Do the math to verify your concept, and nominal conductor widths, but do not
specify laminate thicknesses. Specify the overall board thickness only. Try
using a note similar to this
example:

X. CONTROLLED IMPEDANCE CONDUCTORS: SUPPLIER SHALL
    PLACE A REPRESENTATIVE TEST COUPON ON EACH PANEL
    AND INSURE COMPLIANCE. SELECTIVE ADJUSTMENT OF
    INDICATED CONDUCTORS IS PERMITTED, AND ARE EXEMPT
    FROM MINIMUM CONDUCTOR WIDTH REQT (SEE NOTE X).
            LAYER             WIDTH              OHMS
                 1                    .019                 45-55
                 2                    .012                 45-55

Cliff Lamson
PCB Design Manager
Plexus Technology Group
Raleigh Design Center
919-807-8047


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of Luther,
Brandon
Sent: Monday, April 16, 2001 4:47 PM
To: [log in to unmask]
Subject: [DC] impedance control specs

Good Day!
  We are having a raging debate about specifying controlled impedance on our
fabrication drawings.
  The main part of the debate is over laminate thickness specifications.  I
would like to know how other companies are handling these issues.
        Are you specifying the individual laminate thickness?
        Are you specifying the individual laminate thickness tolerance?   If
so, how do you handle deviations between different fab houses beyond the
tolerance?
It seems that every time we use a different fab house, they like to use
their standard laminates.  While this is also a benefit to us, it causes
problems when our boards are microsectioned and the board doesn't agree with
the print.
We will continue to specify trace width, copper thickness and impedance, all
with tolerances.
Any input will be appreciated!
        Brandon Luther
        Dataram Corp.
(609) 799-007  x2310