Earl

We don't try to adjust the cure to fine tune the dielectric properties of
the finished board.  I didn't mean to imply that.  I apologize if I didn't
make that clear.  You still have to advance the cure of the prepreg enough
that the board sticks together and has the necessary mechanical and thermal
properties for the board to withstand assembly and perform reliably.  I
don't have data to back this up, but I suspect that the amount of
undercuring necessary to cause a significant shift in the dielectric
constant would not be satisfactory.

For the finished board electrical properties, more important than the degree
of cure is how you get there.  The press cycle parameters (how fast do you
heat up, what temperatures do you hold it at and for how long, when do you
apply pressure and how much) determine the amount of resin flow and the
curing kinetics.  All of these effect the pressed finished thickness and
thus the dielectric constant of the prepreg opening.  Two different
manufacturer's can build the exact same board out of the exact same
material, each using a different press cycle and the boards will have
different impedance.  Neither is doing anything wrong, just different.

How much different?  Probably a few tenths of a mil per opening.  On a 6 or
8 mil dielectric spacing this is probably insignificant.  But if you are
looking at 4 mils or less, it starts to be significant if you're shooting
for a 10% or less spec.  What's important is for the fabricator to know how
their boards are going to turn out and do it repeatably.  And to be able to
predict the impedance on a design before it's going to be built.

As for our plans, that's where our focus is.  SPC in conjunction with our
suppliers on incoming material and pressed prepreg thickness to characterize
and minimize variability.  SPC on line width control.  And improved
understanding of TDR measurement and modeling.



> ----------
> From:         Earl Moon[SMTP:[log in to unmask]]
> Sent:         Monday, December 03, 2001 12:03 PM
> To:   [log in to unmask]; [log in to unmask]
> Subject:      Re: Dielectric constant vs degree of cure , FR4
>
> Michael,
>
> Your very eloquent answer probably is quite correct. However, how many of
> us
> work in an uncured or over cured world.
>
> I know the question was stated as the difference between cured and
> uncured,
> but how can we make boards with less or more cure that are acceptable as
> specified. Out of the window is out of spec, right?
>
> Having worked with very low dielectric constant materials (styrafoam as an
> example), approximating air, I know it is not practical to attempt, at
> this
> time, building circuitry with such materials. My answers to the initial
> question were based on what is required to make a circuit practical in
> terms
> of manufacturability, acceptance, and performance.
>
> I realize you folks at Merix are on the cutting edge. If you could share a
> bit of your future plans, based on your excellent answer to the original
> question concerning degree of cure, we would all appreciate this.
>
> Respectfully,
>
> Earl Moon
>

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