About Dr. Ron

Materials expert Dr. Ron Lasky is a professor of engineering and senior lecturer at Dartmouth, and senior technologist at Indium Corp. He has a Ph.D. in materials science from Cornell University, and is a prolific author and lecturer, having published more than 40 papers. He received the SMTA Founders Award in 2003.

RoHS’s Side Benefits

I have mentioned numerous times that the first purpose of RoHS is to help make recycling easier. So RoHS was developed to support WEEE. One would imagine that, in doing this, the EU was primarily concerned with recycling in the EU.


Fortunately, thousands of folks in the Third World will benefit from RoHS, as much recycling is performed by poor people in these countries. When they recycle non-RoHS-compliant scrap electronics, they are being poisoned by lead, cadmium, mercury, and smoke from non-banned organic compounds. This sad situation was again recently brought out in a New York Times article.

As more and more waste electronics becomes RoHS compliant, the amount of toxic material that these people are exposed to will become less and less. It still shocks me that, when I point out this benefit, a person comments something like this:

“You mean I have to put up with RoHS just to help these people?”

It is my fervent hope that very few of us feel this way.

Cheers,

Dr. Ron

The Successful Process Engineer Quiz, Redux

Folks,

Answers to the quiz of a few weeks back …

Phil and Rob had agreed to ask the GM if it was OK to ask the tech and engineers at some of their subcontractors to take the test anonymously. Over a period of two months Phil and Rob got 52 people to agree, almost all of them after Phil or Rob agreed to take them to lunch. They asked Patty to grade the “exams.” Today Patty would reveal the results.

“Phil, this is one of the best bets I have ever made,” teased Rob.

Everyone at the lunch table chuckled, but the look on Phil’s face said he expected to lose. Rob has said that he thought the average score would be less than 70%, Phil insisted that it would be greater than 85%. In asking the different folks to take the test, invariably Phil started asking questions not on the test. He was surprised that no one knew what tin pest was. He even asked how to time balance a chipshooter and flexible placer, only one in 20 knew.

As Patty approached the lunch table, the ensemble held their breath.

“OK, Patty, tell us the bad news,” Phil said in a resigned tone.

“Rob wins, the average score was 58%,” Patty said getting to the point. “Here are the answers and percentages on each problem,” she went on:

1.    What is the composition of SAC305?
96.5% tin, 3.0% silver, 0.5% copper. 60% got this right.

2.     What are tin whiskers?
Tin whiskers are metal whiskers that can “grow” from tin plating on component leads. They are mitigated by 2% bismuth in the tin, a nickel overplate of the lead copper, a matte tin finish, and a few other mitigation approaches. (40%.)

3.    In a stencil aperture, what is the area ratio?
The ratio of the area of the aperture opening divided by the area of the side walls. This ratio is typically used for circular and square apertures. It is equal to D/4t, where D is the diameter of square side and t is the stencil thickness. (40%)

4.    What is an approximate peak temperature for a reflow oven in lead-free assembly?
Any answer 235° to 250°C accepted. (90%)

5.     A board is inspected after wave soldering and one lead is not soldered to the board. The board is run through the wave solder machine again and has the same defect on the same lead. What is the most likely cause of the defect?

a.       The solder temperature is too low.

b.      The pad on the board is oxidized.

c.       The preheat temperature is too high.
(b. 70%)

6.     What are local fiducials on a PWB for?
Local fiducials are located near the pads of a component with fine lead spacings to ensure accurate placement. (70%)

7.     What does “thixotropic” mean in regard to solder pastes?
The viscosity decreases with increasing shear stress. Hence, during printing the viscosity drops as the paste is forced through the aperture, aiding good aperture fill. It increases as the printed deposit rests, minimizing slump. (20%)

8.     A chipshooter places passives at a rate of 36,000 per hour. It is placing 300 passives on a PWB, how many seconds will the chipshooter take to place the passives on one board?
300/36000 = 1/120 hr = 30 seconds. (90%)

9.     A reflow oven belt speed is 100 cm/min. The PWB is 40 cm long. What is the minimum cycle time that the oven can support?
The amount of time that the belt needs to cover 40 cm is 40/100 = 0.4 minutes = 24 seconds. This is the minimum cycle time the oven can support. (40%)

10.   What is “tombstoning”?
Tombstoning is observed when a passive component’s terminations experience unequal wetting forces strong enough to lift one end of the passive so that it looks like a tombstone. (60%)

Overall average score: 58%.

