[SI-LIST] Re: Via stub math help needed....
- From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>, Eric Bogatin <eric@xxxxxxxxxxxxxxx>
- Date: Fri, 13 Jan 2012 16:13:50 -0800 (GMT-08:00)
I do very well by representing vias as small capacitors, which is what this
discussion is implying. More local C would look like a higher dk.
I have measured many of them and find that a 12 mil drilled hole in a 100 mil
thick PCB with the usual number of planes for such a thickness averages about
0.3 pF. Using this in simulations pretty well matches what one sees when
making measurements of physical hardware.
These local capacitors are often mistakenly called stubs.
-----Original Message-----
>From: Scott McMorrow <scott@xxxxxxxxxxxxx>
>Sent: Jan 13, 2012 6:47 AM
>To: Eric Bogatin <eric@xxxxxxxxxxxxxxx>
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Via stub math help needed....
>
>Eric
>I do not like the term "effective Dk" in this context.
>
>Effective dielectric constant is generally used to describe quasi-TEM
>layered dielectrics.
>
> - Microstrip
> - inhomogeneous stripline
> - coplanar waveguide
>
>Effective Dk is a rather imprecise term for loaded interconnect. For
>example, a multidrop bus is often described as having an "effective Dk".
> It doesn't. Dk does not change anywhere in the interconnect. Neither the
>through traces, or the stub traces, or receiver capacitance does anything
>to alter dielectric constant of the underlying material or the
>instantaneous wave propagation through those sections. We can speak of
>"loaded delay" of "loaded impedance", but as I look in Pozar I see no
>reference to Effective Dk when he discusses periodic structures.
>
>Call the phenomena "correction factor for physical stuff that happens
>around a via", but not Effective Dk.
>
>Scott
>
>
>On Fri, Jan 13, 2012 at 7:13 AM, Eric Bogatin <eric@xxxxxxxxxxxxxxx> wrote:
>
>> Scott-
>>
>>
>> I thought I would chime in on the question of the higher Dk for
>> vias. I learned a lot about vias working on a project with Bert a
>> few years back and continue to learn more every time I look at
>> them, so I always appreciate your comments and others on this
>> topic.
>>
>>
>>
>> I think the first question is, is there a higher propagation
>> delay through a via, if you take as its start and finish the top
>> and bottom pad region where the transmission line feed hits the
>> annulus clearance hole?
>>
>>
>>
>> It's really hard to measure directly, but easy to simulate in
>> many 3D tools. I find that when you keep the stack height fixed,
>> by just changing the clearance holes, NFPs and other physical
>> features inside the via pad stack, you can get a variety of
>> propagation delays, as extracted from the phase of S21.
>>
>>
>>
>> I have to say that I did not use generalized modal S-parameters,
>> however, this higher delay is seen even when the return loss is
>> less than -25 dB, where you would not expect a phase distortion
>> from reflections.
>>
>>
>>
>> I interpret this higher delay as a higher "effective Dk". I don't
>> think the intrinsic material properties have changed, nor is
>> there an issue of the anisotropic effects of the Dk from glass
>> weave. I think this has been shown to be only on the order of ~
>> 20% at most.
>>
>>
>>
>> As an aid in describing the via propagation delay, I interpret
>> the extra delay as a higher Dk. In some multi layer via
>> structures, I extract a Dk_eff as high as 16, if there are NFPs.
>> You see this clearly in the lower stub resonance frequencies.
>>
>>
>>
>> Why does the prop delay increase thru a via? I think it is due to
>> the non TEM fields. In a TEM propagation, I think we all agree
>> that the delay is due to the physical length and the bulk Dk.
>>
>>
>>
>> In a non TEM propagation, I think all bets are off. The speed of
>> the signal will depend a lot on the shape of the fringe fields.
>> The more non-TEM, as you get with all those fringe fields from
>> the barrel and pads to the planes, the more different the speed
>> is from the bulk Dk.
>>
>>
>>
>> I think the confusion is over the use of the term Dk. It may be
>> less confusing thinking of this as an effective Dk, useful to
>> predict the prop delay based on the physical length.
>>
>>
>>
>> I can model many complex via structures as uniform transmission
>> lines up to the 10 GHz range, using a Z0, the physical length and
>> the Dk_eff. This helps in estimating the stub resonances.
