[SI-LIST] Re: PCB Corners - Need help finding research paper
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "Howard Johnson" <howie03@xxxxxxxxxx>
- Date: Fri, 8 Aug 2008 15:24:53 -0700
If you look at the current distribution at the right angle bend (e.g. set
up a simple simulation in Sonnet Lite for that), you'll see that most
current is crowding at the inner corner of the bend, and very little
current flows along the outside. Which explains why removing just a little
bit at the outside won't make much difference, you really have to taper it
down all the way into the high current region. Again Sonnet or a similar
field solver can be used to find out how much.
On the other hand, for typical trace dimensions the parasitic capacitance
of the bend is negligibly small up to many GHz. E.g. for a 10mil (0.25mm)
wide trace in FR-4 it is of the order of 0.02pF, or a time constant of
0.5ps.
I believe one place this myth may have originated from is from
RF/Microwave, where the traces tend to be much wider in order to be able
to carry a lot of power (the power of a digital signal is typically a few
mW). E.g. on a trace that is 10 times as wide (2.5mm), the parasitics are
10 times as bad (area increases with square of scaling, while distance =
thickness of the substrate increases only linearly), for a parasitic C of
0.2pF, time constant 5ps. Combine this with the fact that RF people have
been dealing with frequencies up to tens of GHz for a long time, and you
see why they are afraid of such right angle bends (and in their case, it's
not a myth but reality).
Wolfgang
"Howard Johnson" <howie03@xxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
08/08/2008 12:45 PM
To
<si-list@xxxxxxxxxxxxx>
cc
Subject
[SI-LIST] Re: PCB Corners - Need help finding research paper
Dear Chris,
Regarding right-angle bends in traces, this paper might help:
"Who's Afraid of the Big, Bad Bend"
www.sigcon.com/Pubs/edn/bigbadbend.htm
From the article: "Layout professionals often point out that modern
layout
systems already round off all the outside corners, assuming that this
rounding eliminates the square-corner effect. It doesn't. Rounding the
corners removes 21% of the copper in the corner. Edwards shows that you
must
remove 70 to 90% of the copper from a right-angle bend to neutralize (to
first order) the excess capacitance. Rounding removes only a small
fraction
of the required amount of copper. Rounded-corner right-angle bends work
well
in digital designs not because they are rounded, but because the corners
are
too tiny to cause significant problems in the first place."
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505, howie03@xxxxxxxxxx
www.sigcon.com -- High-Speed Digital Design seminars, publications and
films
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of DAVID CUTHBERT
Sent: Friday, August 08, 2008 11:49 AM
To: Chris.McGrath@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCB Corners - Need help finding research paper
Chris,
I can run a quick SONNET sim for you to show what happens with sharp 90
degree corners and with chamfered corners. The sharp corners add some
capacitance and lower the Z0. For anything that I have done this effect
was
almost negligible.
Howard Johnson's book has a section on this, I think.
Dave Cuthbert
On Fri, Aug 8, 2008 at 12:11 PM, <Chris.McGrath@xxxxxxxx> wrote:
> I used to have a copy of a public paper that debunked the myth that
> stated the evils of using 90 degree corners when routing PCB traces.
> I can't seem to find it on the web, so I'm hoping that one of you may
> either have a copy of it or can point me to the link. Feel free to
> email me directly with the attachment since the SI List server will
block
it.
> Thanks!
> Chris
>
>
> -------------------------------------------------------
> Chris McGrath
> Applications Engineer
> chris.mcgrath@xxxxxxxx
> Office: 480.704.7061
> Blackberry: 480.747.3848
> www.smsc.com
> -------------------------------------------------------
>
>
>
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