[SI-LIST] Re: PCB Insertion loss prediction
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Thu, 8 Nov 2012 09:12:46 -0500
Jeff
A few quick comments. Although the tanD of Meg 6 is stated to be flat, it
is not if you measure it. The manufacturer reported characterization in
the data sheet is not correct. Causality is violated when tanD is flat.
Loss is generally due to molecular dipole losses in the material. It can
be low for high Er, as is the case with ceramic.
Hygroscopic loss is due to molecular polarity. Polar molecules "glom" on to
water molecules, which are also polar. Same property makes the material
extremely "sticky."
The paper that Jason Miller of Oracle and I wrote for DesignCon last year
covers some of the impact of temperature and humidity on measured losses.
I don't have access to my storage server right now, otherwise I'd give a
paper citation.
regards,
Scott
On Fri, Nov 2, 2012 at 11:37 AM, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
> I realized we hadn't answered the basic question - "why does a high resin
> prepreg give lower loss?" The prediction of loss vs. resin content isn't
> trivial; as Steve said, a tool which allows you to model loss for the
> various scenarios should be on your Christmas wish list. Here are the
> factors that I know of (thanks to Richard Kunze for clarifying things for
> me, and I welcome others' data/opinions):
> * Resin has a lower Er than glass
>
> * loss is approximately proportional to Df * sqrt(Er), so lowering Er
> lowers loss
>
> * lower Er allows wider traces for the same impedance - this may
> decrease loss also
>
> * But, resin is more lossy than glass, so Df may increase
>
> * for standard FR4 constructions, this is especially true. The data
> sheet for IS370HR, for instance, shows Df varying from 0.0177 to 0.0247
> (1GHz), depending on the resin content
>
> * for low loss materials, this doesn't hold. The data sheet for Meg6
> shows Df constant (0.002 @ 1GHz) for all its flavors of prepreg
>
> * Where the factors dominate will depend on your relative conductor vs.
> dielectric loss effects: for FR4, dielectric loss dominates at >~1GHz; for
> low-loss materials, conductor loss dominates up to much higher frequencies
> (as much as 10GHz).
>
>
>
> In your particular (low loss) case, the lower Er of the resin-rich case is
> trumping the Df change (or lack of) so you get lower loss.
>
>
>
> Only a tool which takes into account the properties of the specific
> material under consideration can be expected to give an accurate prediction
> of insertion loss for various resin contents.
>
>
>
> There are also environmental effects (I haven't heard or seen these
> stressed at this point, though that may change soon):
>
> * Higher resin content will absorb more moisture, and thus your loss will
> be more susceptible to humidity effects
>
> * There's a difference in how the various materials' Df changes w/
> temperature - more at DesignCon
>
>
>
> I hope this helps,
>
> Jeff Loyer
>
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Terry Ho
> Sent: Monday, October 29, 2012 5:52 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] PCB Insertion loss prediction
>
>
>
> Hello experts,
>
> I'm from PCB house. Recently we have producted some insertion loss test
> boards(16L, SET2DIL coupon, IS415/IT150DA/I-Speed Mid/low loss material
> with RTF copper foil). We found that the multiply core and high resin PP
> will result a lower loss result. It's a trouble to MI engineer. I would
> like to know how to predict the loss base on stackup. Please help to
> suggest (papers, script, free software etc ). Thanks a lot!
>
>
>
> Best regards,
>
> Terry Ho
>
>
>
>
>
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