“Wait a minute Patty; your answers are too demanding,” Phil shouted.

“Calm down Phil, I gave full credit for anything close,” Patty responded.

In unison, almost everyone at the table sighed, “Yikes.”

Patty interjected, “One person who received a 70% commented after completing problem 9 said, ‘I didn’t think I would need a Ph.D. in math to do this quiz.’ ”

All agreed that organizations committed to electronics assembly education, like the SMTA and IPC ,were more needed than ever.

Cheers,

Dr. Ron

Changes Coming in Computing?

Folks,

I was in the Dartmouth College computer store in 2004 or 2005 and I saw a signed that proclaimed: “After July 1st computer services will no longer support Apple Macintosh.”

By 2008, however, I noticed more and more of my students using Macs. This trend was important to me as I use Minitab software in two of my classes. Newer versions of Minitab only support Windows.

This year, so many of my students use Macs, that I have to hold the classes in which I teach Minitab in the engineering department’s computer lab, as I want the students to follow me on a computer.

I was recently told that 75% of the incoming freshman at Dartmouth will be using Macs. All of this in spite of Macs significantly greater cost over a Windows based PC.

Since the students at our colleges and universities will be tomorrow’s leaders, this may portend big changes ahead in computing.

Cheers,

Dr. Ron

Getting ‘Up’ for ’60 Minutes’

Rob bolted upright in bed. He had that terrible feeling that he had overslept for an important appointment. His eyes quickly found the clock and it said 10:30!

“Wait a minute!” he thought, “It’s Sunday.”

He looked a Patty peacefully sleeping and decided to let her sleep. They had had a tough two months. Ever since they proposed increasing uptime to greater than 60% on two “experimental lines”, they were working 90 hour weeks. They just felt they needed to constantly monitor two lines, to assure that things were going smoothly. They felt satisfaction that they achieved 68.8% uptime in a two month period, compared to the company average 30.4%, which is still very good.

The local newspaper got word of this effort and did a story on Rob and Patty’s work. The article was well written and very complimentary to both he and Patty, as well as ACME. Sam Watkins, the site general manager, was very pleased with the good PR. The accompanying photos were really nice too.

The big shocker came this past Tuesday. 60 Minutes wanted to do a segment on “The US Competing with the Far East in Electronics Manufacturing.” In agreeing to be interviewed, Rob and Patty insisted that members of their ACME team be included. In addition, they felt it was only fair to include the efforts of Rita from their stencil printer and reflow oven supplier, and States, their colleague from the component placement company. And they couldn’t forget Eric, from ACME’s prime solder paste supplier. These three folks helped Rob and Patty and their team to develop the plan to achieve the 60+% uptime.

An even bigger shocker came when the 60 Minutes crew told them that Andy Grove would be in the segment because of his recent article in Business Week, “How America Can Create Jobs.”

Grove insisted that to participate in the piece, he wanted to visit ACME to see what Rob and Patty were doing. So the 60 Minutes crew was visiting ACME’s plant this week as were Rita, States, Eric and now “Andy.”

“Maybe we should call him Mr. Grove,” Rob thought.

Rob had suggested that he and Patty go to Berdick’s in nearby Walpole, NH, for Sunday brunch and then to play golf. Rob had to chuckle, it was mid July and he and Patty had played golf 27 times (she kept a spreadsheet). He had beaten her 14 times and she was miffed. Even during their 90=hour weeks they would take three breaks a week to play 9 holes.

On Monday they were meeting with site GM, Sam Watkins, to discuss what they would tell 60 Minutes.

Rob and Patty’s Sunday was delightful. The brunch was delicious and relaxing and they both golfed well, Patty’s 68 beating Rob’s 69.

It seemed like no time at all and Rob and Patty were in Sam’s office.

“Just assure me that this 60 Minutes thing is not some expose that will embarrass ACME or put me in jail,” he teased.

