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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