and I can tell if they are Ivy U material,” The Professor stated.

“How is that possible?” Patty asked.

“I look for two signs. The first is if their parents are much more interested than they are, that is a bad sign. The other is that if a high school student finds someone like me interesting, that’s a good sign,” The Professor chuckled and then continued.

“I know, to the typical 17 year old, I will seem like a boring nerd, however, to someone passionate about learning, I will likely be seen as a fecund resource, even if they are only 17,” he finished.

Patty chuckled a little herself, thinking that only The Professor, would use the term “fecund resource.”

Patty said farewell to her mentor and called Pete to make arrangements to leave for the Manchester, NH, airport, about an hour from their office in Exeter.

By the end of the day they were at their hotel in Sherbrooke. They had dinner at a French restaurant and both agreed to try and speak only French. Each of them slipped in a little Spanish inadvertently, a common problem among those who speak several Romance languages.

After a good night’s sleep, they met for breakfast. At breakfast they agreed on a few things:

  1. They would try and speak French at the meeting.
  2. They would discuss using preforms to solve the QFN voiding problem first as they expected this topic to be more controversial.
  3. The profitability problem, they would leave for last as they anticipated that this would take time, but were expecting less controversy.

After a short drive from the hotel, they were at the facility. Pete commented on the logical way that exits were numbered on Canadian highways, by the number of kilometers from a reference point.

As they approached the receptionist, Pete proclaimed, “Bonjour, comment ca va? Nous sommes là pour répondre à Jacques? (Hello, how are you? We are here to meet with Jacques.)

In a short time, Jacques appeared.

“Bonjour Jacques, mon nom est Patty et c’est Pete. Nous aimerions parler en français si c’est acceptable.” Patty cheerfully said. (Hello Jacques, my name is Patty and this is Pete.  We would like to speak in French if that is OK.)

“Ah, my friends, French probably won’t work for us. You speak with a Parisian accent, suggesting you learned European French. Our French has many different words, we almost always speak in English with our customers and partners from France,” Jacques responded.

Patty thought a minute and it made sense. Quebec has been separated from France for 250 years, but then it occurred to her that the US and Great Britain were separated for about the same amount of time. Maybe this is why some people say that the US and Britain are two cultures separated by a common language, she thought.

They went to a conference room and began discussing the QFN voiding issue.  Jacques presented his data and Patty and Pete gave a presentation on how solder preforms can minimize QFN voiding. Patty gave Jacques a copy of Seth Homer’s paper on the topic.  Both Pete and Patty were surprised at how receptive Jacques was to using preforms.  It seemed that this trip may be easier than they thought.

“Jacques, is it OK if Pete and I walk around and observe the manufacturing process for a while,” Patty asked.

“Sure, take a couple of hours and then we can go to lunch,” Jacques responded.

So Patty and Pete headed off to see the 3 SMT and through-hole assembly lines.  Upon entering the facility, they were stunned to see what appeared to be scores of rework operators.  Patty went over to observe more closely.  It appeared that right after the PCBs were assembled they were visually inspected.  Many of the boards went directly to a rework station.  The boards that appeared to pass the visual inspection, went to an in-circuit testing.  Most of these boards, also went to rework stations. The so-called first-pass yield was obviosuly measured after all of this repair work.

“Pete why don’t you check out the rest of the processes, I’ll stay here and see if I can get a true first-pass yield count,” Patty suggested.

So Pete went off to observe the other parts of the SMT and though hole processes and Patty stayed and counted boards to determine first-pass yield.  After a little more than an hour, they met  in the break room to sum up the situation.

“Well, for the hour I was there, 150 boards were assembled on the one line I was watching. The first-pass yield was only 24%,” Patty groaned.

“I can top that!” Pete replied.  “They have a pencil pusher,” he chuckled.

Patty choked on her ice tea.  As she recovered, she was able to say, “Just like in Mexico?”

“Yep! Same scenario,” Pete responded.

Several years ago, Patty and Pete were at a shop in Mexico, and observed an operator pushing a component, on a board that had exited a component placement machine, with a pencil.  The component was out of alignment and the operator was straightening it.  No one knew how to program the placement machines to correct for this error.

