----------
From:  D'Auteuil, Gerry
Sent:  Wednesday, April 23, 1997 9:20 AM
To:  TechNet-request
Subject:  RE: Controlled Impedance Backplane

To the person at Lockheed Martin in Canada if you contact Robert Beland   
at 1-508-933-1924 he has an answer on manufacturing this board to the   
required impedance.

 ----------
From:  TechNet-request[SMTP:[log in to unmask]]
Sent:  Friday, April 18, 1997 3:26 PM
To:  Technet
Subject:  DES: Controlled Impedance Backplane

     I am trying to specify the stackup for a controlled impedance
     psudo-VME backplane. This backplane has controlled impedance traces   
of
     100 Ohms and 50 Ohms. The current finished board thickness is 0.093"   

     and consists of ten layers, 4 of which are controlled impedance
     layers.
       

     In discussion with a local fab shop I am finding that in order to
     create 100 Ohm traces, the width of the dielectric (for FR-4) is
     prohibitive.
       

     The only alternatives are to cut back on the number of layers or
     substantially increase the PWB thickness (which causes a large
     headache in finding components with sufficient lead length).
       

     I have two questions concerning this and I hope someone out there   
can
     help:
       

     a.) Are there any major drawbacks to producing a backplane with,   
say,
     4 layers only? i.e. Is there a limitation on the minimum number of
     layers for a backplane PWB that I don't know about?
       

     b.) How are standard 0.093" OTS VME backplanes produced? They seem   
to
     me to have a fair number of layers without resorting to thick PWBs   
or
     superfine traces.
       

     I hope that someone out there has had experience with this kind of   
PWB
     and can help me.
       

     Thanks in advance,
       

     --
     Max.P.Harris
     Hardware Design Engineer
     Lockheed Martin Canada, Inc.
       

     'Nil illegitime carborundum'

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