My own experience has been a little different. The real separation you
need is more to do with not interfering with the electric field around
the microstrip, and that has a lot to do with the distance beneath the
microstrip trace to the ground plane underneath. The thicker the
dielectric the wider the electric field around the microstrip and the
more loading you will get from adjacent ground. A thin dielectric will
result in a 'tighter' electric field and so fewer problems. That's the
reason thin dielectrics give you less EMI - the electric field is more
contained within the dielectric. Trouble is of course, the thinner
dielectric makes it harder to maintain the right microstrip impedance.
Muy own rule is that the edge of the microstrip should be five times the
separation from the ground plane away from an adjacent ground pour. That
works well. In a pinch, three times the dielectric thickness will also
work but now you start getting into edge effects, which may or may not
be important depending on the sensitivity of the circuit to impedance
mismatches and to capacitive loading.
Andy Kowalewski
Sychip Inc
Office (972) 202 8852
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Denis Lefebvre
Sent: Thursday, February 05, 2004 12:10 PM
To: [log in to unmask]
Subject: Re: [DC] copper pour stand-off
When we've had a copper pour around microstrips the rule was that the
clearance should be no less than the width of the transmission line. In
your case, that is 10mils Min.
-----Original Message-----
From: Ray Dwyer [mailto:[log in to unmask]]
Sent: Thursday, February 05, 2004 9:58 AM
To: [log in to unmask]
Subject: [DC] copper pour stand-off
Greetings,
I am concerned about rules for stand-off distance for copper pour on an
outer layer to provide added isolation to a pair of sensitive low
current
traces. There are other traces on the top layer as well. The basic
info is
50 ohm impedance, 10 mil wide microstrip, digital TTL signal current as
low
as in 10s of mili-amps, FR-4 material, 1 oz copper.
Ray Dwyer
972-371-4046
Maxim
4401 Beltwood Parkway
Dallas, Tx. 75244
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