I agree with the others and add that the most critical parameter is the
dielectric constant of the substrate and any solder mask, conformal
coating etc. The DC can vary from one batch of laminate to the next,
even from the same manufacturer, and even from day to day, depending on
the hygrometry of the air. Well over a generation ago (c. 1970), I made
some 4-layer experimental boards for Motorola to test their MECL2
technology, working at 50 ohms characteristic impedance and we made
adjustments, using TDR to ensure optimum matching. In the end, we
selected terminating resistors to obtain minimal reflections and, a few
days later, found they had to be replaced with another value. Of course,
this was not for a practical application, but for purely lab work. The
difference between 49 and 51 ohms, for example, would have little
practical effect in a well-designed circuit.
Brian
Avigdor JAkubinsky wrote:
> Hello friends,
>
> It is always the question of controlling (CI) with Coupon measurement on the
> stack.
>
> Would any of you The Experts trust a meticulous Calculated approach without
> actually measuring the outcome impedance Out of the coupon?
>
> I would appreciate your pointer/Link to any written facts publishes at any
> given year along with your personal opinion.
>
> Best Regards
>
> Avigdor Jakubisnky
> Engineering Mgr.
> www.fibrolan.com
>
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