Lee

The frequency at which "skin depth" of Cu equals 0.5oz(17.5um) is only ~13MHz. For 1oz(35um) ~ 3.5MHz, so a 2oz track or foil in
free space will start to be less useful above 3.5MHz.

Becomes pretty significant for power circuits (remembering of course the 0.4% TCR of Cu!), and somewhat limiting if you need to
push GHz signals any distance(just as well dielectric losses other things become more of an issue in the GHz, or we would all be
hung up over "skin effect")

Getting back to the original Q, assuming a covered microstrip over return path plane then the difference in characteristic
impedance for 0.5oz and 2oz will be in the order of about 8% (rough guess). Most of the signal will be propagating in the space
between the trace and plane so the track thickness is not terribly important

Best Regards

David Greig
______________________________
GigaDyne Ltd
Buchan House
Carnegie Campus
Dunfermline KY11 8PL
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: 09 December 2005 22:09
To: [log in to unmask]
Subject: Re: [TN] External Plating - Impedance Calculation

John

I believe the Polar web site shows the typical geometry of the electro-magnetic flux lines extending from the signal trace to
the reference ground. They are distributed along the sides of the trace from the top of the copper to the dielectric. I always
use the entire copper thickness of the trace for the impedance calculation. You may have in mind the skin effect which is a
different issue altogether. If the frequency is in the range of gigahertz the skin effect becomes important when calculating the
ohmic contribution of  the resistance ("R") since most of the current is transported within the outer portion of the trace; not
the core, the effective copper thickness is greatly reduced.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
  ----- Original Message -----
  From: Roger Mouton<mailto:[log in to unmask]>
  To: [log in to unmask]<mailto:[log in to unmask]>
  Sent: Friday, December 09, 2005 4:04 PM
  Subject: Re: [TN] External Plating - Impedance Calculation


  John,

  Let me add to what Jeffrey has said by pointing out that the plated thickness uniformity must be reasonably close so that the
values you are assigning will have validity.  I've seen plated thickness uniformity vary from 1.1 mils to over 3.0 mils on the
same board.  I hope others will weigh in on this because I'm told plated thickness can significantly affect the impedance
values.

  Roger Mouton

  EIMC - Advanced Plating Technologies
  949 481-5194
  www.smartcatshield.com<http://www.smartcatshield.com/>
  [log in to unmask]<mailto:[log in to unmask]>



  -------------- Original message from Babcock John <[log in to unmask]<mailto:[log in to unmask]>>: --------------


  > Hi All,
  >
  > When figuring impedances with the field solvers...for the external layer
  > copper thickness does one enter the finished plated value or the
  > unfinished?
  >
  > If you specify a top and bottom layer as 1.5 oz plated to 2 oz, which
  > value would you put into the field solver for the impedance calc?
  >
  > I would figure you put in the finished plated value...but not positive.
  >
  > Thanks!
  >
  > John
  > PCB Designer III
  > Isothermal Systems Research
  >
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