[SI-LIST] Re: Right Angle Bends
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 16 Jul 2011 09:35:29 -0700
Hi Guys,
I assume we are talking about right-angle bends in ordinary (not
differential) traces.
In that case, I am in agreement that a right-angle bend, on a trace as
typically fabricated with today's (2011) small geometry, at the speeds where
we operate (2-20 Gbps) does not present much of a signal integrity issue.
There is, however, a perfectly valid engineering reason behind the old adage
that right-angle bends should be avoided. It is explained in the following
short editorial article, the complete text of which is copied below.
H. Johnson, "Who's Afraid of the Big, Bad Bend?", EDN, 5/11/2000
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
--------------- TEXT OF ARTICLE -------------------
Who's afraid of the big, bad bend?
(Originally published in EDN Magazine, May, 2000)
Right-angle bends in PC-board traces perform perfectly well in digital
designs in speeds as fast as 2 Gbps.
In most digital designs, the right-angle bend is electrically smaller than a
rising edge. For example,the delay through a right-angle bend in an
8-mil-wide, 50-ohm microstrip trace in FR-4 is on the order of 1 psec.
That's less than 1% of a 100-psec rise time. For any object of this tiny
physical scale, a lumped-element model should suffice. Years ago, TC Edwards
presented good lumped-element models for right-angle bends for the microwave
industry (Reference 1). These models indicate that a right-angle bend has
two primary effects: a slight delay plus some excess lumped capacitance. You
might imagine that, as a signal traverses a right-angle corner, the trace
appears to grow wider at the corner. This simple idea explains why you see
an excess capacitance (lower impedance) near the corner.
[Ed. note: Edwards goes on to show a three-element L-C-L equivalent
circuit for
the bend, and discusses means of correctly chamfering the corner to
mitigate
the effect.]
For an 8-mil-wide, 50-ohm microstrip transmission line in FR-4, the excess
lumped capacitance works out to 0.012 pF. Assuming that you are using
100-psec rise and fall times, the size of the reflected signal that bounces
off this capacitive discontinuity is 0.30% (that's 0.003) of the incoming
step amplitude. I conclude from this analysis that the reflection from a
single corner is too small to worry about. (The reflected signal size scales
in proportion to the trace width and inversely with rise and fall times.)
Some people worry that conduction electrons are traveling so fast that they
won't be able to make it around a square corner. Perhaps they might reflect
back or fly off into space. Such arguments are ridiculous. Sure, individual
electrons move at high speeds, but their aggregate drift velocity is less
than 1 in./sec as they bounce from atom to atom. Your average electron
smacks into something and changes directions billions of times in a length
of 10 mils. Electrons don't have any trouble banging around a corner.
Might the electric-field concentration at a sharp, pointy corner create a
lot of radiation? Hogwash. As a trace rounds a corner, it stays a constant
distance from the underlying reference plane the whole way. The electric
field intensity from trace to plane doesn't radically vary at any point
along this track except for a modest perturbation in the vicinity of the
actual pointy tip of the corner. It's true that a microscopic electric-field
probe directly adjacent to the corner would detect this field concentration.
However, measurements taken from farther away account for the average of
everything that happens along the whole trace, not just at the corner. The
corner, because it's so small, cannot noticeably affect the far-field
radiation.
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.
Today (circa 2000), only microwave designers need to worry about right-angle
bends. At microwave speeds, roughly 10 times the rate of most digital
designs, parasitic capacitance presents 10 times more of a problem. Also,
microwave designers often use big, fat, 100-mil traces to reduce skin-effect
losses, so their corners appear electrically 10 times bigger. They also tend
to linearly cascade multiple stages. Cascading sums the imperfections in
each stage, making microwave designs about 10 times more sensitive to tiny
imperfections. Overall, contemporary microwave designs can be 1000 times
more sensitive to right-angle bends than are digital designs.
As digital designs push toward higher speeds, you may eventually reach a
point where the right-angle bends begin to matter. For example, corners are
just beginning to affect the design of 10-Gbps serial connections, and they
also contribute perceptibly to skew in certain poorly routed differential
pairs. If you accumulate a lot of corners, as in a serpentine delay
structure, you may begin to see a little extra delay. Other than these
extreme applications, right-angle bends remain electrically transparent.
Some manufacturing engineers complain about the use of right-angle bends
when using wave-soldering equipment. They worry that wayward solder balls or
solder flux will get trapped in the inside corners. With reflow soldering
and good solder masking, neither is a problem. I have heard no other
credible negative comments about the manufacturability of right-angle bends,
but I am always happy to hear from others whose experience may differ.
Please write.
REFERENCE
1. Edwards, Terry, Foundations for Microstrip Circuit Design, John
Wiley & Sons, 1981,1992.
-------------------- END ARTICLE ---------------------
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lee Ritchey
Sent: Friday, July 15, 2011 12:15 PM
To: Vinu Arumugham
Cc: Moreira, Jose; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Right Angle Bends
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!
--------------------------------------------------
From: "Vinu Arumugham" <vinu@xxxxxxxxx>
Sent: Friday, July 15, 2011 11:28 AM
To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
Cc: "Moreira, Jose" <jose.moreira@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Right Angle Bends
> Are 90 degree turns not an "acid trap" problem especially for 3-4 mil
> wide traces?
> Lee Ritchey wrote:
>> There is a paper on my web site authored by Doug Brooks using lab
>> measurements performed by Todd Hubing when he was at UMR as part of
>> the EMI department that clearly shows that right angle bends in
>> signal traces are not detectable out to something like 20 GHz.
>>
>> You can download it from www.speedingedge.com. It is titled "90
>> Degree Corners, The Final Turn".
>>
>> This article has the info you are looking for. I'm sure Doug would
>> not object to you sending it around. Anything to stop the crazy
>> rumors about this topic.
>>
>> It is truly amazing the stories I hear about how evil things happen
>> at right angle bends. In every case, the author of the rumor has no
>> data to back it up or makes a statement such as "I measured this at
>> my last company and
>> cannot share it without their permission". Doesn't that sound a little
>> fishy?
>>
>>
>> --------------------------------------------------
>> From: "Moreira, Jose" <jose.moreira@xxxxxxxxxx>
>> Sent: Friday, July 15, 2011 9:48 AM
>> To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
>> Subject: [SI-LIST] Re: Right Angle Bends
>>
>>
>>> From my experience (I also get that statement from my customers) it
>>> comes from the thinking that on a right angle bend the signal would
>>> "see" a larger trace width when crossing "the bend" and because of
>>> this different trace width it will have a different impedance at
>>> that point so there is a discontinuity that is bad for the signal.
>>>
>>> I remember an old conference paper showing no measureable effects to
>>> 10 Ghz but I cannot find it again.
>>>
>>> I would be interested if anyone know of a good up to date reference
>>> (with
>>> measurements) that I can also send to my customers when they have
>>> the same statement.
>>>
>>> Jose
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx
>>> [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>> On Behalf Of Lee Ritchey
>>> Sent: Friday, July 15, 2011 6:33 PM
>>> To: si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: Right Angle Bends
>>>
>>> Does anyone know the origin of the myth that right angle bends cause
>>> signal integrity problems?
>>>
>>> I though this myth had died a long time ago, but had a conversation
>>> last week with a well known fabricator who was told by a customer
>>> that right angle bends were a no no.
>>>
>>> This was a customer who should know better.
>>>
>>> Lee Ritchey
>>> Speeding Edge
>>>
>>>
>>>
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