Patty took the lead and explained what they had done. They trained the operators on the importance of line uptime, they worked with Rita, States, and Eric to develop a plan to assure that there would be minimum unscheduled downtime. They had to order extra spare parts and solder paste to assure no stoppages due to parts or paste shortages. One obvious thing is that they would be using two times or more the normal amount of solder paste. The two lines in the high uptime experiment had an average of one change over per day, consistent with ACME’s business.

They also increased routine maintenance on all machines. Both this maintenance and added spares was an increased cost, but these costs were second order effects compared to the dramatic profit increases due to almost 70% uptime.

Preparation for the next three jobs for each line was meticulous, so that setup time was minimized. Feeder racks were used extensively in minimizing setup time for changeovers. In addition tape splicing was employed to minimize any assist time for component placement. States’ help was crucial in the component placement part of their efforts, Rob pointed out.

Patty went on to describe how Rita helped them in their efforts to develop minimum assist times for the stencil printing process.  The reflow oven presented the least concerns in assist or unscheduled downtime.

The solder paste they selected was robust in that it had a very good response to pause, excellent tack, and minimal slump.  The paste also had the best track record for minimizing defects like head-in-pillow and graping. Eric also participated as an enthusiastic partner in the effort.

Patty mentioned that their colleague, Phil, had agreed to monitor uptime for two standard lines during the two month trial to compare downtime metrics to the high uptime experiment. These would be experimental “controls.”

She then showed the uptime data for the two high uptime lines and Phil’s control lines. The control lines had ACME’s respectable 30% uptime, but the high uptime lines had almost 70% uptime. Rob went on to explain all of the things the team did to minimize downtime, most of it was common sense. Sam was especially interested in one downtime category.

“What is floundering time?” Sam asked.

“That is time when the line is not operating due to some unplanned error,” Rob answered.

“Can you give an example?” asked Sam.

“Sure, you know how we have a quite organized approach to setups?” Rob responded.

“You mean our use of white boards to write down all of the things needed for the next three jobs on each line?” Sam came back.

“Yes, that is one of our biggest sources of floundering time,” Rob replied. “Someone will write that they have the stencil for the next job, when they just think they know where it is. When it comes time for that job the stencil cannot be found and an hour is lost.”

“Phil also noted a case where a job was finished on a line at 11:15AM, since lunch was at 12 noon, the changeover for the new job was not started until after lunch. Forty-five minutes was lost, forever,” Patty added.

Sam gulped.

“So we are losing more than 25% uptime to ‘floundering?’” Sam weakly asked.

“According to the Professor, it’s endemic in the industry,” Patty interjected. “He coined the term, ‘Floundering time’.”

Sam then mentioned how the “bean counters” at ACME were impressed with the two high uptime lines. ACME’s CEO wants a concerted effort to transition all of ACME’s assembly lines in North America  to higher uptime performance. Manufacturing in North America would also mean no two to four weeks of transportation time from the Far East. Patty, Rob, and their “team” were to form a new group in ACME to do this. Patty would be director of the group.

As the meeting was about to close, Sam asked what surprises Patty and Rob had during this experiment.

Rob then shared, “It relates to floundering time. We found that even among the engineers, no one appreciated the value of one hour of production time. We asked a group of operators what an hour of production was worth and the figures ranged from $50 to $500. ACME runs two shifts at 30% uptime; that’s about 1500 hrs per year. Our typical line produces $30 million per year; that’s $20,000 per production hour. When we told the operators this, floundering time dropped significantly.”

Patty added, “The other thing we saw is that a “watchdog” is needed. If someone isn’t constantly watching things, floundering and assist times will go up. Since productivity is doubled with a high uptime line, the added cost of a watchdog is insignificant.”

Epilogue: The 60 Minutes segment was a great success. Patty was made director of corporate productivity, but was also asked to manage Pete, who would take over her old group. No one seemed to worry that Patty was Rob’s boss, except maybe Rob!

Cheers,

Dr. Ron

Can the US Compete with Low Wage Countries?

Folks,

The adventures of Patty and Rob continue …

“No way Rob. I can’t buy this one,” shouted Phil.

“I’m telling you I did all of the calculations. I know it can work,” Rob shot back.

“No way! You’ve been hanging around The Professor too long,” Phil continued.

“Guys, you’re not arguing again are you?” Patty said as she approached the lunch table.

“Your hubby’s gone crazy Patty, why don’t you trade him in for me?” Phil teased.

