We use the following: 

0.25 oz - same top to bottom
0.35 oz - same top to bottom
0.5 oz - same top to bottom
1.0 oz - .0075" smaller top than bottom
2.0 oz - .0015" smaller top than bottom
3.0 oz - .0020" smaller top than bottom

The line calculation that our model uses is ((B-T)/2)+T

Where T=Top and B=Bottom



Jeffrey Bush
Director, Quality Assurance and Technical Support

                          76 Technology Drive - POB 1890
                             Brattleboro, Vermont 05302-1890
                                Tel. 802.257.4571.21 Fax. 802.257.0011
                                    [log in to unmask]
                              http://www.vtcircuits.com	



-----Original Message-----
From: Mark Larson [mailto:[log in to unmask]] 
Sent: Monday, May 03, 2004 12:05 PM
To: [log in to unmask]
Subject: [TN] impedance calculation


Hi,

when doing an impedance calculation of a trace more accurate results
come from using the trapizoidal shape of the trace cross section rather
than a square one, is there a rule of thumb people use for what the top
width of the trace is in relation to the base? obviously this will be
based on the copper thickness. Since it is such a small variable, is
there ever a situation where it is really important?

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