RE: [SI-LIST] : Dielectric Material Comparison

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From: Mark Geddes (mark@AudeSi.com)
Date: Fri Apr 14 2000 - 15:32:21 PDT


I'd suggest this paper by Lee Ritchey (available on his website).

http://www.speedingedge.com/docs/tutorial.doc

Here is a summary of what I got out of the article for my applications:

- The relative dielectric constant (er) will affect the wave velocity, so
all materials with an (er) around 4 will all have similar wave velocities.
For anything most digital designers are doing today FR4 is adequate (5.82
inches/nsecond).
- (Tg) glass transition temperature has mainly to do with the thermal
characteristics & how much the board will expand when heated. You may have
to use material other then FR4 for boards over 90mil thick, to avoid
cracking vias during the soldering process. If you are making standard 62mil
boards, that aren't being used in high heat applications, then FR4 is great.
- The (DBV) dielectric breakdown voltage lets you know at what point the
dielectric will start breaking down when exposed to high voltage. If you are
dealing with a few mils between layers & less then a couple thousand volts
on your PCBs, then almost any "glass woven" product, such as FR4, will do.

As far as signal quality is concerned I believe that (er) is probably your
main concern. Check the edge rates & trace lengths on your PCB. I would
doubt you are doing designs that can't be designed using FR4.

These are mostly my impressions on what's important on my designs. I'm
definitely not an expert.

Mark Geddes
(I am not an FR4 salesman :-)

> Does anyone know of a good paper or text that gives an introductory
> explanation of how dielectric materials affect signal quality? I have
> information comparing the physical properties of commonly
> used dielectric
> materials (FR-4, GETEK, Teflon, etc) but I don't understand
> the relationship
> between these properties and what will actually happen to a
> signal that's
> traveling through that material.
>
> Thanks,
>
> Lisa DeSandoli
> Test Technology Engineer
> PMC-Sierra, Inc
>
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