Decoupling caps and cold failures

High-K ceramics have horrible tempcos. It was probably changing the waveform of the ringing at the VDD pins, so that the marginal situation became a failure.

It's pretty poor engineering, letting something basic like that get through. Didn't anybody think to check the supply noise at the uC? I hope nobody's life depends on the units you already have in the field.

Cheers

Phil Hobbs

Understood It was a design I inherited. I do remember reviewing the original design and remarking that the decoupling on one channel was too far from the chip and was told as much, to shut up... Difficult to respond to that when the chief designer is the company president. I wanted to do a four layer board but got shot down so it was done in two layers. No- it's not life support related and low energy.

Life is a series of compromises, it's true. Sounds like that old line "I won't have yes-men in this company! I want people to tell me the truth even if it costs them their jobs!"

Cheers

Phil Hobbs

Did the board have power and ground planes, or was that stuff routed as traces? Boards with solid ground and power planes seldom have bypassing problems, and the distance between parts and caps doesn't usually matter.

Blame is apparently in the eye of the beholder. (You might have a look in your makeup mirror.)

The point I was making was not that Oppie is a bad dog, or a bad engineer. I was just pointing out that that sort of problem is a screwup, not a mystery. I screw up often, but I'm paranoid enough about it that it nearly always gets caught before going out the door. Fortunately, I don't have anybody who can tell me to shut up about it. :-(

Cheers

Phil Hobbs

the

tion

C? =A0I

ld.

I am very sensitive to making claims about life/death designs. Nobody is good enough to field a life/death design based upon their experience alone. The poor engineering is the testing, not the design, when it comes to letting bad designs out into the field.

Based on your comments it seems that you let your experience be your guide to not screwing up. If a design is a life or death design, the proper way to not let it get out is not based on how experienced the designer is but on how good the testing profile is before it goes out the door. My designs are crap until I can show my peers the test procedures and test data for the design. And even after comprehensive testing, most products still have some problem that will need to be fixed after they are out in the field.

You need to read more carefully. I didn't say anything about my designs being life-or-death. In fact, all my consulting contracts have language explicitly forbidding such use. Joerg and Co. do medical stuff, but I don't. Ever. For any reason. Capiche?

You seem to have had a bad week. Have a beer and chill out.

Cheers

Phil Hobbs

ssage

ing the

ituation

get

he uC? =A0I

field.

our

look

You stated that you hoped that no ones life was dependent upon the design being discussed. So you did bring up life/death design. And yes, I do believe that you have not done life/death designs because of your assessment as to why the design got fielded.

I have not done contracting work , but if I did for a life dependent design, I would only sign up to pass a set of tests and let the party who bought the design be responsible for whether that testing profile was adequate.

about

You know, Brent, I don't know what's got you so worked up, but I'm pretty sure it isn't anything I've actually written. You're kibitzing on an exchange between Oppie and me--SED being a discussion group, after all--but you're getting all bent out of shape about nothing.

If you rag on people like that at work, for a throwaway comment, I sure wouldn't want to be your colleague.

Cheers

Phil Hobbs

Are you a test engineer?

You may perceive that I am getting bent out of shape, and you have taken several posts to point out my bad behavior, and yet the point that I have challenged you on you have not commented on. Not directly addressing the point or challenge that is being made , but focusing on the behavior of the person bringing up the point,.. well.. that is not such good behavior.

In other words, if you were my colleague and I brought up a concern that I had about your design and your response would be to criticize my tone of voice and not address the point being made, then that would be bad behavior in my book.

he

ion

? =A0I

d.

It does not matter if I am a test engineer. What is your opinion about the best way to ensure that a critical design is fielded with the least risk?

hnology.com=A0 jlarkin at highlandtechnology dot com

Besides the cap tempco, the chip itself is probably noisier at low temp due to the mobility tempco. Basically faster rise and fall times. Five deg C isn't all that cold though.

If you make a chip that touchy, you get in a special category known as the jerk whose chip requires bypass caps on the contactor rather than DUT board. [If you need bypass on the contactor, then you have a dedicated contactor per chip.] It happens, but you really try not to need exceptional bypass since invariably some less than optimal boards are going to be produced.

You did well to find the problem yourself.

BIST perhaps?

Brent: Let me make it simple for you.

I. Was. Not. Talking. About. What. You're. Talking. About. You. Put. That. In. Yourself.

Give it a rest, OK?

Cheers

Phil Hobbs

ing

ter

Then. don't. reply. to. my. comments. with. smug. insults. and. no. content.

OK.

Are you?

What is your opinion

Really good design, reviewed and checked by really good engineers. It's hard to test quality into a product, especially firmware.

Temperature testing is a good thing to do.

age

he

uation

et

? I

ield.

I am a design engineer. As I get older I do more testing a procedure development. When I was younger all the older guys were the test engineers. I now understand the wisdom of this. The design is done by the younger guys because in many ways it is the easy part. Making sure that crap does not get fielded is far more important and requires better judgment.

Really good design is the objective.

Yes, the review process is very important.

It is hard to test quality into a project because a good testing program is hard. But I think it is even harder to get good quality without testing.

A good test also makes the designer write down in certain terms what is acceptable performance.

Testing firmware is becoming a huge field unto itself, and it is a pain in the can too. But all aviation equipment now requires rigorous firmware testing .

Temperature testing is the best and relatively easiest thing to do . it will uncover marginal designs better than anything else.

hnology.com=A0 jlarkin at highlandtechnology dot com

it's probably not the change of the capacitor characteristics with temp, but probably a change in the chip characteristics with temp.

logic chips can go faster etc when cold and so may need better bypassing...

Mark