This is a very good question. I have had this discussion
        with many different end users. It always boils down to what the 
        trace was designed for. If the circuit design is one headed for
        RF or Microwave and is an outerlayer (Microstrip) or the first 
        signal layer down. (Embedded Microstrip), then the "Effective"
        linewidth will be the base of the trace. (Your 6.5 mil noted)
        In the situation of an innerlayer located between two reference
        planes (Stripline or Dual Stripline), then the cross sectional
        area that effects the electrical performance may be slightly
        different. (ie, the average)
                If the PCB is not intended for these types of environments
        then the commercial tolerance of +/- 20% of master pattern is
        used and generally the trace widths are fairly large. 
                The Seventh Working Draft of IPC-A-600 Rev E shows two
        different ways to measure the trace. One is to use the etched
        trace base or face depending on which is larger. The other is
        to use the base for impedance controlled product. Most of the
        PCB designs that I am fabricating are targeted for this market.
        (CI Product) We have standardized the measurement to be taken
        from the base due to this reason. 

        Groovy
______________________________ Reply Separator _________________________________
Subject: Conductor Width Measurement
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    10/4/95 5:11 PM


As a manufacturer of bare printed circuit boards, it is common 
for us to see minimum line widths called out on fabrication 
drawings.  A question has been raised as to the proper method 
for measurement of finsished line width.
     
When the circuit is etched, the copper is typically thinner at 
the top than at the surface of the laminate. (Bare copper inner 
layer)
     
(Crude cross-section graphic to follow)
     
     
     
     
     
            |______6.6 mils______|   
            |                    |
            | |___ 5.5 mils____| |
            | |________________| |
            | /                \ |
            |/                  \|
 -----------'--------------------'----------------
     
     
     
In the above graphic, the line width at the base of the circuit is 
6.5 mils.  At the top of the circuit, it measures 5.5 mils.  If the 
drawing requirement is 6.0 mil minimum, would this meet the drawing 
requirments?
     
In the past, I have always considered this to be a 6.5 mil circuit. 
Is this correct thinking?
     
A review of IPC-T-50 terms and definitions says that the conductor 
width is "The observable width of a conductor at any point chosen at 
random on the printed wiring board normally viewed from directly 
above unless otherwise specified."  
     
If this circuit is viewed from above with backlighting, the 
observable width is 6.5 mils.  When viewed from above with top 
lighting, the appearance is 5.5 mils.  Is this issued clarified 
in any of the IPC specifications?  Is there a consensus as to 
the meaning of "observable" as it applies to conductor width 
measurements?
     
Jim McNeal
     
=====================================================
     
Jim McNeal  Q.A.and Eng. Mgr.
Electro Plate Circuitry, Inc.
Carrollton, Tx
Ph. 214-466-0818
EMAIL  [log in to unmask]