[SI-LIST] Re: 90 degree bend

• From: "richard moffat" <richard.moffat@xxxxxxxxxxxxxxxxxxx>
• To: <si-list@xxxxxxxxxxxxx>
• Date: Tue, 31 Aug 2004 09:56:37 +1200

```Hi all

With all due respect, we may as well explain WHY this is not true, and
give a model answer for the homework assignment!

Argument #1:  Electrons will fly off the end of an acute angle trace.

There are essentially two electron velocities: the Fermi speed and the
drift velocity.

The Fermi speed is the actual electron speed, and is in the order of
106 ms-1 for copper.  It is not related to voltage or the change in
voltage over time (the voltage rise time).  The electron drift velocity
is the velocity that an electron travels along the medium, and is
related to the voltage, is approximately of 0.004 ms-1 for one volt in
copper.  It also is not dependant on the voltage rise time.

Argument #2 :  Acute traces can radiate

This can happen due to impedance mismatch, but at a scale that is not
important.  Referring to Fig.1, The corner-to-outside radius of a
right-angled trace is approximately 21% larger than the trace width, and
it is correct that this will cause an impedance mismatch.   However, the
distance that the
mismatch occurs is less than the trace width itself.  (Very large
traces, ie, in high RF, this will  _start_  to have an effect.)

Assuming my maths is correct:
A rise-time of 100ps will give a first-harmonic wavelength of
approximately 60mm.  This is far greater than the mismatch distance, and
combined with only approximately 20% impedance difference, any radiation
will be extremely negligible.  Impedance mismatches from pad-trace or
pin-trace will far exceed the value from acute angle traces.

Cheers,
Richard

__________________________
Richard Moffat
Allied Telesyn Research Ltd, NZ.
ph. +64 (3) 3393000
richard.moffat@xxxxxxxxxxxxxxxxxxx

>>> "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx> 31/08/2004 4:38:52 a.m.
>>>
This is one of the pieces of misinformation that circulates about
right
angle bends.  It is simply not true.  Those who repeat it need to do
their
homework.

Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX-    707-568-3504

I just used the energy it took to be angry to write some blues.
Count Basie

> [Original Message]
> From: Kanakaraj <k.kanakaraj@xxxxxxxxxxxxx>
> To: <pcb_user@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 8/30/2004 7:17:42 PM
> Subject: [SI-LIST] Re: 90 degree bend
>
> Hi,
> The simplest thing is that when you make 90 degree turns on the pcb
track,
> the electron moving in it cannot make sudden turns..( remember
inertia of
> direction in physics and a bus making a 90 degree turn at full
speed)...
so
> some of the electrons will jump out of the pcb track and will cause
emi
> issues..
>
> and Andy, Andrew, Ray anderson and the seniors will give you more
inputs
on
> that.
> Well I am too like you. a beginner :-)
> Kanak
>
> ----- Original Message -----
> From: "New Bee" <pcb_user@xxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Monday, August 30, 2004 5:19 PM
> Subject: [SI-LIST] 90 degree bend
>
>
> > Hi
> >
> > I am a new PCB designer.
> >
> > I have a silly doubt - really do not know if I ask it in this
forum.
> >
> >
> > why is it suggested not to have 90 degree bends for the traces
while
> routing ?
> >
> > If some one can give me the electrical/electronics and
manufacturing
> reasons I will be greatful to them
> >
> > If this is not the right forum to address this please let me know
where
> >
> > Kam
> >
> > Yahoo! India Matrimony: Find your life partner online.
> >
> >
> > Yahoo! India Matrimony: Find your life partneronline.
> >
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