[SI-LIST] Re: PCB tracks

  • From: "john lipsius" <johnlipsius@xxxxxxxxx>
  • To: "SI-LIST" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 7 Aug 2002 04:47:33 -0700

Philippe,

For a detailed answer, you should find it in a textbook.  
There's been much prior discussion on this; see the si-list archives 
//www.freelists.org/archives/si-list/ . 
Or see perhaps  www.signalintegrity.com or www.gigatest.com .

I assume your problem area is current digital technology.

1. For digital boards, even dealing with 10Gbps, 90 deg. is fine.  
There's just not a significant effect requiring even 45 deg., much less 
curved traces.  

2. That said, why is 45 is "better" than 90 deg., and curvilinear is best?
All for the same reason:  with sudden geometry changes, the TEM 
(transverse electromagnetic) wave sets up "evanescent modes" other 
than the main one to compensate for the change.  Why?  Remember, 
the EM field is a solution to partial differential equations with both 
space and time as variables.  Therefore, sudden spatial boundary changes 
force different boundary conditions, forcing different local micro field 
changes, thus local impedance changes.  The locus of these changes is 
so much less than the wavelengths of importance that the signal 
integrity effect is unimportant.  The energy loss in these evanescent modes 
is, likewise, minimal in the overall noise budget.  This assumes you have a
limited number of trace bends. 

That's my $.02.  Due to inflation and the passing of time, it may now be 
$.015 or less. 

---


----- Original Message ----- 
From: "Philippe Robert" <PhilippeR@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, August 07, 2002 2:22 AM
Subject: [SI-LIST] PCB tracks


> Hi there,
> 
> I am currently in the process of doing a new PCB, at the routing stage. In
> the router that I am using, there is an option to have 90-degree or
> 45-degree angles on the tracks. I know that it is better to have the option
> set to 45 to avoid changing too much the impedance of the tracks. I designed
> few board using that option.
> 
> In the router, there is an other option to have curves instead of angles on
> the tracks. Theoretically, it should be better in terms of impedance as the
> width of the track vary even less.
> 
> Could someone tell me if curves are better than 45-degree angles and why ?
> 
> Best regards.
> Philippe.
> 
> 
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