This has been addressed in various ways with different degrees of
success. The goal is to have test specifications for the board fab that
actually indicate the fab will work properly and that are readily
verified by the shop. That goal seems to be slipping away. It makes for
competitive advantage to first tier players. It makes a burdensome
problem for the electronics industry as a whole.
Steve.
On 8/18/2015 2:15 PM, L R (Redacted sender digitalchkn@yahoo for DMARC)
wrote:
 Hi folks,
See lots of discussion this month on whether SI has become commoditized.
I'll hold off my thoughts on that... and instead ask my
"too-ashamed-to-ask-but-will-ask-anyways" question.
The question is this. Why do we still insist on board impedance data from our
PCB manufacturers?
Here's the rationale behind this question As we all know too well, PCB
trace impedance is not a nice constant real value (say 50 ohms) so we can't
really expect a single number(right?). And with that said, PCB
manufactures never provide us with the details of their measurement (edge
rates, launches, ref planes anyone?). They just give us a number saying
they are within 10%.Â
Great.  We know nothing from this other than I can sleep better at night
knowing my, say, single ended clocks will work fine. Well, how useful is
all that for links designed for my fancy 10Gbps or more? Wouldn't it be more
useful for them to give us S11/22? Â
- Lenny
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