[SI-LIST] Finding the impedance of a PCB trace

  • From: "Graham Davies" <GrahamDavies@xxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 29 Apr 2005 21:28:30 -0000

This is, of course, related to my search for a good 6-layer stackup.

What is the impedance of a PCB trace (microstrip) with the following 
Width ......................... 6.00 mil
Height above return plane ..... 3.20 mil
Thickness ..................... 1.35 mil (1 ounce copper)
Dielectric rel. permittivity .. 4.2

These are the answers I get from various tools:

41... ohms (http://www.icd.com.au/)
41.69 ohms (http://www.emclab.umr.edu/pcbtlc/microstrip.html)
41.69 ohms (TraceSim version 1.0.0)
41.69 ohms (http://www.technick.net/public/code/cp_dpage.php?
41.7 .ohms (http://www.sunmantechnology.com/resources/cal_cat00.shtml)
46.14 ohms (ZTOOL Impedance Calculator V1.2)
47.31 ohms (http://www.rogers-corp.com/mwu/mwi_java/mwij_vp.html)
50.89 ohms (http://www.csgnetwork.com/boardrunimpcalc.html)
51.27 ohms (TxLine version 1.1)

Does anyone have an idea as to why these calculators give results 
that vary by 20% and which are more reliable?


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