[SI-LIST] Re: FR4

  • From: Ritchey Lee <leeritchey@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 11 Jul 2001 14:45:59 -0700

Depends on what conditions are when the measurement is made.  At the glass to
resin rations used in multilaeyr PCBs and at the edge rates we all deal with,
4.1 is much closer to the real number.  Remember, there are two uses for er,
impedance calculations and velocity or propogation delay.

For the high laeyr count PCBs that I design daily, the resin ratio is so high
that the er is closer to 3.7.  This is probably true for most of us in 14 plus
layer situations.

Lee

Rich Peyton wrote:

> Thanks All,
>
>   Before I got any replies I found a Reference that stated FR4= 4.5 +/- 0.5
> which is pretty close to all replies.
>
> Thanks,
> Rich
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Martyn Gaudion
> Sent: Tuesday, July 10, 2001 4:57 PM
> To: si-list@xxxxxxxxxxxxx
> Cc: Rich Peyton
> Subject: [SI-LIST] Re: FR4
>
> Rich,
>
> Its well worth taking a look at the laminate vendors web sites.
>
> It is also worth noting that FR4 is a resin glass
> composite based on Glass with Er of around 6 and
> resin with Er of around 3.
>
> If you are designing differential traces on fine
> spacings you need to take this into account.
>
> Typically most fabricators use 4.3 or 4.2 for single
> ended designs, but for close spaced differential traces
> they use 3.6 to 3.8
>
> An explanation of this effect is here:
>
> www.polarinstruments.com/support/cits/AP139.html
>
> Kind regards
> Martyn Gaudion
>
> At 13:57 10/07/01 -0400, you wrote:
>
> >Hi
> >
> >what is the dielectric constant for FR4?  Or a web site with listing of
> >typical materials and there constants?
> >
> >Thanks,
> >Rich
> >
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