[SI-LIST] Re: Need advice on basic 6-layer stackup

  • From: "Sol Tatlow" <Sol.Tatlow@xxxxxxxxxxxxxxx>
  • To: <GrahamDavies@xxxxxxxx>
  • Date: Tue, 3 May 2005 09:43:52 +0200

Hi, Graham.

In response to the question of

"... I don't understand where this 50 ohms came from ..."

here is a link to an article by Dr. Howard Johnson on the subject:


In practice, 50 Ohms seems to be a readily achievable impedance for
most layer stacks both today and in the recent past, avoiding both:

a) very wide tracks that restrict the layout unnecessarily,
   and present a low, heavy load, and
b) very narrow tracks that cause problems with manufacturing.

As manufacturing capabilities improve, allowing ever thinner tracks,
the same improvements lead to thinner cores and prepregs, meaning
that the 2 changes (impedance-wise) tend to balance each other out -
even with 30+ layers in less than 3mm thickness, and pin densities
close to 200/sq in (no HDI, just normal PCB technology) leading to
high density routing, 50 Ohms is a readily achievable goal.

What is interesting though (if I am right), is that the first
typical 4 and even 6 layer boards would have tended towards more
like 65-75 Ohms. But perhaps, back then, 50 Ohms was neither readily
achievable (for typical industrial applications) nor desirable?

Sol Tatlow, M.Eng. (Oxon)
ProDesign Electronic & CAD Layout GmbH
Product Developer
Albert-Mayer-Str. 16
D-83052 Bruckmuehl
Phone: +49 (0) 8062-808-302
Fax:   +49 (0) 8062-808-333

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