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 <mailto:jean.bratton@xxxxxxxxxxxxxx>
Visit us at http://www.freedomcad.com
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