[SI-LIST] Re: 90 degree turn in PCB tracks
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Tom Dagostino" <tom@xxxxxxxxxxxxx>, "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Wed, 26 Jul 2006 09:58:32 -0700
Tom,
I agree that when traces are very wide, corner effects are visible. Can't
be sure that they are significant though. If the original question had
been about high power microwave, I would have had a different answer.
Long ago, I designed microwave PCBs and did radius corners on very wide
traces to get the best VSWR.
I think the consensus of all the replies to the original question
concerning traces used in logic PCBs is that right angle bends are not
detectable sources of degradation.
The curious thing all along had been that engineers fretted over right
angle bends in traces while ignoring them in their IC packages. This was
especially true with DIPs and flat packs. My olde partner, Dan Murphy,
called this "looking where the light is good" or if I can't see it, I won't
worry about it.
For those who want to design microwave PCBs, maybe we need another forum.
> [Original Message]
> From: Tom Dagostino <tom@xxxxxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>; Scott McMorrow <scott@xxxxxxxxxxxxx>
> Cc: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 7/25/2006 3:48:59 PM
> Subject: [SI-LIST] Re: 90 degree turn in PCB tracks
>
> Lee, I've constructed test boards to measure the effect of non-mitered =
> 90
> degree bends and they are very visible with 50 Ohm traces on boards =
> 0.062
> thick with 0.110 wide traces.
>
> Tom Dagostino
> Teraspeed(R) Labs
> 13610 SW Harness Lane
> Beaverton, OR 97008
> 503-430-1065
> tom@xxxxxxxxxxxxx=20
> www.teraspeed.com=20
>
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> 401-284-1827
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
> On
> Behalf Of Lee Ritchey
> Sent: Tuesday, July 25, 2006 1:04 PM
> To: Scott McMorrow; scott@xxxxxxxxxxxxx
> Cc: Matthias Bergmann; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: 90 degree turn in PCB tracks
>
>
> Way back in the dark ages when we used ECL to do high speed designs, I
> contacted the author of the book to see why we couldn't duplicate his
> measurements with an identical setup. The response was that the picture =
> was
> wrong, but technical publications was in a hurry to go to press, so they
> left it in!
>
>
> > [Original Message]
> > From: Scott McMorrow <scott@xxxxxxxxxxxxx>
> > To: <scott@xxxxxxxxxxxxx>
> > Cc: <leeritchey@xxxxxxxxxxxxx>; Matthias Bergmann
> <MBergmann@xxxxxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> > Date: 7/25/2006 12:14:18 PM
> > Subject: [SI-LIST] Re: 90 degree turn in PCB tracks
> >
> > Y'all,
> > I just went and pulled up the old app. note. you can find it here=20
> > http://www.onsemi.com/pub/Collateral/HB205-D.PDF
> >
> > I think the problem people have had is in not putting the old Motorola
> > measurements in perspective. Clearly if you read the previous pages=20
> > you'll find that on pages 140 to 142 the design being tested is that =
> of=20
> > a microwave hybrid divider on a substrate that was 62 mils thick and =
> had=20
> > an Er of 5.3 (Er(eff)=3D 4.5 for microstrip). A hybrid divider takes =
> a 50=20
> > ohm trace and divides it into two 100 ohm traces. If you do the=20
> > calculations, that 50 ohm trace was somewhere around 110 mils wide.
> >
> > As we go on to page 144, we find the famous corner tests, where=20
> > clearly
> > there are corner discontinuities shown by the TDR. If I were a =
> betting=20
> > man, even though the material thickness and trace widths were not =
> shown,=20
> > I'd bet that they are similar to the previous hybrid divider. So, we=20
> > have a case where there is a really fat trace and therefore a big=20
> > discontinuity. If we do the math, as I did previously, that=20
> > discontinuity should be:
> >
> > t(discontinuity) =3D 85 x sqrt(2 x 4.5) x .11 =3D 28 ps.
> >
> > In a TDR this time will be doubled, due to the round trip across the
> > corner, so we would expect a hump with a duration of approximately 50 =
> to=20
> > 60 ps. Even though the plots do not show a scale, I'd guess that =
> around=20
> > a 50 to 60 ps pulse duration would be about right, when you add in =
> the=20
> > additional TDR rise time, itself.
> >
> > My conclusion is that there is nothing wrong with the old Motorola=20
> > application note. In fact, if you were to replicate this experiment=20
> > with 100 mil lines on a thick FR4 substrate today, you'd get pretty=20
> > much
> > the same result. The problem is that people have generalized the data =
>
> > to cases with smaller line widths without scaling. If you scale the=20
> > Motorola measurements by 1/22 (5 mils/110 mils), those corner=20
> > discontinuity impedance bumps will just blend into the noise. They=20
> > still exist ... but they are really, really tiny ... and not important =
>
> > for modern digital board design ... just as Lee advocates.
> >
> >
> >
> > regards,
> >
> > 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
> >
> > TeraspeedR is the registered service mark of
> > Teraspeed Consulting Group LLC
> >
> >
> >
> > Scott McMorrow wrote:
> > > Lee
> > > The experimental design used in Ultracad corner test is quite
> > > sub-optimal, given the incredible launch discontinuity. I'd be=20
> > > surprised if better than a 35 to 50 ps rise time edge actually makes =
> it=20
> > > into the board. Then, given the length of the traces involved, I=20
> > > suspect that the actual bandwidth seen at the corners is somewhere=20
> > > around 3 GHz, at best. Unfortunately, there do not appear to be any =
>
> > > structures available to determine the actual measurement bandwidth. =
>
> > > Just because Todd Hubing was responsible for the measurements, does =
> not=20
> > > mean that the experimental design is as good as an 18ps launch and =
> 20=20
> > > GHz makes it sound. If you can get a fast launch into a board, and =
> can=20
> > > use low loss materials, it is possible to see corners. Gus Panella =
> and=20
> > > I did so about 8 years ago on a test vehicle that we designed using=20
> > > Rogers 4350 material.