Why? Is he crazy? asked Patty.

“He thinks that, if we can get uptime high enough here in the USA, we can compete with assemblers in low wage countries. He’s gone too far this time,” answered Phil.

“Hear me out,” responded Rob. “Our uptime is about 30% on our lines. Patty and I made some measurements in China and uptime there is about the same. I did some calculations with ProfitPro and I’m convince that if we can get our uptime to greater than 60% we can compete, even if they pay only $0.75/hr vs our $12.50,” Rob finished.

Rob showed the ProfitPro output and sure enough the numbers didn’t lie.

Jan Curtis chimed in, “But Rob, that is theoretical. Realities might set in that will cause unforeseen problems.”

Phil agreed, “The only way to prove it is to do it.”

“You’re right,” said Patty, “Let’s do it!”

“Whoa girl! How can we do it?” asked Rob.

“Let’s develop a plan and go to Sam the GM and ask if we can do it,” was Patty’s answer.

All agreed it was a gutsy move, but worthy, and the only way to prove the point one way or the other.

So, Patty and Rob made an appointment with Sam Watkins, the site GM. They were admittedly a little nervous about the meeting. They both agreed that Rob should speak since it was his idea.

“To what do I owe the honor of a meeting with our recently married superstars?” Sam cheerfully greeted them at their meeting.

Rob explained their idea and showed Sam the ProfitPro output. He then requested that they be allowed to manage two assembly lines at ACME to show if increasing uptime to 60% was possible.

“So basically you are asking me for permission to improve our uptime on two lines to help us make a lot more money?” Sam asked. “Do you have a plan?”

Rob and Patty proceeded to show Sam their plan, and the two assembly lines on which they hoped to implement it.

Is it possible to pay US-type wages and compete with low-wage countries? Will Patty and Rob be successful? Stay tuned for the results.

Cheers,

Dr. Ron

The 10 Questions for Successful Process Engineers

Patty had just finished some emails and was ready to head off to meet Rob and some of their buddies for lunch. When she and Rob returned from China a month ago, Sam, the site GM, told them he was giving them an extra week of vacation for their honeymoon. Their China trip had been an unqualified success in helping the local teams achieve more productivity and higher yields. Sam had received numerous positive reports from the Chinese managers involved.

There were several requests to have Patty and Rob stay a year in China to help with the many process issues that the China team has. Fat chance of that happening, Sam needed Patty and Rob here! Sam also mentioned that he knew that the trip was a little stressful coming so close to their wedding, so the extra week was ACME’s gift to the young couple for their sacrifices.

The wedding went off without a hitch. Patty was touched at how choked up her dad was in “giving her away.” The wedding reminded Rob and Patty how close they were to their parents. They both agreed that the support of their parents was crucial in any success that they had in life.

For their honeymoon they decided to tour France, Italy, and Germany. Rob was really proud that he handled the languages a little better than she did. Of all the things that they saw, they were most impressed with Pompeii. Because the city was covered in hot ash in a matter of moments, it was as if Pompeii was frozen in 70 AD. Visiting Pompeii was like stepping back into the time of the Caesars.

Truth be told, Patty was happy things were back to “normal.” It was pleasant to have their working schedule and to go home to their apartment at night. A couple nights a week, and most Saturdays and Sundays, she and Rob played golf. He had improved somewhat and she was a little annoyed that so far this year he had beaten her more than half of the time … and yes, he was rubbing it in.

As Patty approached the cafeteria she heard a friendly but heated discussion.

“No way can you evaluate an assembly company with just 10 questions,” Phil Anderson stated emphatically.

“I’m really convinced we can, I’ve thought it through a lot,” responded Rob.

“What’s the spirited debate about?” asked Patty as she sat down.

“Rob thinks you can evaluate an assembly company by asking a lead process engineer only 10 questions. Phil thinks he’s nuts,” responded Patty’s best friend Jan Curtis.

“I’ve thought about this quite a bit,” said Rob. “I’ve just finished reading Malcolm Gladwell’s ‘Blink.’”  “Gladwell claims that often the best judgments can be made quickly with just a sampling of data.”

“Be specific,” challenged Phil.

