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

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text
in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for
additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700
ext.312
################################################################

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################