[SI-LIST] Re: PCB Insertion loss prediction
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "pcb_layup@xxxxxxx" <pcb_layup@xxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Nov 2012 15:37:46 +0000
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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