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