[SI-LIST] Re: Conductor surface roughness
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Sukumar <sukumarm@xxxxxxxxxxx>
- Date: Sun, 9 Mar 2014 09:28:04 -0400
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® is the registered service mark of
Teraspeed Consulting Group LLC
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