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]