FreeLists / si-list / [SI-LIST] Re: Traces don't cause EMI - really?

["Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>]

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


----- Original Message ----- 
From: 
To: weirsp@xxxxxxxxxx;leeritchey@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
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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