Since this is an interpolation of the dielectric constant of two
different materials forming a suspension (of a solid within a liquid,
hence, fiberglass within epoxy resin) which varies directly with the
proportions of either substance depending upon the process of the
laminate manufacturer, why not consult your PCB vendor and get the exact
figures of their laminate supplier? Keep in mind that the mean
dielectric constant of prepreg may be somewhat different than that of
rigid laminate. Your PCB vendor can also help you with the implications
of the combined thickness of prepreg and laminate in your particular
multilayer stackup. After you find the values of your particular
stackup, capture the complete stackup combination in a dimensioned cross
section of your PCB on the fab dwg. This initial effort avoids the added
cost of specifying the impedance of a trace group and having your vendor
produce the desired impedance. It also insures that subsequent PCB lots
shall be built in a like fashion to the original, with like performance.
Remember that you must factor a tolerance of +/- 10% in any overall
laminate thickness specification. This tolerance directly applies to the
impedance since the impedance of a microstrip or stripline trace is
determined by the dielectric spacing of the trace to the ground plane.

Chris Taylor
Senior Printed Circuit Designer
LXE, Inc.
125 Technology Parkway
Norcross, GA 30092
voice: (770) 447-4224 X3242
fax: (770) 447-6928
e-mail: [log in to unmask]


>----------
>From:  Olson, Jack C[SMTP:[log in to unmask]]
>Sent:  Tuesday, March 03, 1998 12:21 PM
>To:    [log in to unmask]
>Subject:       [DC] dielectric constant
>
>WARNING: this is a very unexciting question...
>
>My software does some basic signal integrity analysis,
>and I just noticed it defaults the dielectric constant for
>regular FR4 to 4.7
>
>I was under the impression that most FR4 is at 4.2-4.3
>these days (more resin? thinner cores? can't remember...)
>
>Even a brief response will be appreciated!
>                                                                Jack
>
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