[PCB_FORUM] Re: Impedance controlled line widths

Hi Jean,

I wouldn't use any impedance calculator.  I'd get the stack-up from the
fabricator (which I would confirm that the trace width/gap you appear to
already have is in fact from the fabricator's calculations), and use what
the fabricator specifies for the trace width and gap.  You'll probably need
to setup rules for outer layers, and inner layers, since they typically are
different for the same impedance.

To get it to work from just trace width/gap, and not an impedance rule you
should probably remove the impedance rule.  I've never used the impedance
rule, so that is just a guess...but if it only has one rule to work from,
trace width/gap, that should work.

When using impedance only to get trace/gap in any particular tool, is they
all give different answers...and an answer may not be what the fabricator
can use to achieve the desired impedance.  Most fabricators use Polar, which
is quite expensive.  The fabricator will be responsible for maintaining
impedance, and they know their process better than anyone else, and what
they need to be able to maintain that impedance.  They will add coupons to
your board so they can test the impedance with a TDR after fabrication, and
you will only get boards with correct impedance, typically +/- %10, and that
should be specified on your fabrication drawing.

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 10:10 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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