[PCB_FORUM] Re: Impedance controlled line widths

Hi Jean,

My guess is the stackup isn't specified correctly or completely in Allegro.
Do you know what Dk was used for each layer and do you have all the correct
thicknesses/materials etc. specified in the stackup?

Regards,

Austin
  -----Original Message-----
  From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx]On Behalf Of Jean Bratton
  Sent: Wednesday, August 29, 2007 10:33 AM
  To: icu-pcb-forum@xxxxxxxxxxxxx
  Subject: [PCB_FORUM] Re: Impedance controlled line widths


  So, short of deleting the impedance property the customer added to his
schematic, is there a way to get it to just use the rules and not what it
calculates the traces should be? (By the way, the Allegro numbers are not
even CLOSE to what the board shop has given us. That's why we were looking
for an independent impedance calculator, to see which one was out of whack.)



  From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of David Kelly
(davidkel)
  Sent: Wednesday, August 29, 2007 10:26 AM
  To: icu-pcb-forum@xxxxxxxxxxxxx
  Subject: [PCB_FORUM] Re: Impedance controlled line widths



  Jean



  We usually target an impedance, line width and stackup. Then we go to our
fab house and

  have them tell us what they can build. Usually we then tweak these
parameters in the board

  to match what the fabricator can build.



  I have found that the Allegro impedance calculator is not a reliable gauge
of the actual impedances.

  It help when you make changes to see their impact but I wouldn't trust the
actual numbers.



  Dave



  David Kelly

  davidkel@xxxxxxxxx

  720-562-6316






----------------------------------------------------------------------------
--

  From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Jean Bratton
  Sent: Wednesday, August 29, 2007 8:10 AM
  To: icu-pcb-forum@xxxxxxxxxxxxx
  Subject: [PCB_FORUM] Impedance controlled line widths

  Hi folks,

  We have a design with impedance rules built in at the schematic level. The
stackup appears to be correctly defined as provided to us, but Allegro is
not calculating the trace widths at the same width as the stackup indicates
they should be. The impedance value appears to be the prevailing rule, not
the trace width we have set up in the constraints. If we delete the
impedance property, we can get the board to route with the trace widths we
specify, but we're wondering if there's a way to change the priority.to have
it look at what the design rules specify for trace width rather than what
the impedance rule would automatically define.



  Also, does anyone know of a good impedance calculator online? The one we
used to use isn't set up for a board as thick as this one is, so we need
another. We need to know which is accurate, the stackup provided to us, or
the calculations Allegro is making.

  Thanks for any insight,

  Jean



  Jean Bratton

  Sr. Printed Circuit Designer

  Freedom CAD Services, Inc.

  603-864-1349

  jean.bratton@xxxxxxxxxxxxxx

  Visit us at http://www.freedomcad.com




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