“Any other interesting phenomena?” Patty asked.

“They use the same paste and print parameters, no matter what the stencil. It’s no wonder their first pass yields are low,” Pete finished.

As they summed things up, they were a little down, as they recalled past adventures when they had to deliver bad news.  Patty, then had an idea.

“Pete, why don’t we offer to have you come here for a week or two to help them?” Patty asked.

“Sounds like fun,” Pete replied.

“But we have to get them to agree that first pass yield is measured as the boards come off the assembly line.  Without this metric they can’t assess where their processes need improvement,” Patty added thoughtfully.

“And we need to plot the defects on a Pareto Chart to develop a continuous improvement plan,” Pete commented.

Figure 1. A typical SMT Board assembly Pareto chart.

“It is amazing that their line balancing and uptime are so good,” Pete added.

They were both apprehensive as they met with Jacques.  They remembered some of the times that folks became hostile when bad news was delivered.

Patty did the best she could to keep it positive. She started with their strengths (uptime and line balancing) and complimented them on how strong these important metrics were.  She then shared their “opportunities for improvement” and offered Pete’s help.

“My friends, thank you. What a gracious offer.  I accept,” Jacques said gratefully.  “I guess the workshops I attended on uptime and line balancing paid off. They were presented by this interesting chap everyone calls The Professor,” he finished.

Patty and Pete were stunned by how well this trip went. They enjoyed a delicious French lunch at a café near the plant, with Jacques. On the trip home they chatted about how important it is to the measure first-pass yield before any rework is done, and to plot the defects in a Pareto Chart to lay the foundation for improving yields. Patty now understood what The Professor meant when he said, “look at the rework costs,” they were reworking before they measure their yields.

Epilogue:  Two months later true first pass yield was at 94%.  Costs plummeted with less rework and business soared. As a result of the increased business, full employment was maintained. Patty’s company did end up purchasing this facility. In addition, Patty and Pete became fast friends with Jacques.


Dr. Ron

Note:  As always, this story is based on a true incident.


In Search of a Problem to Solve

It has been a while, let’s look in on Patty …

Patty had to admit that she was very fortunate. She had yet to turn 30 and she was a Senior Vice President at ACME.  There was even a small article about her in Fortune magazine. But she had to admit that, at some level, she was bored. She missed the action of being out on the line and solving problems.

With these thoughts she headed toward the lunch room. She had avoided eating lunch with the execs and still ate lunch with the young engineers that were her age. No one thought it strange. Pete was occasionally the old-timer in the group, as he was approaching 45 years old.

As she sat at lunch with her friends, Patty also had to admit that she was jealous of all of the group’s talk about solving technical problems. She was now responsible for corporate strategies and seldom got her “hands dirty.” So she missed the technical challenges on the shop floor.

After lunch she stopped Pete.

“Hey, Pete, could you stop by my office?” Patty asked.

“Kiddo, for you anything … even that,” he answered and they both chuckled.

As Pete sat down in Patty’s office, she asked him, “How do you like your new job?”

“What’s not to like? Twice as much money and working with you!” Pete answered.

“But don’t you miss … ,” Patty stopped and struggled to gain her composure.

Peter helped her, “Working on the shop floor solving process problems?”

“Yes, so much so that I could almost cry,” Pete finished.

They were silent for awhile.

Then Pete suggested, “Why don’t I see if I can find us a problem.”

Patty smiled. Pete was always well connected.

A few days passed and Patty had just about forgotten about their meeting. There was a knock on her door and Pete stuck his head in.

“Hey kiddo, we have an assignment,” Pete shouted cheerfully.

Patty perked right up.

“What’s the scoop?” she asked.

“You know the new program that rewards cost savings?” Pete asked.

“Sure, I think it is a great idea,” Patty responded.

“There is a conflict in our plant in Santa Clara. Management wants to give a $10,000 reward and the senior purchase manager is blocking it,” Pete elaborated.

“Why?’ Patty asked.