“OK, I actually developed 10 proposed questions to evaluate a assembler. Let me list them and then defend them. Maybe you guys have better ones,” said Rob.

Patty thought, as she heard this, that it was good news that ACME was looking to buy more assembly companies to handle their ever-increasing workload, unlike AJAX, which was laying folks off.

Rob had come prepared; he actually had some printouts. His 10 questions were:

1. What is the composition of SAC 305?

2. What are tin whiskers?

3. In a stencil aperture, what is the area ratio?

4. What is an approximate peak temperature for a reflow oven in lead-free assembly?

5. A board is inspected after wave soldering and one lead is not soldered to the board. The board is run through the wave solder machine again and has the same defect on the same lead. What is the most likely cause of the defect?

a. The solder temperature is too low.

b. The pad on the board is oxidized.

c. The preheat temperature is too high.

6. What are local fiducials on a PWB for?

7. What does thixotropic mean in regard to solder pastes?

8. A chipshooter places passives at a rate of 36,000 per hour. It is placing 300 passives on a PWB. How many seconds will the chipshooter take to place the passives on one board?

9. A reflow oven belt speed is 100 cm/min. The PWB is 40 cm long. What is the minimum cycle time that the oven can support?

10. What is tombstoning?

“You have got to be kidding,” shouted Phil. “Everyone will score 100% on that test.”

Jan chimed in, “I’m not so sure. We hang around people all day who study this stuff. I’m not sure the typical process ‘engineers’ have enough time to study and learn new things. Remember the ‘water in the solder’ and the ‘isopropyl in solder paste’ incidents?”

At this comment, Phil spit up his ice tea and started choking from laughter. One of their friends, Sally Herman, had been sent to a recently acquired company to help with assembly process issues. One of the “process engineers” introduced himself by bragging that he was saving the company money by taking used, dried solder paste and mixing it with isopropyl alcohol so that the paste could be used again. Later in the day, the same chap shared that he thought he had a solution to the poor hole fill problem in lead-free wave soldering: the solder was too thick, and if it was mixed with water it would fill the holes better, he opined.

Jan added, “As a minimum, these questions act as a good screening process.”

Rob interjected, “That’s my point. I’m not saying this tells us everything, but you will agree that if a lead process engineer can’t handle these questions, it is unlikely he or she would be able to solve graping, or the head-in-pillow defect, right?”

All at the table murmured agreement.

“On second thought, maybe you have something here Rob,” Phil said. “What do you propose as a passing score?”

“Seventy percent,” Rob answered.

Are Rob’s questions reasonable to evaluate an electronics assembler? What are the answers? Comment with your answers. Stay tuned to find out.

Cheers,

Dr. Ron

Risk Priority and Tin Whiskers

Folks,

Tin whiskers continue to generate considerable interest. People often suggest that their risk is great and yet unknowable. RPN may help to clarify the TW risk. What is RPN? It is the risk priority number from failure mode and effect analysis.

As this link tells us:

A failure modes and effects analysis (FMEA), is a procedure in product development and operations management for analysis of potential failure modes within a system for classification by the severity and likelihood of the failures. A successful FMEA activity helps a team to identify potential failure modes based on past experience with similar products or processes, enabling the team to design those failures out of the system with the minimum of effort and resource expenditure, thereby reducing development time and costs. It is widely used in manufacturing industries in various phases of the product life cycle and is now increasingly finding use in the service industry.

RPN is an important part of FMEA. It is the product of three numbers that range from 1 to 10. The first number is the severity (S) of a possible fail. A “10” would be given if the failure injured someone, “7” would be assigned if the failure caused a high degree of customer dissatisfaction, whereas a “2” would be given if the failure has only minor negative effects.

The second number is occurrence (O) of a fail. The highest rating is a “10,” which would be a failure every day (reminds me of Windows ME!) or one fail in 3 events, whereas a “7” would be a failure every month or one in 100 events. A “2” is a six sigma fail rate.

The last number is detection (D) of a potential fail. A”10” would suggest that the detection of a potential fail is either not performed or not possible. A “7” is a manual detection approach that may not be reliable, whereas a “2” is 100% effective potential failure inspection.

So obviously a product with a RPN of 10 x 10 x 10 = 1000 is a disaster, its failure is dangerous, frequent and incapable of being detected beforehand. Industry rules of thumb suggest that and RPN of 200 needs to be addressed and an RPN of 75 is usually considered acceptable.

