Electronic Design
What’s All This Perfection Stuff, Anyhow?

What’s All This Perfection Stuff, Anyhow?

(first published in the April 11, 1991 issue)

Once upon a time, a lady went shopping for shoes. She went into a shoe store and said to the salesman, “My good man, I would like to buy some alligator shoes.” The salesman showed her some alligator shoes. But this one had a flaw, and that one had a blemish, and that one had a scratch. He went back and got more shoes. But when the lady inspected them, she found an imperfection that was unacceptable to her every time. Finally, he brought out every pair of shoes he had, and she was not prepared to buy shoes with such defects. In great exasperation, the salesman told her, “Look, lady, you’re not perfect. I’m not perfect. How do you expect an alligator to be perfect?”

One of my latest ventures involves the writing of a book. On October 30, 1990, I handed in a complete, polished manuscript. I don’t know how many words, or how many pages, but it was about 700,000 bytes and the Fed Express guy had to struggle to get it on its way. This was a refinement of some stories on the troubleshooting of analog circuits I wrote a couple years ago. I took the old stories and added a lot of new info, tweaked and refined a lot of the text, and corrected some typo errors, finally rounding it into pretty good shape. With a little luck, the book will come out on schedule this month.

On November 14, I sent to the publisher a package of new corrections and changes. Only one envelope, but there were 28 items there. When I talked with the publisher, I could tell she was smiling, seeing that I was so willing to put so much effort into getting things right. On November 30, I assembled an envelope with more corrections, changes, refinements, and upgrades. I mean, some of the people had moved in the last month, so I certainly had to put in an address correction. In several cases, the distributors who sold things didn’t sell them anymore. In other instances, I remembered a new technique that I had forgotten previously. So, when I send in this package, I’m really making progress—because there are only 27 items.

Next week I expect to remember a couple more items. I’m not sure exactly when the smile will completely disappear from my publisher’s face, but it’s only a matter of time. She will say, “It’s time to shoot the engineer, and put this to bed and print what we’ve got.” And I’m sure I will have to agree with her pretty quickly. But meanwhile, I know I have to correct the spelling of one guy’s name. We can’t leave that wrong. And an old friend is mailing me some more info on some diodes. And there are curves that don’t look quite right… and there are some photographs that still need to be developed, not to mention the ones that haven’t yet been taken….

“Time to shoot the engineer.” That’s a phrase that has been around for a long time. Almost as soon as I got out of school and into industry, I began to hear people explaining that the need for perfection was all very fine, but it must not go on much further. When is the circuit going to be good enough? Perfection isn’t necessarily justifiable. What is good enough? And whose opinion is to be relied on? Sometimes the engineer is correct that there are some improvements that have to be made. Other times, it’s not so clear.

For example, we were recently trying to release a new product, but the distribution of one parameter was not quite centered. The yields might occasionally fall off more than we liked. So we proposed an optional metal mask that would bring the distribution back close to center. But this might cause some dynamic problems. And even if it didn’t cause serious problems, it might get some of our customers cross if they had to re-qualify our product, because we had made a change in a mask, even though it would be a very tiny change.

What’s the right thing to do? Accept the yield loss? Change the data sheet? Delay the release of the product and risk the loss of market share? If we wanted to compromise, where would we do it? Hey, I don’t know how you run your business, but in our business, there aren’t any easy questions. If you’re making alligator shoes, and your QC department says you can ship only shoes that have no visible flaws under a magnification of 5X, you’re not going to be selling a lot of shoes. But, most of the time, our shoes are on our feet, at least 3 or 4 feet away from our eyes or anybody else’s eyes. From that vantage point, “imperfection” is quite different.

Now, I’m not proposing that we refuse to ship amplifiers because they’re not “perfect.” I mean, if a perfect amplifier is one that has less than 0.5 mV of offset voltage, we have a lot of perfect amplifiers. Just 10 years ago, customers were pretty happy to buy that kind of “perfect amplifier.” But these days, even 15 µV isn’t “perfect.” And if you build lots of amplifiers with less than 10 µV, and they all test out good, and then you allow them to warm up, and you temperature cycle them through an oven, are they still “perfect”—better than 15 µV? If you want to buy a “perfect” amplifier, do you require big safety factors against every possible condition? You may wind up going barefoot: The price for a “perfect” pair of shoes might be more than you would be willing to pay. Sometimes we have considered that amplifiers at this level of precision might have a looser AQL than normal—perhaps 0.5% instead of the typical 0.01%. But your QC people don’t want to concede to that.

If we test an amplifier for noise, and we do it 30 times, the data might show low noise on 27 of the passes. But on 2 or 3, there might be a small deviation—an increase (or a decrease) in the test result—whether or not the amplifier actually made more noise. Heck, you can’t design a noise test to be perfectly repeatable—that’s inherent in the nature of noise. Now, if a unit passes a test at its “class” or final test, it must never be allowed to fail its guaranteed specs if you re-test it. That means you must have wide guardbands, as wide as the deviations of the system’s noise. If an amplifier that reads 1 µV pk-pk most of the time can read 0.8 µV other times, that would tell you that you must have a guardboard of at least 0.4 µV, and maybe 2 or 3 times that. So you could sell to a guaranteed spec of 2.2 µV—even though most of the parts are 1 µV pk-pk. Is that the spec you want to buy?

Once, a long time ago, a bright young engineer was working on semiconductors in England. The transistors were passing a 27-V breakdown test very reliably. But this semiconductor company was a subsidiary of a large plumbing-supplies company. And if you’re making boilers or gauges or pipes, you’ve got to have a 3:1 ratio between the working pressure and the bursting pressure. So, this semiconductor company wrote their data sheets for a 9-V transistor, while their competitors were selling to a 24-V spec. Needless to say, the young engineer knew there was no future in a business where perfection and safety factors make the playing-field so badly tilted. And that was why Bill Frusztajer left England and came to the United States, where he became the head of Teledyne Crystalonics.

All for now. / Comments invited! / RAP / Robert A. Pease / Engineer

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