“The engineer deserving of the reward purchased a solder paste that improved uptime,” Pete said.

“Sounds great, what is the issue?” Patty asked. “Let me guess. The better solder paste costs more?” she asked.

“Yep!” Pete responded, “One penny per gram.”

“Mike Madigan wants someone to negotiate the situation. Why not us?” Pete asked.

Patty quickly sent Mike an email offering to help. He gave her the go ahead shortly thereafter.

In a matter of days the arrangements were made and Patty and Pete were on a jet from Boston’s Logan Airport to San Jose, California.

Their flight had taken off and they were enjoying a snack, when Pete commented, “Let’s hope we don’t find someone there like the guy who wanted to assemble the boards without the boards,” Pete chuckled.

At this comment, Patty almost choked on her sparkling water. About four years ago, when Patty was just starting out, they were working on a critical project. The manager in charge wanted the boards to be assembled on a certain date.  Unfortunately, the PWBs did not arrive on time, even though all other components, connectors, and the other hardware where ready. The manager, in frustration, came out to the line on the scheduled start date and was furious that the boards were not being assembled.

The manager asked the lead engineer, “Why aren’t the boards being assembled?”

The lead engineer responded, “The PWBs did not arrive from the vendor.”

To this the manager responded, “Aren’t you going to assemble them anyway?” (See note below.*)

This was their favorite story about the occasional comedy in electronics assembly.

It seemed like no time at all and Patty and Pete were sitting in the conference room that had been reserved for the meeting. They introduced themselves to a young engineer who was sitting in the room waiting for the meeting to start. His name was Dave Ferris.

“So Dave, you are the cause of this meeting, eh?” Pete teased.

“I guess so. I can’t believe how hard it is to sell productivity here. The amount of time the new solder paste saves enables us to produce 1,000 more units per year on each line. And these boards are super expensive, with high margins. Admittedly the solder paste costs $0.01 more per gram, but the additional profit is over $800,000 per year for each of our three lines,” Dave Ferris explained.

“How did you perform the calculations,” Patty asked.

“I went to a workshop run by this quirky, cheerful guy everyone calls ‘The Professor.’ He was amazing,” Ferris replied.

Pete and Patty both chuckled.

“We know The Professor well,” they chimed in unison.

“We assume you used ProfitPro for the calculations?” Pete asked.

“Yes,” Dave responded with a surprise in his voice that they would know about such things.

Will Patty and Pete save the day?  Will Dave get his award?  Stay tuned to see.


Dr. Ron

*As hard as it is to believe, the story about building the boards without the PWBs is true.  Thanks to ITM.

Common Cause vs. Special Cause Defects

In teaching process optimization and failure analysis, one of the most helpful concepts is understanding he difference between common cause and special cause defects. A special cause defect, in a well tuned process, occurs when something unpredictable changes. As an example, let’s say you get a batch of printed wiring boards (PWBs) that have oxidation on the pads. This is a defect and the boards shouldn’t  used, however we will assume that somehow they made it through the company’s receiving inspection process. It should not be too surprising that when the boards are assembled that they have a poor first-pass yield, say 35%. Typical first pass yield in this optimized process is 95%. It is obvious that the poor yield was due to this “special cause,” the oxidized pads.

Common cause failures are a little more difficult to explain and comprehend.  In a process, there are multiple entities that can vary, within the specifications, such as  the solder paste viscosity, the temperature and humidity of the room, the reflow profile, the wettability of the component leads and PWB pads, etc.  Statistically, within the specifications, the variation can be such as to result in a small number of fails … say the 5% we get with this process when everything is as it should be.  These types of fails are called common cause fails.

It is fundamentally crucial to understand the differences between special and common cause fails to successfully monitor and improve processes.  One of the tragedies that  I often see when the failure rate increases, due to special cause fails, is the process engineers changing the process parameters (e.g., raising the reflow temperatures when the pads in the special cause example above did not wet).  In a well optimized process the process parameters are determined by designed experiments, any collapse of process yields is the result of a special cause.  You can only fix special causes by identifying and rectifying them, not by changing the process!