> AND:  Should the specification for the Impedance to the Board house
> be in stack data, or Impedance value?
>

        The answer, like any PCB question, is "it depends".

        The short answer is to use one or the other but not both,
        as this ties the fabricator's hands.

        More esoteric applications may have concerns beyond simple
        impedance and require a "controlled stackup" in order to
        achieve modeled results.  Also, when using a fabricator
        who is not impedance facile, or if you need to get iden-
        tical product from multiple sources, this approach may be
        for you.  Of course you'll need to have specific details
        on materials and weaves to achieve uniform results.

        Generally it is simpler and more cost effective to merely
        seek an impedance, assuming this is the root need.  Then
        the next question is whether you are comfortable with the
        vendor's approach to that or do you need proof, as in TDR
        testing.  One facet of that question is whether you've
        put in a specification framework that covers impedance lot
        testing or relying on simple "good commercial practice".

        If you choose to control the impedance it makes sense to
        at least include a reference view of the stackup, so the
        fabricator can at least see what you were trying to do.

        HTH,
--

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

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