[SI-LIST] Re: Right Angle Bends
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 25 Jul 2011 11:54:20 -0700
Dear Mr. Ritchey,
It is so nice to hear that you have noticed my article, "Who's Afraid of the
Big, Bad Bend", and that you seem to like it because you believe it supports
your cause. You say that the article, "makes a compelling case", and
"confirms my own measurements."
I should point out that the full text of that very same article was posted
to this list just last Saturday. At that time, you challenged the article,
saying, "Has any of this been verified with measurements? Doesn't sound
like it from what I can read out of this. If so, can we see them? If not,
the proper approach is to assume the conclusions are not valid."
(//www.freelists.org/post/si-list/Right-Angle-Bends,34)
I wonder which of your two responses represents your true feelings? Does the
article make a "compelling case", or are the conclusions "not valid?"
(The full text of the article, by the way, appears below in this very
thread,
along with Mr. Ritchey's response to it.)
The point of the article was to explain the origin of the myth that right
angle bends cause signal integrity problems, and show that the original
motivations for early prohibitions against right-angle bends, while correct
for some microwave designs, do not (in my experience) apply to typical
digital circuits as we make them today.
www.sigcon.com/Pubs/edn/bigbadbend.htm
Mr. Ritchey, regarding your statement that you used your pcb layout tools
to "design some of Howard's boards", suggesting that we had in the past some
sort of working relationship, please allow me to refresh your memory.
I believe you are referring to your work with Ultra Network Technologies
circa 1988-1989. At the time, I was responsible for design of the Ultra
Network 1 Gb/s serial transceivers. As part of my due diligence for that job
I interviewed you at length about your board layout design approach and
capabilities, as I did several other layout shops, fab shops, microwave
designers, book authors, and EE faculty.
As a result of those interviews I chose not to involve your company in any
of my high-speed board designs. While your board shop remained involved in
other Ultra Networks assemblies, I hand-drew all the serial transceiver
designs on vellum and had the designs translated to gerber format by Dave
Hawkins at DVK. He did a great job.
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 Lee Ritchey
Sent: Friday, July 22, 2011 10:17 AM
To: Howard Johnson; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Right Angle Bends
Howard and all,
I had a Colombo moment after my last reply. I have one other question!
I was thinking about all the columns and other documents I have collected
over the years to use as references.
I found a column in the May 2000 issue of EDN titled "Who's Afraid of the
Big, Bad Bend, 90 Degree Corners?" which made a compelling case that the 90
degree corners in traces of the widths used to route modern PCBs represented
at most a 1 pSec delay that few if any logic could see. That confirmed my
own measurements.
Much to my surprise, it was authored by Howard Johnson. Now I am confused
by the below references.
For those of you who wonder why anyone would route PCBs with right angle
bends since modern design tools make it easy to do 45s or round corners,
these tools did not do this originally. It took too much code and computer
time. My company sold such a tool for many years and had to deal with the
request over and over. We used that tool to design some of Howard's boards.
That is where we met.
The CAD companies had to add features to the tools to remove right angle
bends in order to sell their products. Many CAD tools were discarded
because they could not do 45 degree corners even though it was already know
that is was not an issue.
Lee Ritchey
--------------------------------------------------
From: "Howard Johnson" <howie03@xxxxxxxxxx>
Sent: Saturday, July 16, 2011 5:15 PM
To: <si-list@xxxxxxxxxxxxx>
Cc: "'Lee Ritchey'" <leeritchey@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Right Angle Bends
> Dear Mr. Ritchey,
>
>
> --------------(1)-------------------
>
> Edwards says the following about right-angle bends in his 1st edition
> on page 109, "Capacitance data has been determined theorectically by
> Silvester and Benedek[13] and inductance data by Thomson and
> Gopinath[5]. Fairly extensive measurements have also been
> conducted.... "
>
> The book continues to discuss small parasitic effects that happen at
> other discontinuities, such as the apparent slight elongation of an
> open-circuited endpoint due to the parasitic capacitance at the end of
> an open-circuited trace, discontinuity effects at step changes of
> impedance (above and beyond the expected reflection due to the change
> in impedance), and other details of great interest to microwave
> designers, for which I assume this material is standard practice.
>
> [13] Silvester and and Benedek, "Microstrip discontinuity capactiances
> for right-angle bends, T-juntions and crossings," IEEE Trans., MTT-21,
> No. 5, May 1973, 341-346
>
> [5] Thompson and Gopinath, "Calculation of microstrip discontinuity
> inductance", IEEE Trans., MTT-23, No. 8, August 1975, 648-655.
>
>
> The book by K.C. Gupta, "Microstrip Lines and Slotlines", Artech
> House, 2nd.
> ed. 1996. Page 194 shows a comparison of theoretical results from
> Thomson and Gopinath with experimental measurements by Easter[33].
>
> [33] Easter, "The Equivalent Circuit of some Microstrip
> Discontinuities", IEEE Trans., Vol. MTT-25, 1975, pp. 655-660, a paper
> based, I believe, on his earlier work, "Resonant Techniques for the
> Accurate Measurement of Microstrip Properties and Equivalent
> Circuits," Proc. 1973 European Microwave Conf., paper B. 7.5.
>
>
> I can't imagine what better data one could ask for than Gupta's
> comparison of theoretical models with experimental measurements.
>
>
> ------------------(2)----------------------
>
> Regarding differential microstrip lines, the coupling between the
> lines introduces an additional complication. That has to do with mode
> conversion between the differential and common modes of propagation
> that happens when a differential pair passes through a corner. The
> outer trace being slightly longer than the inner trace, some of the
> differential mode is converted to common-mode, and vice-versa. If the
> next bend breaks to the opposite direction, all is restored. In an
> uncoupled (or loosely coupled, meaning
> widely-spaced) differential pair, the mode conversion is completely
> reversible. A tightly-coupled pair does not work that way. The even
> and odd-mode impedances of the tightly coupled pair are not the same,
> and crucially, the even and odd-mode impedances may not be the same,
> so it seems to me obvious that, were any power converted to a mode
> with a different impedance than the original, some small reflection
> must be generated.
>
> I've not personally heard of any measurements corroborating such
> differential reflections, however I am certain that if one were to
> fabricate enormous traces as was common in the microwave industry in
> 1973, perhaps 100-mils wide on a 50-mil thick substrate, the small
> reflections so generated would rise to a level of concern in a
> high-quality microwave application (such as radar).
>
> I restricted my earlier remarks to the case of single-ended traces
> precisely because there exists a wealth of well-established literature
> documenting their performance.
>
>
> 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: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Saturday, July 16, 2011 9:56 AM
> To: Howard Johnson; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Right Angle Bends
>
> Has any of this been verified with measurements? Doesn't sound like
> it from what I can read out of this. If so, can we see them? If not,
> the proper approach is to assume the conclusions are not valid.
>
> Why would it matter whether a trace is single ended or differential?
> A transmission line is a transmission line either way.
>
> --------------------------------------------------
> From: "Howard Johnson" <howie03@xxxxxxxxxx>
> Sent: Saturday, July 16, 2011 9:35 AM
> To: <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Right Angle Bends
>
>> 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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