Let’s look at a “ball park” RPN for tin whiskers. We will assume the application is a critical IC in a PC. Let’s assume that a severity rating of “S” of 8 (failure renders the unit unfit for use) is reasonable. TW are hard to inspect for future fails, so detection, “D,” could be as high as a 10. At this point we are at 8 times 10 equals 80 for both. A bad start.

Occurrence (“O”) for TW failure modes is dramatically different. When trying to assess the occurrence of TW fails, one is often directed to NASA’s web page . Many reference this web site that lists a little more than a score of TW fails. What escapes me is that people don’t seem to appreciate the rarity of less than 100 fails in decades of data collection. Surely TW fails are not common. I could find no report of a failure of a RoHS compliant product anywhere on the internet. So it would be hard to rate “O” any higher than a “2.” I suspect that the reason few TW fails have apparently occurred is due to TW mitigation techniques that are widely practiced.

I would expect that “modern” process defects like the head-in-pillow or graping defects could have a much higher RPN than TW, if assembled without proper process controls and materials. However, there is little need to worry about these defects either, if you use the right solder paste and practice some assembly process precautions.

Patty and the Professor: Heading Off HIP

Patty, Rob, and The Professor finished their tasks in Shenzen and were flying to Shanghai for their last set of challenges in electronics assembly. Then they would head back to the US, Rob and Patty being only a week away from their wedding day.

As usual Rob, conked out as soon as the plane lifted off. Surprisingly, The Professor also drifted off to sleep. Patty was too excited to sleep. Rob’s mother had given her and Rob their wedding presents early … an iPad for each. They decided to bring only one laptop and one iPad. Patty was a little nervous about using the iPad for presentations but it worked quite well. She was still surprised that the iPad did not have a USB port. The Professor also gave each of them an early wedding present, a Pickett slide rule for Rob and a K&E slide rule for her. She must be the only person in the world right now that was watching a movie on an iPad and solving a math problem with a slide rule!

True to form, The Professor was passionate about how learning to use a slide rule helped improve a person’s innate math ability. He showed Patty and Rob how to use them and gave them several assignments. Rob was better with his slide rule than Patty due to the amount of “one on one” time he had with The Professor. She had to admit that using the “slip stick” gave one more of a feel for calculations and it was consistent with one of The Professor’s adages: “Always know approximately what the answer to a calculation should be … it will help you to avoid errors.”

In addition to the iPad and slide rule, Patty was excited to be going to Shanghai at the time of the World Expo 2010. Our trio had scheduled some time at the expo into their busy schedule.

Their plan was for Rob and The Professor to work on some productivity issues and for Patty to take on some of the process materials related problems. The three of them again met with the site GM for ACME’s newly acquired plant in Shanghai, a Mr. Wong. Wong was relieved to find that they all spoke Mandarin, as his English was a little rough. When The Professor addressed him in excellent Shanghainese, everyone was speechless. Patty was determined to ask him about this later. No American spoke Mandarin, Cantonese, and Shanghainese!

They again agreed to stick to Mandarin. Patty headed out to the line, accompanied by a young Chinese engineer, Zhou Chang, who seemed to be taking more interest in her than expected. She tried to make her engagement ring visible, but she wasn’t sure he recognized its significance. When she got to the line that was experiencing yield problems, she was met by the engineering manager, Fei Ding. He showed her some of the fails and she quickly identified head-in-pillow defect as the likely culprit. After investigating more fails, looking at stencil printing, some of the BGA components, and component placement, she asked Zhou Chang what spec was used to thermal profile the line.

“I don’t understand what you mean,” Zhou said in Mandarin.

“How do you determine what the reflow profile should be?” Patty responded.

With more discussion, Patty determined that they had one profile for all products! Fortunately most of the products were of similar, small thermal mass.

“What solder paste do you use for this line?” Patty asked.

The embarrassed silence suggested that Zhou did not know. They grabbed a tube and Patty was relieved to see that it was one of her favor solder pastes. Since profiling was so rarely performed, Patty and Zhou had to go to another part of the complex almost a mile away to find a reflow profiling unit. After taking the profile, the likely solution appeared. The 11-zone oven was very long and the reflow profile had a long thermal “soak” before the temperature went above liquidus. This long soak probably exhausted the flux, so that when the PWB went above liquidus, there was little flux left, resulting in oxidation and poor reflow.

