Re: [SI-LIST] : Different board vendors different impedances

Jeff Seeger (
Fri, 26 Mar 1999 01:39:34 -0500


All the prior posts are true. Some perspective sounds in
order. Perhaps I can help.

FR4 is a compromise material, and frankly the resin con-
tent varies within a single sheet, never mind across an
entire lot.

There is also variation in process methods; different
etching technologies produce different cross-sectional
shapes - from trapeziod to (yes) rectangular.

I've lost the beginnings of this thread but it sounded
like the problem was one that I call "controlled stackup"
used primarily to be certain the impedance doesn't vary
within a board. Moving this level of specification from
vendor to vendor will produce surprisingly different im-
pedance results. Some design folks will try to use this
technique to buy a "cheaper" impedance controlled board.
More correct to call it "impedance contained".

Next striation I make is between controlled impedance and
etched components. For digital domain (up to a point)
simply targeting the impedance and specifying the test
method is adequate. In this realm it is only appropriate
to verify that your intended stackup is robust and provides
a reasonable window for any fabricator to reach intended
performance. Up to and including differential impedance.

When reaching the limits of that technique, or when you
seek explicit sub-parameters, you must specify those
parameters - impedance alone is not enough. The fabrica-
tion community will reveal its capable vendors when you
reach into the specifics. I'd rather guess that we're
not hearing from the SI engineers on-staff at those ven-
dors in this thread because of current trade shows.

Correction of pad geometries for etch factors versus art-
work usually falls in the manufacturing domain. Done all
the time, at least at a certain tier of supplier.

Obviously, your mileage has varied and will undoubtedly
continue to do so.

Hoping the above improves it an MPG or so,

      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

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