[SI-LIST] Re: surface roughness
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: "'Oluwafemi, Olufemi B'" <olufemi.b.oluwafemi@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Nov 2011 21:37:22 -0800
Femi,
I totally agree with you on need of further research in the roughness
characterization. That is exactly why we are discussing the problem here at
SI LIST and at the conferences focused on SI analysis, such as DesignCon.
Professor Huray's and your research results are very important for
understanding of the roughness characterization. If Huray's model works, it
should be available in a software for advanced signal integrity analysis - I
do not see any problem with that. Due to the simplicity of the final
equations, it will probably be in any SI analysis tool very soon. Though,
there are some question.
Can Huray's model be used for any type of rough conductor surface?
If not - how to distinguish. If yes, why surfaces that do not look like
pyramids of snowballs by any imagination can be still modeled with that
particular approximation?
Is there a procedure to find parameters of the model for a given surface
micro-structure - ball radius, base size and number of balls (assuming that
we have equipment to get the details of a surface)?
Why Huray model does not explain the capacitive effect of roughness?
Are there any other models that we have to consider in the signal integrity
software?
A few words in defense of Hammerstad model. First of all, it is physics
based - Morgan's model is physical. A fit to a modeled data can be
considered an empirical macro-model. Morgan investigated just a few profiles
with the common increase in length along the surface equal 2 - that where
the main restriction came from. The shape of the surface had minor effect -
here is a commonality with your conclusion on independence of absorption
from actual geometry of piled balls. Hammerstad coefficient simply describes
increase in attenuation due to transition from regular skin-effect without
roughness to the skin-effect on a rough surface at frequencies where the
details of the surface are much smaller than wavelength of propagating wave.
It worked for microwave applications and saturation at 2 did not matter due
to very small roughness in microwave ICs (only the onset of the curve was
used). PCB and packaging applications are different. A few years ago I
followed the Morgan/Hammerstad approach and performed numerical analysis of
surfaces with the increase in length smaller and greater than 2 using
Trefft'z finite elements (not published yet). The fitted formula had 2
parameters and was close to Hammerstad correction coefficient for surfaces
with doubled length (within 10%). Later I realized that Hammerstad
correction coefficient can be extended in the same way as the new model with
the roughness factor - maximal possible increase in length (or area for
radial waves) comparing to the flat surface. The modified Hammerstad
correction coefficient is simple and it works for practical applications. As
Scott mentioned, with the separation of conductive and polarization losses
based on a causal dielectric model and phase and group delay fit,
technically anything can be used to account for the remaining losses due to
the skin effect on rough surface. Contribution of the rough conductor losses
to the phase is minimal due to the smallness of the internal inductance at
frequencies with well-developed skin-effect.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
Office +1-702-876-2882
Cell +1-206-409-2368
Skype: shlepnev
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Oluwafemi, Olufemi B
Sent: Monday, November 28, 2011 10:34 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: surface roughness
All,
Maybe it's going to take adding the alphabets A thru z as coefficients
behind Hammerstad equation, before the SI community realizes that the
equation is fundamentally flawed.
Here are some reasons why the equation is flawed:
1) It is an empirical fit to Morgan's data that was published in 1948. There
is no physics to it. "Effect of surface roughness on eddy current sources at
microwave frequencies" Journal of Applied Physics V.20 pp 352-362, 1948
2) The ratio you get from the equation is going to saturate at 2; no matter
how rough the copper is.
3) The only input to Hammerstad equation is rms value. As you all know, it
is possible to have a rough and relatively smooth copper have the same rms
value. Thus Hammerstad equation will give you the same ratio for both.
Apart from providing an equation to use, what Huray et all presented at
DesignCon 2010, was to explain the physics of surface roughness from an EM
perspective.
The presentation also debunked the myth that says, current flows in and out
of the rough structures.
As a community, I think we need to start going back to the physics of the
problem, rather than, rules of thumb or equations that have been passed down
that might not work. We can't keep patching...
I'm not trying to convince anyone to use the Huray Model, but to look at the
physics of the problem.
By the way the Huray model correlates in phase as well. Please check pg
83-84 'Surface Roughness and Its Impact on System Power Losses' Olufemi
Oluwafemi, ProQuest, Ann Arbor , MI.
My two cents,
Thanks,
Femi Oluwafemi
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Tuesday, November 22, 2011 4:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: surface roughness
All
I'd like to be clear. Any simulation model for dielectric and copper
surface roughness modeling must be a good match to measured data in
amplitude and PHASE to be an accurate model. It is clear to me that the
papers presented on the Huray method have thus far not shown any phase
correlation data.
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
Teraspeed(r) is the registered service mark of Teraspeed Consulting Group
LLC
On 11/20/2011 1:59 PM, Yuriy Shlepnev wrote:
> 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=60
> 38367&
> openedRefinements%3D*%26filter%3DAND%28NOT%284283010803%29%29%26search
> Field%
> 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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