[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency
- From: Andy Kuo <andyk@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 24 Nov 2003 17:04:37 -0800 (PST)
Thank you everyone for replying to my question.
Seems that FR4 might not be a good material to use for PCBs running at
over 1Ghz frequency. Another question I have is: which other materials
do people typically use instead of FR4 for high speed PCB designs?
Thanks in advance,
Andy
On Mon, 24 Nov 2003, Alex Horvath wrote:
> Exactly, it seems to be fairly common misconception that skin effect losses
> always dominate. Dielectric losses are proportional to frequency, although
> they start out lower they will always exceed skin effect losses between 500
> and 1Ghz or so. The biggest factor affecting this seems to the resin content
> of the PCB.
>
> I'd like to hear more about the properties of the resin, if it can be
> manipulated for less loss and what are the tradeoffs. I wasn't able to find
> out much about resin properties on the web.
>
> Thanks
> steve weir <weirsp@xxxxxxxxxx> wrote:
> Stuart, for common dielectrics, somewhere in the high hundreds of MHz
> dielectric loss overtakes loss from skin effect.
>
> Steve.
> At 05:16 PM 11/24/2003 -0500, Stuart Brorson wrote:
> >True enough, but aren't the dielectric losses in FR-4 more or less
> >frequency independent (i.e. to the <10% level that the dielectric
> >function is constant)?
> >
> >Anyway, Tom is right: you need to consider the frequency variation of
> >both Dk (the dielectric function) and Df (the dielectric loss
> >function). However, both are typically dominated by skin effect
> >losses, particular in the GHz range.
> >
> >Stuart
> >
> >
> > > To make the answer complete you need also to include dielectric losses
> > which
> > > also are frequency dependent.
> > >
> > > Tom Dagostino
> > > Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
> > > 2926 SE Yamhill St. Device Modeling Division
> > > Portland, OR 97214 13610 SW Harness Lane
> > > Beaverton, OR 97008
> > > http://www.teraspeed.com 503-430-1065
> > > tom@xxxxxxxxxxxxx
> > >
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Stuart Brorson
> > > Sent: Monday, November 24, 2003 1:22 PM
> > > To: andyk@xxxxxxxxxx
> > > Cc: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz
> > > frequency
> > >
> > >
> > > If I am not mistaken, your question is equivalent to this one: Is the
> > > dielectric function constant over the 1 MHz -- 1 GHz range? The
> > > answer is: It is not constant, but varies by several percent or more
> > > over that range for many FR-4s. (FR-4 is not a single material, but
> > > is a family of materials, each different depending upon your PCB
> > > vendor, and whatever batch of materials he was shipped on any
> > > particular day.)
> > >
> > > However, the bigger problem for you to think about is the loss due to
> > > the skin effect in your copper traces. The skin effect losses vary as
> > > the square root of the frequency, IIRC. The frequency-dependent loss
> > > due to the skin effect dominates over the frequency variation of the
> > > FR-4's dielectric constant.
> > >
> > > Here are some references:
> > >
> > > http://www.tdasystems.com/library/appnotes/siml0803.pdf
> > > http://signalintegrity.com/Pubs%5Cnews%5C4_5.htm
> > >
> > > Stuart
> > >
> > >
> > > > Hi all,
> > > >
> > > > I am designing a high speed PCB at over 1Ghz frequency using FR4
> > material=
> > > > .
> > > > I am wondering how much the dielectric constant will be affected by the
> > > > frequency over 1Ghz as compared to 1Mhz?
> > > >
> > > > Thank you
> > > >
> > > > Regards,
> > > > Andy
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