[SI-LIST] Re: Traces don't cause EMI - really?
- From: steve weir <weirsp@xxxxxxxxxx>
- To: MikonCons@xxxxxxx, leeritchey@xxxxxxxxxxxxx
- Date: Mon, 02 Feb 2004 08:41:49 -0800
Mike, I agree wholeheartedly about the dangers of generalizations. But,
demanding an outer ground layer is just such a generalization. That outer
layer is nice, but I don't think we need legislation to force it.
I don't know how to make the world safe for the naive. Rules have and will
continue to be applied inappropriately by people who don't understand where
the rules came from, and hence their limitations. Even if someone uses 20
ground layers and 4 signal layers they can create a mess by a host of bad
practices that we all know too well.
Regards,
Steve.At 11:18 AM 2/2/2004 -0500, MikonCons@xxxxxxx wrote:
>Steve:
>You (and Lee) are absolutely correct that techniques have changed (for the
>better), and indeed a "buried microstrip" will reduce the radiated emissions
>relative to a surface microstrip. I also use that technique when
>appropriate. The
>tests I referred to only had a soldermask thickness of ~3 mils, but the
>relative dielectric constant was ~5.5. The present day soldermasks tend to
>fall in
>the 1.5-2.0 mil thickness and have an Er value of ~3.5, which would worsen
>the
>test results I obtained for surface microstrips.
>
>Equally important is the placement of surface high-speed traces in-board of
>the PCB edges. As much as 20 dB reduction in emissions can be achieved
>relative
>to traces very near (<50 mils) to the PCB edges. My tests were of traces
>in-board by more than 500 mils; therefore, the results would have been far
>worse
>for traces near the PCB edges.
>
>Note that these not so obvious details are critical to achieving acceptable
>performance as Lee has reported. Again, my concern is that less experienced
>designers will ignore the radiation potential of surface traces and fail
>to meet
>regulatory requirements without a respin of the PCB.
>
>
>
>Mike
>
>Michael L. Conn
>Owner/Principal Consultant
>Mikon Consulting
>
>
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