> > >
> > > Having said that, 3GHz bandwidth is typical of the fastest standard
> > > logic signals on most conventional PCBs. Your assertion that =
> corners
> do=20
> > > not matter for digital designs is true.
> > >
> > > The original source of the corner information comes from the=20
> > > Microwave
> > > literature. Gupta, et. al., Microstrip Lines and Slotlines, is a =
> good=20
> > > reference. It has been well known that, in Microwave design, right=20
> > > angle bends are not a good thing. If you're trying to decrease =
> return=20
> > > loss in a microwave design, you will most assuredly use radiused
> corners=20
> > > or the optimal chamfer. Why? Because microwave boards use low loss
> > > materials, like PTFE, and very wide trace width on thick substrates =
> to=20
> > > reduce total power loss. The corner discontinuity on a 5 mil wide
> logic=20
> > > signal trace on a conventional high density PCB may not be an issue,
> but=20
> > > on a low loss microwave board that uses 50 to 200 mil wide=20
> > > microstrip
> > > traces, that silly little corner is a killer.
> > >
> > > The magnitude of the corner discontinuity is proportional to it's
> > > duration. A corner discontinuity lasts for approximately 85 x =
> sqrt(2 x=20
> > > Er(eff)) x w ps:
> > >
> > > Where
> > > Er(eff) =3D is the effective Er of the material
> > > w =3D the width of the trace in inches.
> > >
> > > If we run the numbers,
> > >
> > > For a 5 mil stripline trace on FR4 with and Er(eff) of 4, the
> > > discontinuity lasts for 1.2 ps.
> > > For a 100 mil microstrip trace on Duroid with an Er(eff) of 2, =
> the
> > > discontinuity lasts for 17ps.
> > >
> > > If we then use the rule of thumb that a discontinuity is important=20
> > > only
> > > when it approaches 1/10th of the risetime, then our little 5 mil =
> corner=20
> > > has an effective operating bandwidth of .35/(1.2 e-12 x 10) =3D 29 =
> GHz. =20
> > > Clearly out of the region where we are interested for digital logic.
> > >
> > > But for the 100 mil trace, we have an operating bandwidth of=20
> > > .35(17e-12
> > > x 10) =3D 2 GHz. This is quite frankly not a very good microwave =
> design.
> > >
> > >
> > > Best regards,
> > >
> > > 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
> > >
> > > TeraspeedR is the registered service mark of
> > > Teraspeed Consulting Group LLC
> > >
> > >
> > >
> > > Lee Ritchey wrote:
> > > =20
> > >> The paper published by Ultracad tests the effects of right angle=20
> > >> bends
> in a
> > >> logic signal trace with an 18 pSec edge which is equivalent to=20
> > >> about 20 GHz. There is no detectable effect. These tests were=20
> > >> conducted by
> Todd
> > >> Hubing of UMR in a well equipped lab and are to be trusted. There=20
> > >> have been amy other similar tests done with the same result.
> > >>
> > >> I believe the notion that right angle bends are a source of=20
> > >> problems
> stems
> > >> from an error in the Motorola ECL handbook published in 1974 and=20
> > >> still
> in
> > >> print with the error. Now O Semiconductor publishes it.
> > >>
> > >> This may be one of those cases where simulation shows a change in=20
> > >> the
> field
> > >> distribution around the right angle bend, which we expect. The
> question is
> > >> whether the change is significant. One of our jobs is to=20
> > >> distinguish between visible and significant.
> > >>
> > >> Once again, we have lies, damn lies and simulations. Simulations
> without
> > >> validation may well be more dangerous than no simulations at all in
> some
> > >> cases.
> > >>
> > >>
> > >> =20
> > >> =20
> > >>> [Original Message]
> > >>> From: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>
> > >>> To: <si-list@xxxxxxxxxxxxx>
> > >>> Date: 7/25/2006 1:42:10 AM
> > >>> Subject: [SI-LIST] Antwort: Re: 90 degree turn in PCB tracks
> > >>>
> > >>> Hello,
> > >>>
> > >>> Following the advices I read the article about 90 degree bends on=20
> > >>> ultracad.com as well as the appropriate chapter in Eric Bogatins=20
> > >>> book. I am quite surprised about the conclusion that 90 degree=20
> > >>> corners don't matter. I remember that once in a simulation of a 50 =
>
> > >>> ohm
> microstrip-lin=3D
> > >>> e
> > >>> with
> > >>> a chamfered 90=3DB0 corner in ADS Momentum, I never got a S11 =
> better
> than=3D
> > >>> 15 dB
> > >>> at frequencies higher than 15 GHz, even 10 GHz made problems. The=20
> > >>> mentioned articles don't consider the frequency respectively just=20
> > >>> consider designs where the right-angle bend is electrically=20
> > >>> smaller
> tha=3D
> > >>> n a
> > >>> rising edge.
> > >>> Would be interesting to know how two 45 degree corners behave at=20
> > >>> higer frequencies.
> > >>>
> > >>> Regards, Matthias
> > >>>
> > >>> ____________________________________
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