[SI-LIST] Re: Via modeling & de-embedding
- From: David Siadat <dsiadat@xxxxxxxxx>
- To: gstokes@xxxxxxxxx
- Date: Wed, 21 Jul 2004 09:00:15 -0700
Hello Geof,
It applies to a single frequency, high or low.
Regards,
David
At 12:56 PM 7/20/2004 +0100, Geoff Stokes wrote:
>Hi David
>
>Thanks for your response. Presumably your "single inductance value" applies
>at low frequency rather than high?
>
>Cheers
>Geoff
>
> > -----Original Message-----
> > From: David Siadat [mailto:dsiadat@xxxxxxxxx]
> > Sent: 19 July 2004 16:23
> > To: Geoff Stokes
> > Cc: 'si-list@xxxxxxxxxxxxx'
> > Subject: Re: [SI-LIST] Re: Via modeling & de-embedding
> >
> >
> > Hi Geoff,
> >
> > I agree with your comments.
> > I should also point out that it is quite acceptable to
> > replace multiple
> > inductance value of a distributed model with a single
> > inductance value.
> > This does not mean that we are modeling the object with a
> > lumped elements.
> > 3D field solvers use this to calculate characteristic
> > impedance of the object.
> >
> > Regards,
> > David
> >
> > At 09:21 AM 7/19/2004 +0100, Geoff Stokes wrote:
> > >Hi David
> > >
> > >In your point no. 1 you mention higher order modes. This
> > was described very
> > >well by Hassan O. Ali. Your point no. 2 mentions
> > "characteristics of the
> > >via". The RF current is confined to the surface because of
> > the skin effect.
> > >At the via this surface current changes direction abruptly
> > from horizontal
> > >to vertical. The discussion seems to indicate to me that there is no
> > >purpose in considering characteristics of the plated via
> > hole alone, because
> > >in the vicinity of the via there are various wave modes. Therefore
> > >conventional de-embedding becomes impossible, because it
> > uses TEM wave
> > >analysis. If the via is sufficiently far from other wave
> > discontinuities in
> > >the signal path, there will be some value in a TEM model of
> > a via together
> > >with its connecting traces, and this can be de-embedded to a
> > reasonable
> > >distance, but not right up to the edge of the anti-pad where
> > the higher
> > >order modes are quite strong. The varying concentrations of
> > field and
> > >surface current can be studied in various 3D tools such as
> > Ansoft HFSS, CST
> > >Microwave Studio or EM3DS. Generally, the TEM mode settles down at a
> > >horizontal distance many times larger than the dielectric
> > height. In some
> > >cases it might be best not to de-embed closer than say 3h,
> > and other cases
> > >could benefit from a yet larger de-embedding distance. The
> > higher order
> > >modes provide interactions that are not represented on a two-terminal
> > >schematic - at higher frequencies there is no such thing as a simple
> > >inductance: that's an approximation suitable only for lower
> > frequencies. In
> > >the case of a straight wire or plated via, if the impedance
> > is significant,
> > >it is because the length is a significant part of a
> > wavelength, even if the
> > >significant part is only a tiny fraction. The lumped
> > inductor model is
> > >better replaced by a transmission line model, or better still, by a
> > >multi-element distributed physical analysis as provided in
> > CST Microwave
> > >Studio or the like. In practice, for good accuracy, such
> > analysis demands
> > >single-mode stimulus and termination.
> > >
> > >So finding the "inductance of a via" or the characteristic
> > of the via alone
> > >is to chase a rainbow.
> > >
> > >Cheers
> > >Geoff
> > >
> > > > -----Original Message-----
> > > > From: David Siadat [mailto:dsiadat@xxxxxxxxx]
> > > > Sent: 16 July 2004 18:04
> > > > To: hassan@xxxxxxxx
> > > > Cc: Ivan Ndip; si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Re: Via modeling & de-embedding
> > > >
> > > >
> > > > Hello Hassan,
> > > >
> > > > There are two related issues that should be discussed separately.
> > > > 1. The required length of the transmission line before the
> > > > higher order
> > > > modes from
> > > > the excitation wave port settle down as Giancarlo
> > > > indicated in his email.
> > > > 2. How much of the transmission line can be de-embedded to
> > > > investigate the
> > > > characteristics
> > > > of the via its feeding.
> > > >
> > > > So in a simulation model, you must have long enough
> > > > transmission line to
> > > > satisfy the first issue
> > > > and you should de-embedded to the edge of your anti-pad
> > > > radius (assuming
> > > > you have a reference
> > > > ground plane below the signal layer).
> > > >
> > > > The impedance of the via consists of the inductance of the
> > > > transmission
> > > > stub hanging off the via and
> > > > the via length as well as capacitance of the via which is
> > > > function of via
> > > > diameter, pad size (and number of them
> > > > in the stack up. you could minimize that by removing pads on
> > > > some layers),
> > > > anti-pad size ( you could choose
> > > > different size at different layers to optimize your impedance).
> > > > Lets not forget the via stub length (non terminated
> > > > transmission line) that
> > > > can oscillate at its resonant frequency.
> > > >
> > > > I should also point out the importance of return path for the
> > > > SINGLE via
> > > > model which impacts the inductance value
> > > > at lower frequencies.