>>
>>
>>
>> What do you all think?
>>
>>
>>
>> --eric
>>
>>
>>
>>
>>
>>
>>
>> *******************************************************
>> Dr. Eric Bogatin, Signal Integrity Evangelist
>> Bogatin Enterprises
>> Setting the Standard for Signal Integrity Training
>> web site: www.beTheSignal.com <http://www.bethesignal.com/>
>>
>> beTheSignal Blog: www.beTheSignal.com/blog
>> 26235 W 110th Terr
>> Olathe, KS 66061
>> e: eric@xxxxxxxxxxxxxxx
>>
>> for class information: email to beTheSignal@xxxxxxxxxx
>> v: 913-393-1305 cell: 913-424-4333 skype: eric.bogatin
>> ***********************************************
>>
>>
>>
>> Msg: #1 in digest
>>
>> Date: Thu, 12 Jan 2012 16:41:44 -0500
>>
>> Subject: [SI-LIST] Re: Via stub math help needed....
>>
>> From: Scott McMorrow <scott@xxxxxxxxxxxxx>
>>
>>
>>
>> Bert and Ralph
>>
>> There are layered-anisotropic variations in Er for many
>> materials,
>>
>> especially those that include fiberglass weave. However, Er does
>> not
>>
>> change with non-TEM modes or different TEM modes (stripline,
>> via-coaxial,
>>
>> circular cavity ... etc). Different propagation modes merely
>> concentrate
>>
>> the field in different directions and select a different set of
>> localized
>>
>> material characteristics.
>>
>>
>>
>> I've read the papers and seen the claim that the dielectric
>> constant of
>>
>> layered fiberglass material is higher for propagation through a
>> via, due to
>>
>> the direction of the field, however, I've not seen a systematic
>> study of
>>
>> this. (Adjustment of material Er(effective) to obtain a match to
>> modeling
>>
>> does not constitute proof.) My experience for launch vias with
>> coaxial
>>
>> ground rings has been that the resonance computed by full wave
>> solvers
>>
>> matches measurements quite well in a multitude of materials, if
>> the
>>
>> dielectric has been characterized correctly. I find that most of
>> the
>>
>> mis-correlations that I've seen are due to improper material
>>
>> characterization. I do not discount the possibility of a higher
>> localized
>>
>> Er region around a via in some measurements, its just that I find
>> little
>>
>> evidence for fiberglass being the sole culprit. In many cases
>> I've found
>>
>> that mismatch in stub resonance could be easily accounted for by
>> adding the
>>
>> correct amount of soldermask to the bottom pad in modeling. In
>> other cases
>>
>> I've found that material variations between layers were not
>> correctly
>>
>> modeled.
>>
>>
>>
>> As a thought experiment, take a section of a PCB with a via along
>> the
>>
>> z-axis that is fully surrounded by a coaxial metal wall.
>> Calculate the
>>
>> average Er from top to bottom, and then calculate it radially
>> out. The
>>
>> volume of material is the same. The composition of the material
>> is the
>>
>> same. Thus the average Er is the same. The only way to come up
>> with a
>>
>> higher Er for the radial direction is to conclude that somehow
>> the
>>
>> drilling process selectively removes more epoxy than fiberglass
>> from the
>>
>> mixture. There are layered variations as we travel down the via
>> passing
>>
>> through fiberglass rich, and then epoxy rich layers. But I see
>> no reason
>>
>> why they would not average out. I can make a case that
>> individual pairs of
>>
>> signal and ground vias can have Er variation, just as I can for
>> traces, but
>>
>> I cannot come up with any reason why the Er would not average out
>> in the
>>
>> limit.
>>
>>
>>
>> There is one other potential reason why a via could have a higher
>> localized
>>
>> average Er. But it has nothing to do with the fiberglass itself.
>> I will
>>
>> probably use it as a topic for next year's DesignCon paper, as a
>> follow up
>>
>> to the paper I'm involved with this year.
>>
>>
>>
>>
>>
>> regards,
>>
>>
>>
>> Scott
>>
>>
>>
>>
>>
>>
>>
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>
>
>--
>
>Scott McMorrow
>Teraspeed Consulting Group LLC
>121 North River Drive
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>(401) 284-1827 Business
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>
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>
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