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Date: | Fri, 18 Dec 1998 11:51:37 -0500 |
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Hi Steve,
It has been my experience that, with FR-4 materials (sorry, but don't
recall whether you ever mentioned the material) you will never get Dk
reduced to a constant. Different fab shops use different material
suppliers, and varying combinations of prepregs to achieve a given
thickness. This alone lends variability to Dk. Further, the design
and density of circuitry on a given layer will cause prepreg resin to
flow away from or be retained around traces, thus changing the Dk in
specific regions. Other variables such as signal speed and retained
moisture make it even less constant.
I would suggest, if the impedance tolerance is as crtical as you say,
that you select one fab house to dial in the performance you are
looking for and then stick with that one shop. In this way, you will
eliminate the majority of the variation. You may want to move to a
more Dk-stable material, such as Getek or Rogers 4003 - somewhat lower
Dk value, but less susceptable to the secondary influences.
Other than that, I agree with previous submissions that x-sectioning
is really the only way to get accurate measurement. Unfortunately,
this only gives you a single point on a trace - generally on a coupon
- that may or may not be truly representative of the majority of the
layer.
Just my thoughts...Andy Slade
______________________________ Reply Separator _________________________________
Subject: Re: [TN] Dielectric Spacing Measurement
Author: "Joy; Stephen C" <[log in to unmask]> at SMTPLink-Hadco
Date: 12/18/98 7:31 AM
Les,
The problem with TDR is that everyone has a good P/T but can't correlate to
anyone else.
My search for a non destructive method of checking dielectric spacing is to
find
a mechanical way to measure impedance.
I am only concerned with trace perimeter and dielectric thickness. Dk is
more
difficult to measure and I hoped I could make it a constant at the nominal
impedance value we fix and the construction we specify. I know we are not
the
PCB experts, but the tolerances are so tight that we need at least to create
a
consensus from the PCB manufacturers for construction.
Is there any hope here or am I just wasting my time?
Thanks for all the responses on this.
Steve
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