[SI-LIST] Re: Traces don't cause EMI - really?
- From: MikonCons@xxxxxxx
- To: weirsp@xxxxxxxxxx, leeritchey@xxxxxxxxxxxxx
- Date: Mon, 2 Feb 2004 11:18:46 EST
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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