All during their time together, she had mentioned that her fiancée Rob was with her on the trip. This information seemed to do the trick.

“Zhou, why don’t you look up the solder paste spec on the web and then set up the right type reflow profile,” Patty suggested.

It was clear that Zhou was troubled. It became obvious Zhou did not know how to profile a reflow oven. Patty set about working with Zhou to accomplish this mission. Within an hour they had re-profiled the oven and, over the next two hours, 300 PCBs were manufactured with the yield improved to 95%.

Patty asked Fei if she could give a brief presentation on the head-in-pillow defect to his team and he cheerfully agreed.

Her major points were:

HIP is caused by the failure of the BGA sphere to reflow with the solder paste. There are three major reasons for HIP:

1. Supplier Issues
a. Solder BGA sphere oxidation
b. Silver segregation to the BGA sphere surface

2. Process Issues
a. Stencil
i. Registration accuracy
ii. Insufficient solder paste
b. Component Placement
i. Off pad
ii. Out of plane
iii. Non optimum pressure
c. Reflow
i. Inappropriate reflow profile
ii. Flux exhaustion
iii. PWB warpage

3. Material Issues
a. Poor solder paste transfer efficiency
b. Insufficient solder flux oxidation barrier
c. Solder paste slump
d. PWB or BGA warpage

Patty went on to say that she had investigated all these issues with Zhou, and that the reflow profile was not optimum as the very long soak time had exhausted the flux. The other possible issues in the list did not seem to be a concern.

At the end of the day Patty, Rob, and The Professor met at the GM’s office to leave together for dinner and the Expo. Patty had to ask, “Professor, how can you possible know Mandarin, Cantonese, and Shanghainese?”

“Actually, I speak Min reasonably well, too,” he replied.

“How can this be?” Rob inquired.

“Mother and father were missionaries with Wycliffe Bible Translators,” The Professor answered. “I grew about around many languages during my youth. Mother and father speak more than I do.”

Patty went on to tell about the interest that Zhou Chang seemed to have in her, and how she had to discourage him.

“The burdens of being a beautiful young woman,” Rob teased.

Patty elbowed him, but they all left the taxi laughing as they headed for a restaurant near the Expo.

Best Wishes,

Dr. Ron

Formula Accuracy

Ken writes:

Dr. Ron, Thanks for your helpful post. I get close (-1.1%) with your formulas for an alloy I am working with. I think the crystal lattice packing factor for some of the individual elements is throwing off the result since it is different than the alloy. I tried to take this into account, but I get an error on the opposite side (+1.6%) of the actual. Any thoughts on if your formula can be made more accurate by taking element and alloy crystal lattice packing factors into account?

The solder alloy calculation assumes that the metals mix with no interaction, much as miscible liquids, of different densities would. There are numerous phenomena that could cause errors. They include:

1. Metals can come from different crystal systems. Lead, silver and copper are face-centered-cubic, whereas tin, the base metal for most solders, is of the tetragonal system.

2. Some metals form intermetallics with tin, such as copper and silver. These intermetallics have different densities than the metals or the resulting alloy.

3. Grain boundaries can leave some (probably small) empty space.

So I think Ken’s 1% accuracy is very good. The biggest mistake one can make however, is the most common: assuming that the density is simply given as the sum of the metal mass fractions times the metal densities. To many, it seems logical, but it is wrong.

My original posting on how to derive the formula for solder alloy density is here.

Cheers,

Dr. Ron

Graping in Shenzhen

Folks,

After a bit of a break, the adventures of Patty, Rob and The Professor continue.

The plane droned on as it made its slow march from Detroit to Tokyo. Patty looked down at Rob as he slumbered peacefully. She caught a glimpse of The Professor, he looked at both of them from across the aisle with a satisfied smile. The proud mentor looking at his protégés.

This was the first time in a while when Patty didn’t feel totally stressed. She had resisted going to China only three weeks before her wedding, but senior management insisted. She would arrive home only 5 days before the big day. She and Rob had their first real fight, she got angry with him because he wasn’t appreciating the pressure that she felt. However, with one long weekend with their moms, she was able to get most of the tasks done and finally felt relaxed that the wedding plans were in good shape.