> > > >
> > > > Regards,
> > > > David
> > > >
> > > > At 12:17 PM 7/16/2004 -0400, Hassan O. Ali wrote:
> > > > >Ivan,
> > > > >
> > > > >To know how long is long enough, you need first to
> > > > understand what a via
> > > > >model should
> > > > >accomplish.
> > > > >
> > > > >Assuming a stripline on either side of the via, a TEM wave
> > > > gets into the
> > > > >via and comes
> > > > >out as a TEM wave. At the via itself, though, the incident
> > > > signal in TEM
> > > > >mode gets
> > > > >converted into several other (higher-order) modes and then
> > > > back to TEM
> > > > >mode as it exists
> > > > >the via. The types of higher order modes depend on the
> > > > environment around
> > > > >the via. Some
> > > > >of the modes are evanescent (non-propagating) and some are
> > > > propagating
> > > > >modes. Some of
> > > > >the higher-order propagating modes have much higher
> > > > attenuation constants
> > > > >(i.e. they die
> > > > >off short distance away). Ultimately, one cares only about
> > > > the TEM mode
> > > > >that propagates
> > > > >far into the striplines on either side of the via.
> > > > >
> > > > >So, in this case, the via model should essentially
> > capture the mode
> > > > >conversion at the
> > > > >via. For that reason, you need to place your reference
> > > > planes at points
> > > > >where only the
> > > > >TEM mode exists (substantially). Unfortunately, the
> > > > determination of such
> > > > >points is
> > > > >problem-dependent. Certainly you shouldn't just use the
> > attenuation
> > > > >constant of the
> > > > >first propagating stripline higher order mode to determine
> > > > the location of
> > > > >such points.
> > > > >That's because you also need to be away from the effects of
> > > > evanescent
> > > > >(non-propagating)
> > > > >modes and other modes that propagate through the via in
> > a manner not
> > > > >supported by
> > > > >stripline.
> > > > >
> > > > >For each via configuration, I would suggest you start with
> > > > very short line
> > > > >segments
> > > > >(e.g. of length equal to the pad radius) and increase
> > the length in
> > > > >several steps to
> > > > >determine S21 (insertion loss) convergence for the
> > frequency band of
> > > > >interest. You
> > > > >should have to fix your own convergence criterion. I would
> > > > be comfortable
> > > > >with 0.5% S21
> > > > >covergence.
> > > > >
> > > > >Regards.
> > > > >
> > > > >Hassan.
> > > > >
> > > > >
> > > > >On Jul 16, Ivan Ndip <ndip@xxxxxxxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > Hi Hassan,
> > > > > > How "long enough" must the trace segments be? Do you have
> > > > any experience
> > > > > > in determining the exact length?
> > > > > >
> > > > > > Best regards,
> > > > > > Ivan Ndip
> > > > > >
> > > > > > Hassan O. Ali wrote:
> > > > > >
> > > > > > >It seems that "de-embedding" in this thread has been
> > > > used to suggest
> > > > > that one can
> > > > >create
> > > > > > >an "isolated" via model that can be used in various
> > > > other environments
> > > > > such as with
> > > > > > >different trace widths and thicknesses; different signal
> > > > rates, etc.
> > > > > If that is the
> > > > > > >case, then I think that suggestion is not accurate enough.
> > > > > > >
> > > > > > >I tend to believe that via characteristics (hence its
> > > > model) cannot be
> > > > > isolated from
> > > > >the
> > > > > > >surrounding environments - especially for higher
> > > > frequencies. The
> > > > > trace/via-pad
> > > > > > >transition itself presents a discontinuity that
> > > > contributes to the
> > > > > parasitics of the
> > > > > > >via. The pads have their own parasitics. Neighboring
> > > > ground/power vias
> > > > > also can
> > > > > > >influence the signal via performance. All of those along
> > > > with the via
> > > > > barrel
> > > > >parasitics
> > > > > > >contribute to the overall via characteristics and have to be
> > > > > appropriately considered
> > > > >in
> > > > > > >model creation.
> > > > > > >
> > > > > > >As to de-embedding, I think you won't get accurate
> > > > results if your
> > > > > reference (de-
> > > > > > >embedding) plane is right at the transition. In
> > other words, an
> > > > > accurate via model
> > > > > > >should include trace segments long enough to ensure that
> > > > the field at
> > > > > the segment
> > > > >ends
> > > > > > >are mainly of the dominant mode of field propagation. In
> > > > fact most (if
> > > > > not all) de-
> > > > > > >embedding techniques make that assumption.
> > > > > > >
> > > > > > >Regards.
> > > > > > >
> > > > > > >Hassan.
> > > > > > >
> > > > > > >
> > > > > > >On Jul 15, David Siadat <dsiadat@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > >
> > > > > > >>Hello Ivan,
> > > > > > >>
> > > > > > >>De-embedding the transmission line feeding the via is
> > > > limited to the
> > > > > > >>edge of the anti-pad region that the transmission line
> > > > is referenced to.
> > > > > > >>That could mean an additional 17.5 mil transmission
> > > > line for 35mil
> > > > > diameter
> > > > > > >>anti-pad. The inductance value of this transmission
> > line can be
> > > > > significant
> > > > > > >>compare with the inductance of the via itself.
> > > > > > >>
> > > > > > >>Regards,
> > > > > > >>David
> > > > > > >>
> > > > >
> > > >
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