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December 2001

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From:
"Mcmaster, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 3 Dec 2001 11:49:59 -0800
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I know you've had answers to the contrary, but the answer to the first
question is "yes".

When an electric field is applied to an insulator, components of the
insulator re-arrange themselves in response to this field.  The components
range from the sub-atomic (electrons and nuclei) to, in polymers,
macromolecular chains.  The dielectric constant of the insulator is very
dependent on how much of this re-arrangement can occur.

In an uncured polymer, the polar chains of the monomer can move more freely
under the applied electric field.  When cured, the chains become crosslinked
and movement is restricted.  The material can no longer hold as much charge
and the dielectric constant will go down.

This same mechanism explains why dielectric constant is changed by
temperature and frequency.  A material above it's Tg will have a higher
dielectric constant than it does below Tg.  Above Tg the polymer is freer to
re-orient itself to the electric field.  Frequency looks at the same thing
just with respect to time.  Under DC voltage, the dipoles in the polymer
have an infinite time to respond and will increase the dielectric constant
as they do so.  Start alternating the field though and now the most
restricted parts of the polymer can no longer respond.  The dielectric
constant starts to drop.  As you keep increasing the frequency, less and
less of the dipoles within the material can respond fast enough to keep up.
An analogy is the relative rate at which you can wave your arms up and down
versus wiggling your fingertip.  The less you have to move the faster you
can do so.

The answer to your second question is "yes" also, but for a different
reason.  An FR4 board that was undercured enough to see a significant impact
on electrical properties would have enough thermal and mechanical issues
from the undercuring that it would be unusable.  What press parameters can
influence is the amount of fill and flow of the resin in the prepreg and the
finished press thickness of these dielectric.  The "science" of pressing
PWBs revolves around balancing the needs to have a material that flows then
cures.  As the prepreg is heated up the resin viscosity drops and the
material starts to flow, filling in the circuits on the adjacent layers.
But the crosslinking reaction is also thermally driven.  As the resin cures,
the viscosity drops.  Balancing these competing mechanisms is done by
adjustment of press temperature ramp rates as well as application of vacuum
and pressure.



> ----------
> From:         tech[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.
> Sent:         Thursday, November 29, 2001 7:00 AM
> To:   [log in to unmask]
> Subject:      [TN] Dielectric constant vs degree of cure , FR4
>
> Does the dielectric constant ( and resultant impedance results) of FR4
> vary with degree of cure of the resin system ? I.e can multi layer press
> parameters influence finished board impedances ? Any info. will be
> appreciated.
> Paul Greene
>
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