[PCB_FORUM] Re: Impedance controlled line widths

Jean,
 
The numbers SHOULD be closed. It is a math formula after all.
Check to see if ALL the entries in your stack up in Allegro are correct
(Dk, dielectric thickness, etc).
 
Julian 

________________________________

From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Jean Bratton
Sent: Wednesday, August 29, 2007 7:33 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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