[SI-LIST] Re: Traces don't cause EMI - really?
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "MikonCons@xxxxxxx" <MikonCons@xxxxxxx>, weirsp@xxxxxxxxxx
- Date: Mon, 2 Feb 2004 08:36:30 -0800
Actually, present day soldermasks are rarely more than 0.5 mils thick and the
er is on the order of 3. The impact on impedance is quite small.
Lee
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From:
To: weirsp@xxxxxxxxxx;leeritchey@xxxxxxxxxxxxx
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Sent: 2/2/2004 8:18:46 AM
Subject: Re: [SI-LIST] Re: Traces don't cause EMI - really?
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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