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Date: | Thu, 05 Oct 95 11:51:26 EST |
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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]
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