Ned,
We donât have years of experience with PAM-4 at this point, but here is our
take:
The Channel modeling results isolating the different flavor of vias (carefully
isolation of the via with de-embedding, then creating pristine S-parameter
model and 3D EM corresponding model using good material ID) suggests backfilled
vias (return loss SDD11 lower than -24dB or so) are totally a low jitter, eye
mask solution for PAM-4, even up to 56G. The return loss numbers at the
lower frequencies are really important, so we typically optimize with a goal of
less return loss below Nyquist than we did with NRZ coding. You simply need
lower S/N at RX than you have before.
For a typical backplane application however, the connector SI and via launch
into/out along with the device package impact dominates the picture over
pristine via transitions to other layers. We didnât account for a possible
slight increase of crosstalk, very slight, however.
Having said that multi lamination fabrication is a pain in the neck with
respect to fab times and associated manufacturing risk.
We submitted an abstract-outline in DesignCon 2019 for a tutorial on building
test fixtures for 56G-112G PAM-4 and we will be playing with vias a bit.
- Al
Products for the Signal Integrity Practitioner
Alfred P. Neves
Chief Technology Officer
Office: 503-679-2429
www.wildrivertech.com
2015 Best In Design&Test Finalist
On Aug 7, 2018, at 1:46 PM, <Ned.Dempsher@xxxxxxx> <Ned.Dempsher@xxxxxxx>
wrote:
Hello SI LIST,
I'm looking for any guidance or papers regarding relative differences in SI
performance between stacked microvias (L1 to L6), traditional blind vias, and
backdrilled vias at high data rates - 28G/56G PAM4.
I did not seem to have much luck with Google unfortunately.
Are all three approaches viable if properly implemented at these data rates,
one better than another?
I'm assuming that any differences in layer-to-layer registration or the shape
of the microvia are effects that are in the noise?
I'm assuming that staggered vias , although supposedly better from a
reliability standpoint are verboten for SI?
Not too worried about cost or relative reliability at this point, just SI
performance.
Thanks Very Much Everyone!
Ned Dempsher
L3 Technologies
Camden, NJ
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