[SI-LIST] Re: Microstrip line loss on Rogers 4350
- From: Ming Tsai <ming.tsai@xxxxxxxxxx>
- To: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- Date: Mon, 21 Mar 2005 10:31:38 -0800
Hi Jeff,
Thanks for your message.
My personal experience before was the bad solder mask caused my 2.4GHz
antenna 1-2 dB gain. Therefore I suspected that's the reason. But I did see
two curves right on each other (up to 18 GHz) after I took solder off from
the same traces. I will check with the PCB house about which kind of solder
mask they were using. Apparently it was a good one for high frequency
application.
Ming
"Loyer, Jeff" wrote:
> I had similar results on FR4 when I played around with several
> configurations (single-ended and differential microstrip, single-ended
> asymmetric stripline, and differential symmetric stripline), trying to
> correlate different tools to actual measurements. I could get good
> broadband correlation for all but the single-ended microstrip (and I did
> have well setup TRL structures for all). For single-ended microstrip, I
> couldn't get good correlation at all frequencies with any tool (though
> this wasn't an in-depth study). I suspect that, for this case, the
> trace geometries are complex enough that all the subtleties must be
> properly represented. For instance, neither a simple rectangle nor
> trapezoid is sufficient - the plating process makes the trace appear as
> more of a "mushroom". This complex shape might have significant effect
> on cross sectional area, and how the fields surround the trace (how much
> energy is in dielectric vs. air). I think you might have to
> cross-section your traces to determine their actual complex geometry, or
> take a best guess, based on info. from your fab vendor.
>
> I'm surprised that removing the soldermask didn't have any effect.
> While I haven't done that on actual boards, simulations show it does
> have a significant effect. And, I've added soldermask (putting a layer
> of Kapton tape over a trace), and seen significant effects.
>
> My 2 cents.
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Ming Tsai
> Sent: Friday, March 18, 2005 6:08 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Microstrip line loss on Rogers 4350
>
> Hi All,
>
> I am studying the microstrip line loss on 10 mil Rogers 4350 (Er = 3.48,
> loss tangent = 0.004 @ 10 GHz). The trace width is 22 mils, with half oz
> copper. From some transmission line calculator I expect to see 0.25dB
> /inch loss @ 10 GHz. However I measured under network analyzer and I got
> the loss up to 0.5dB/inch @ 10 GHz.
>
> I did not have well-setup TRL calibration on board, but I took 3
> separate measurements from 3", 5" and 7.5" long microstrip lines, and
> estimated the delta to get 0.5dB/in loss. So I assume the edge-mounted
> SMA connector is not a factor. Also checked under TDR I saw 48 ohms of
> line impedance, which is within the spec. I also scratched out the
> solder mask which did not have any effect. I used full-wave tool to
> verify the radiation as well as skin effect, but no surprise the
> simulation showed that I should get about 0.25dB/in loss.
>
> Does anyone have similar experience or any suggestion? Thanks!
>
> Regards,
>
> Ming
> --
> Ming. Tsai
> Staff Hardware Development Engineer
> Product Technology - Hardware Design Group
> XILINX
>
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--
Ming J. Tsai, Ph.D.
Staff Hardware Development Engineer
Product Technology - Hardware Design Group
XILINX
Telephone: 408-626-6260
E-Mail: ming.tsai@xxxxxxxxxx
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