[SI-LIST] Re: surface roughness
- From: "Todd Westerhoff" <twesterh@xxxxxxxxxx>
- To: "'si-list'" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 21 Nov 2011 07:39:16 -0500 (EST)
I'm with Yuriy and Jeff on this one. The physical details of copper
roughness are complex and hard to measure; we can create simulation models
that represent arbitrarily detailed physical features, but we rarely (if
ever) get the measurement data from PCB manufacturers to plug into those
models. What good is a detailed model if we have no data from the
manufacturer to plug into it?
We've found the process Yuriy describes to be very useful. Starting with
measured data, we adjust the trace roughness factor to correlate to
measured data. The process is incredibly valuable, as we gain great
insight in the characteristics of the same board built by different
suppliers (and even the manufacturing process control within the same fab
run). Once the models are tuned in, automated interconnect extraction and
simulation allows us to validate the behavior of all the links in a high
speed system as a batch simulation process.
Hope that helps,
Todd.
--
Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@xxxxxxxxxx
www.sisoft.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Yuriy Shlepnev
Sent: Sunday, November 20, 2011 2:00 PM
To: nagachander.sing@xxxxxxxxxxxxxxxxxxxx; 'si-list'
Subject: [SI-LIST] Re: surface roughness
Hi Naga,
As Jeff mentioned the theory of conductor roughness is still evolving
research area and there is a lot to do here. However, this may be
important only for the completeness of the theories. For practical purpose
we can use heuristic models that capture the macroscopic observable effect
of the roughness (similar to how we characterize dielectrics). Practically
any roughness theory produces a roughness correction coefficient, that can
be used to adjust surface impedance of a conductor surface locally for
accurate electromagnetic analysis or to adjust skin-effect matrix
extracted with a static field solver (in case if less accurate static
field solver is used for the analysis). In both cases the parameters of a
roughness model can be identified by fitting the parameters of the
correction coefficients to measured data (reflection-less generalized
modal S-parameters are best suitable for such fitting). See a brief
overview of roughness correction coefficients in Y. Shlepnev, C.
Nwachukwu, "Roughness characterization for interconnect analysis". - Proc.
of the 2011 IEEE International Symposium on Electromagnetic Compatibility,
Long Beach, CA, USA, August, 2011, p. 518-523
- paper is available at
http://ieeexplore.ieee.org/search/freesrchabstract.jsp?tp=&arnumber=603836
7&
openedRefinements%3D*%26filter%3DAND%28NOT%284283010803%29%29%26searchFiel
d%
3DSearch+All%26queryText%3Dshlepnev
Sorry for the long link - you can just search for authors names at
ieeeexplore.org, and paper and presentation are also available at
www.kb.simberian.com (in publications and presentations sections,
registration is required).
This paper proposes a simple 2-parameter Modified Hammerstad Correction
Coefficient (MHCC) as an extension of 1-parameter Hammerstad model used in
microwave IC applications for decades. The paper shows how to identify 2
parameters of MHCC by fitting GMS-parameters, assuming that the parameters
of a broadband dielectric model are identified separately. The model shows
excellent correlation with measurements up to 50 GHz. Parameters in
Huray's snowball model can be also identified with GMS-parameters with
minimal knowledge about the surface structure - no expensive conductor
surface investigation is required for such identification. Note, that the
identified Huray's model produces practically the same results as MHCC -
we will show that in our oncoming paper at DesignCon 2013. Both models are
capturing well the effect of additional absorption by surface due to
increase of the surface area.
Considering the roughness effect on PCB or packaging interconnects, I can
draw a parallel here with the wideband Debye dielectric model (also known
as Djordjevic-Sarkar or as Swenson-Dermer) that produces very good
correlation in analysis of interconnects in PCB and packaging applications
over ultra-wide frequency band. The model describes a dielectric with a
continuous spectrum over a wide frequency band. It correlates well with
the measured data and is used in practically all SI software nowadays.
Though, as engineers we do not care much about the internal structure of
the composite dielectrics that produces behavior captured by the model -
all we need is 2 or 3 model parameters that characterize a particular
dielectric. A multi-pole Debye model has more parameters can be used to
describe almost any dielectric used in PCB/Packaging and we do not care
what structure produces those poles - all we need are just values of the
poles and residues that produce good correlation within the spectrum
bandwidth of our signal.
Considering the effect of roughness on degradation of signal in
PCB/packaging interconnects, rough surface leads to increase of
attenuation (insertion loss) and increase of inductance at high
frequencies due to simple increase of the total surface of the conductor.
Both MHCC or Huray's snowball models capture this increase in loss and
inductance if applied appropriately in the electromagnetic analysis.
Conductor surface impedance adjustment for instance produces causal models
that take into account current distribution within a conductor due to
roughness on just one side of plane or strip or due to additional plating
layers for instance. Note that both low and high profile copper roughness
lead to a substantial increase in attenuation above 10 GHz. In addition
roughness also increases the capacitance of conductor - that is clearly
visible in cases with large roughness and thin dielectric layers - see the
paper and presentation cited above. The capacitive effect of roughness due
to surface singularity is practically frequency-independent and was
reported in IC applications a few yeas ago - see for instance this paper
and references there: A. Albina at al., Impact of the surface roughness
on the electrical capacitance, Microelectron. J. 37 (2006) 752-758.
Finally, if you need a software that correlates with the measured data up
to
50 GHz, it has to have at least wideband and multi-pole Debye models for
dielectrics in addition a selection of advanced models for roughness (MHCC
or Huray's snowball models for instance) that can be easily identified
without expensive investigation of the surface structure. Classical
Hammerstad model has low accuracy and not usable above 3-5 GHz in general.
The software has to be validated with measurement over the bandwidth of
your signal. Though, the software is only a part of the story here. In
addition you have to establish a procedure to identify the parameters of
both dielectric and conductor models. Without the rigorous material
parameters identification it is "garbage in - garbage out" no matter how
advanced is the analysis. Models that describe dielectrics and roughness
over a wide frequency band from MHz to 50 GHz are typically not available
from manufacturers at this time and have to be identified. Companies like
Teraspeed Consulting Group for instance can help you to establish such
material identification procedure.
Best regards,
Yuriy Shlepnev
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of nagachander.sing@xxxxxxxxxxxxxxxxxxxx
Sent: Friday, November 18, 2011 11:35 PM
To: si-list
Subject: [SI-LIST] surface roughness
Hi all,
What is the best tool to simulate surface roughness when simulating for
the differential S-parameters for striplines and microstripline in IC
packages?
I am interested in simulating till 50GHz.
What kind of effects are you seeing with surface roughness? i mean things
like does your insertion loss increase? what abt diff return loss?
Any good papers to understand the problems in detail...
Any feedback will be appreciated
Thanks,
Naga
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