Category Archives: 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.

Shear Thinning

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Solder pastes are a very complex “fluid” of high viscosity. Their behavior, when experiencing shear stresses, is “non-Newtonian,” meaning that their viscosity is not constant as the shear stress varies. The viscosity of solder pastes is high when there is little or no shear stress and low when shear stresses are high such as when the paste is forced through a stencil aperture. This property is called thixotropy. The solder paste being thixotropic is ideal, as it enables the stencil printed “brick” of solder paste to retain its shape after it is printed, yet the low viscosity, when stressed, allows good filling of the stencil aperture.

Many might assume that this relatively complex phenomenon is the end of the story. However, there is at least one other well-known property of solder pastes during printing that is important: response to pause. A solder paste with a poor response to pause will stiffen when permitted to idle for as few as 15 minutes. When this occurs, the first print likely will have insufficient solder paste for effective assembly. Hence, response to pause is a critical variable to measure when evaluating a potential solder paste.

Another important solder paste property has only recently become well known: shear thinning. My Indium Corp. colleague Tim Jensen was one of the first to point this out. Shear thinning is a property of some solder pastes in which the viscosity becomes lower and lower as the paste is repeatedly printed (see figure below). The x axis is number of prints, the y axis is the viscosity. It is normal for the viscosity to go down during the print, but the viscosity should recover as the “good paste” does, not have a downward trend as the “bad paste.” The resulting drop in viscosity that the bad paste exhibits will often result in too much paste being printed and potentially lead to defects such as shorts or solder balls. Unfortunately, if not tested for, shear thinning might first be observed after a paste has been implemented on the line.

If you are interested in a method to test a paste for its resistance to shear thinning, send me a note and I will send a test protocol.

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

Just 3 Minutes

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Three minutes is not much time. It’s about the amount of time to get a coffee from the vending machine or maybe not quite enough time to visit the restroom. Three minutes doesn’t seem important.

You have probably spent multiples of three minutes looking for a stencil that was misplaced. The job couldn’t get started until that stencil was found. Three minutes, 10 minutes; what’s the big deal?
Let’s say your company has a two-shift, five-days per week operation, and on each shift three minutes is lost each day. Assuming 250 days per year, this is 1500 minutes or 25 hours of lost production time in a year.

How much is this costing your company? Using ProfitPro cost-estimating software that I developed and information in annual reports, I analyzed the typical subcontract assembler’s profitability. It nets out that each hour lost on a typical assembly line is worth about $3500 of production. So 25 hours per year is almost $90,000 of lost production – all because of just three minutes.

And I’ll bet some of us are losing 30 minutes a day.

The Age of Spiritual Machines?

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That Ray Kurzweil is a smart, creative, inventive and prolific person is beyond dispute. In addition, it cannot be said that he hasn’t made numerous accurate predictions, most with 10-year lead times, such as IBM’s Deep Blue computer beating world champion Gary Kasparov in chess, the growth of the Internet and emergence of flash memory drives. However, he goes way too far in his belief in “spiritual machines” and the advent of a technical singularity.

The spiritual machines argument is quite old (1999) and with little progress in that direction, it surprises me that his 2005 book The Singularity is Near still suggested that, as Wikipedia puts it, “the functionality of the human brain is quantifiable in terms of technology that we can build in the near future.” Why do I say there is little progress? Computers are faster and can perform more tasks than ever, but are still limited to what human programmers tell them to do. In addition, human consciousness is far from being understood today. Our brains are not machines that simply perform clever mathematical operations, developed by computer programmers.

All this somewhat dated information was brought back to me in an interview on Fox News recently. Kurzweil predicts that a computer will passing the Turing Test by 2019, and that by 2029 a computer-based machine would be recognized as having consciousness.

Why am I not a believer? Computers are terrific at mathematical operations. Many felt that computers would never beat a world champion at chess. But consider: chess can be described in strictly mathematical terms. Deep Blue did what computers do well, math, not human thought. As one person said upon hearing of the news of Deep Blue’s victory, “I’ll be impressed when a computer can write and understand poetry.” The best computers today, connected to a vision system cannot do what a six-month-old child can, recognize and follow its mother’s face in a crowd.

All this reminds me of a book I read a few years ago by Robin Cook called Abduction. In this novel, the main characters, while exploring the sea, happen upon an opening to Interterra, a society of humans living under the earth’s crust. Their society is so advanced that when their bodies wear out, a new test tube baby is selected as a replacement. Their memories are downloaded to the babies and hence the babies become them. Their old body is destroyed and they live another life through the baby. Many of Kurzweil’s farout ideas are similar to this type of human/technological immortality.

I can’t be the only person who believes there is something uniquely me and uniquely you, uniquely human, that goes beyond our memories and can’t be downloaded from our bodies.

Interesting stuff. Why am I discussing this? We will be the ones assembling the electronics for Kurzweil’s machines. It will be interesting to watch.

Batteries: Unsung Hero in the Electronics Revolution?

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Folks,

The miniaturization of electronics has had a tremendous impact on our lives. Beginning well before the advent of the Sony Walkman and the 20-lb. “portable” computers of the early 1980s, Moore’s Law has delivered increasingly more powerful and less expensive electronics. However, some of us may have missed a parallel miniaturization and performance improvement that have been occurring for over 10 years: Batteries.

Most of us remember the short battery life of a 1995 mobile phone. If asked we would probably agree that the rechargeable batteries today are significantly better. It is less obvious for our laptop computers, as a battery charge has lasted about 2 hours or so for the past 10 years. The laptop situation masks the dramatic improvement in batteries because today’s laptops use many times the power of one from a decade ago.

The battery bottom line: The miniaturization and energy density improvements in portable electronics rechargeable batteries has been breathtaking in the last decade.

Are there any additional benefits from this battery technology? For the first time in the history of transportation, an electric car couples a practical cruising range (more than 200 miles between charges) with the performance of a sports car. At 135 mpg equivalent, 0 to 60 mph in 3.9 sec., 220 miles per charge, and energy costs of about $0.02/mile, the Tesla Roadster portends the development of a practical, electric powered automobile within five years. It’s pricey — about $100,000 — and, as a sports car, is likely impractical as a family vehicle, but Tesla is showing the way. And it is powered with hundreds of laptop batteries, which will only continue to get better. In a day of $130 per barrel of oil, we should all find this comforting.

It is surprising to me that this breakthrough has not received more publicity.