[SI-LIST] Re: Conductor surface roughness
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Mon, 10 Mar 2014 14:12:03 +0000
Hi Scott,
I agree wholeheartedly with the first two paragraphs, but there were some items
after that contrary to my experience:
* In the study called out in our 2013 DesignCon paper, we found that
the Dk/Df values given by the material vendors allowed very good correlation to
8GHz. We found no cases where they were "dead wrong". Are you talking about
higher frequencies perhaps?
* For microstrip on server boards, soldermask plays a very minor role,
and the OSP or ENIG finishes play none, since they are only applied to very
small portions of the traces (unless there has been a manufacturing error).
Jeff Loyer
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Sunday, March 09, 2014 6:28 AM
To: Sukumar
Cc: chockalingam.s@xxxxxxx; Loyer, Jeff; Lee Ritchey;
suresh.kondepati@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Conductor surface roughness
Sukumar,
First, it is unlikely that the number your manufacturer gave you for surface
roughness has any relationship to what you would enter in your particular field
solver. Solvers that use the classical Hammerstadt equation, or it's variants,
are not calibrated to actual roughness values. Rather, the RMS number entered
is more of a tuning number to obtain the desired result. For solvers that use
the Huray model, you're pretty much on you own. Since the final as-fabricated
values of the material are unknown, the Huray model allows for two tuning
parameters, which you must adjust, without guidance from material suppliers or
your fabricator.
Second, even if the roughness of the copper before lamination is XX microns
RMS, it is likely that this will be altered by the adhesion enhancement process
for lamination. Roughness is always greater than what you started with.
Finally, if your design is such that the roughness of copper will make or break
the design, then using the material parameters provided by your fabricator, or
their laminate vendors, is not the way to go. In my experience, Df, Cu
roughness, and sometimes Dk will be dead wrong. The measurements that laminate
manufactures use for the characterization of dielectric properties was intended
for bulk microwave material characterization. These methods are not accurate
for the typical narrow microstrip or stripline used on most digital boards
today. Actual dielectric material parameters are guaranteed to be higher than
what is published. There are real, physics-based reasons for this.
In the end, if a design is so sensitive that Cu roughness or Df is crucially
important to us, then it is our job to either measure these as-built properties
ourselves, or have a specialist perform the necessary measurements for us.
Oh, and for microstrip, don't forget to include the trace coating used for your
loss calculations, Soldermask, OSP, and Enig will tend to dominate your loss
characteristics.
regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
Teraspeed(r) is the registered service mark of
Teraspeed Consulting Group LLC
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