[SI-LIST] Re: Right Angle Bends
- From: Jack Olson <pcbjack@xxxxxxxxx>
- To: SI-LIST <si-list@xxxxxxxxxxxxx>
- Date: Mon, 18 Jul 2011 09:14:27 -0500
Speaking from the board designers point of view:
I may never understand why people use square corners in their models when
they want to experiment with this subject. Are we talking about 90 degree
bends? or drawing traces with square corners? Maybe it is interesting from a
mathematical point of view, but I've never met a board designer who used
square apertures to draw traces. Every modern board design I've ever seen
(by modern I mean designed in a CAD system , not the old manual tape
layouts) uses round apertures, and when using a round aperture the width is
always constant, no matter how you "bend" the trace.
Furthermore, I haven't seen too many designs where the bends weren't 45
degrees anyway, why would anyone route 90 degree bends? longer runs, in most
cases.... but theoretically I suppose its interesting to discuss.
Real world? not so much.
onward thru the fog,
Jack
.
Date: Fri, 15 Jul 2011 16:56:38 -0400
From: Scott McMorrow <scott@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Right Angle Bends
Lee
It's not so much "misinformation" but "misapplication" of information.
The "no right angle bend rule" makes perfect sense on RF microstrip
boards where the traces are about 1 or 2 mm from the plane, and are
extremely wide. When you're dealing with 50 ohm traces that are 100
mils wide, excess capacitance at the corner is a big deal, since the
discontinuity acts for about 140 mils in distance (the diagonal across
the corner). If built on an FR4-type material with a Dk = 4, this
corner discontinuity has about a 25 ps time duration, which can be
significant. Additionally, RF engineers are always trying to reduce
narrow band return loss to extremely low levels, far beyond that which
is necessary for broadband digital.
However, when we scale the traces down to 5 mil width, the discontinuity
is same relative size, but 1/20th the physical size, and acts for only
about 7 mils, or about 1.25 ps. IF a 1.25 ps discontinuity is a big
deal to an engineer then this might matter, or if you have 20 corners,
the cumulative effect of this discontinuity will span a duration of 25
ps, or 1/4 of a 10 Gbps bit time. But 1.25 ps of excess capacitance
does not matter until we designing for 50 Gbps, and only a fool would
route a trace with 20 corners in it, so effectively corners are not an
issue.
Lee, you are correct.
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On 7/15/2011 3:15 PM, Lee Ritchey wrote:
> That is another urban legend. Never been true. I've seen fabricators say
> this and then etch outer layers with all sorts of surface mount pads that
> have traces entering them that result in right angle corners and never
> complain!
>
> We'll probably all go to our graves before we flush out all this
> misinformation!
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