She had to chuckle at Rob. He was all nervous being with The Professor by himself. The plans that they had made had Rob and The Professor focusing on productivity improvements at ACME’s new acquired plants in China. While they were working on these tasks, Patty would handle some process materials related issues. The rest on the trip went smoothly and after a night’s rest they were off to the first of ACME’s new factories.

This one was located in Shenzen. Our trio was ushered in to see the site GM, Peng Zhou, a native of the area. He addressed them in good English. When Rob and Patty answered in better Mandarin, he seemed shocked. When The Professor answered him in flawless Cantonese he and Patty and Rob were stunned.

“Perhaps we should all speak in Mandarin, since we speak it well,” said Rob.

Rob and The Professor went off to audit a few assembly lines, while Peng accompanied Patty to visit an assembly line that was having a quality problem.

(Dialogue translated from Mandarin)

“I’m very impressed with how well you all speak Mandarin,” said Peng. “Where did you learn it?”

“Thank you,” replied Patty. “Rob and I studied Mandarin in college and we did an internship in China,” she went on.

“Very impressive,” Peng commented. “But I have to tell you, I’ve never heard any American speak Cantonese at all, let alone as well as The Professor does. It’s like he was born here,” he went on.

“He never ceases to amaze me,” Patty responded. Patty and Peng finally arrived at the assembly line. Patty was introduced to the line engineer, Elvis Chang. She chuckled inside, this was the third Asian person her age she had met that had chosen “Elvis” as an English nickname. Elvis was relieved that Patty spoke Mandarin. They went to a stereo microscope and looked at some of the assembled PCBs that had quality issues. Patty was quick to pick out the problem: graping. She looked at the stencil and the pad sizes on the PCB. She performed a few calculations and appeared satisfied that she had the answer.

Patty suggested that, if Elvis would like, she could give a brief presentation on what she thought the problem was.

“Patty, that’s a great idea, but it might be best to wait until after lunch,” Elvis suggested.

Elvis, Patty, and a few other young engineers went together for lunch. They seemed to be fascinated with Patty, especially her ability to speak Mandarin. They all spoke some English and were studying it as they recognized a promotion to a senior level required English fluency. One of them pointed out that she had read that about 250 million Chinese people are studying English, while only 20,000 Americans are studying Chinese. Patty enjoyed Chinese food and was happy to find Sea Cucumber on the menu. One of her friends said it was the only Chinese food he couldn’t eat. She tried it and liked it.

After lunch, Patty asked for a few hours to prepare her presentation. Her main points are summarized below:

1. The aperture size for the pads that experience graping is 8 mils in diameter for the 0.004″ thick stencil.

2. The resulting area ratio (D/4t, D= diameter, t = stencil thickness) for this aperture is 0.50, less than the recommended 0.66.

3. The very small solder paste deposit doesn’t not have enough flux to avoid oxidation of the solder particles in reflow. The resulting defect looks like a bunch of grapes so it is called graping.

4. Likely solutions:

a. Use a square aperture. An 0.008” square aperture provides 27.3% more volume, and it has better transfer efficiency. (Transfer efficiency is the volume of the solder paste deposit divided by the volume of the aperture times 100.) The result would be >30% more solder paste. The more solder paste, the less likely to experience graping

b. The solder paste they were using was not best of breed re: graping resistance. She recommended another one, which she knew performed well in all respects — and minimized graping. This solder paste’s flux was robust and designed to minimize defects like graping. Her presentation was received very well. Fortunately some of this excellent solder paste she recommended was being used for another job in the plant. So with approval from Peng, the team switched to this paste.

After the meeting, Patty thought about how much one of the technical engineers from one of her favorite solder paste suppliers had helped her to understand graping and how to minimize it. His name is Ed Briggs and she had just attended SMTA Toronto where he gave a paper on graping. Much of the information in her presentation came from the paper and his other writings on graping.

Epilogue: Three weeks later, the graping had disappeared from Elvis’s assembly line. They didn’t even need to adopt a stencil with square apertures; the solder paste change itself was enough.

Cheers,

Dr. Ron