[PCB_FORUM] Re: Impedance controlled line widths

Pick one below, compare them all.

http://www.emclab.umr.edu/pcbtlc/

http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_pcb_imp_calculator

http://www.csgnetwork.com/boardrunimpcalc.html

Jean Bratton wrote:
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>

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