[SI-LIST] Re: Buried uStrip question

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: "Brown, Mike (Austin, TX)" <mibrown@xxxxxx>,"Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>, <MikonCons@xxxxxxx>,<leeritchey@xxxxxxxxxxxxx>
  • Date: Mon, 02 Feb 2004 09:23:16 -0800

Mike, whatever term you want to use for it, we agree that the field is 
almost completely confined.  Away from the edges, I think you have to work 
at it to get an antenna of any efficiency.  Ordinary traces aren't much of 
an issue.

Regards,

Steve.
At 11:15 AM 2/2/2004 -0600, Brown, Mike (Austin, TX) wrote:
>Actually, Lee has stated that he uses a pad cap layer, followed by the 
>first signal route, followed by a plane.  That configuration can be made 
>to have little radiation, assuming proper plane continuity under the 
>traces.  Most of the field is confined to the dielectric surrounding the 
>trace.  Whether you can make an efficient antenna with that configuration 
>is less clear to me.
>
>Mike
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
>Sent: Monday, February 02, 2004 10:15 AM
>To: Grasso, Charles; 'MikonCons@xxxxxxx'; 'leeritchey@xxxxxxxxxxxxx'
>Cc: 'si-list@xxxxxxxxxxxxx'
>Subject: [SI-LIST] Re: Buried uStrip question
>
>
>Charles, no as I have read him, Lee promotes putting prepreg w/o foil on
>the outside.
>
>Regards,
>
>
>Steve.
>At 09:01 AM 2/2/2004 -0700, Grasso, Charles wrote:
> >Steve, AFAIK, a buried ustrip is a ustrip with soldermask.
> >Is there another definition? I am unsure as to why one
> >would advocate a buried ustrip over a "true" surface ustrip
> >when (for typical boards) only buried ustrips prevail.
> >
> >Best Regards
> >Charles Grasso
> >Senior Compliance Engineer
> >Echostar Communications Corp.
> >Tel:  303-706-5467
> >Fax: 303-799-6222
> >Cell: 303-204-2974
> >Email: charles.grasso@xxxxxxxxxxxx;
> >Email Alternate: chasgrasso@xxxxxxxx
> >
> >
> >-----Original Message-----
> >From: steve weir [mailto:weirsp@xxxxxxxxxx]
> >Sent: Monday, February 02, 2004 5:44 AM
> >To: MikonCons@xxxxxxx; leeritchey@xxxxxxxxxxxxx
> >Cc: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Traces don't cause EMI - really?
> >
> >Mike, I think the issue is worth reviewing under current
> >circumstances.  Some understandings have improved greatly with regard to
> >antenna design and/or detuning, and geometries have definitely changed  for
> >the better.  Lee advocates a buried microstrip for the outermost routing
> >layer.  That captures the vast majority of the field in the dielectric.
> >
> >It's been a long time since I read those old papers going back to the mid
> >eighties when stripline started getting promoted for EMI.  But I am
> >confident that we will find all of those studies compared surface
> >microstrip to stripline.  It would be interesting to repeat your 50 ohm
> >tests with this "buried microstrip" versus a true surface microstrip for
> >EMI.
> >
> >Regards,
> >
> >Steve
> >
> >At 03:30 PM 2/1/2004 -0500, MikonCons@xxxxxxx wrote:
> > >In a message dated 2/1/2004 9:35:10 AM Pacific Standard Time,
> > >leeritchey@xxxxxxxxxxxxx writes:
> > >Wow!  This topic keeps coming up.  Wasn't so long ago that we had a very
> > >long exchange where proponents of this notion provided the research papers
> > >that  supported it.  I got copies of all of them and looked for some
> > >objective measurement of the EMI caused by a trace routed on an outer 
> layer
> > >over a plane and then moved below the plane.
> > >None of the papers did such an experiment in a way that could be used to
> > >bet money on.  I pointed out that the real source of EMI from a PCB were
> > >the lead frames of components that stick up from the PCB.  While at 
> Maxtor,
> > >we had this very problem with disc drives.  We fixed the problem by
> > >changing lead frames from PLCCs to QFPs- packages that don't stick up very
> > >far from the PCB.  If you want to see this in action, go to Frys or any
> > >place else that sells stand alone disc drives and look at how the PCBs are
> > >designed.  All of the signal traces are on outside layers and all of the
> > >disc drives comply with CISPRB B.
> > >
> > >We used a pedicel of equipment that allowed the PCB to be laid on it and
> > >then scanned to provide a 2D picture of where emission were coming 
> from.  I
> > >cannot remember the name of the tool, but it had a table with a grid on it
> > >on which the PCB was mounted.  The output looked a lot like what one gets
> > >from a thermal mapping tool showing places with higher emissions.  The
> > >sources of EMI were very clear- the IC lead frames.
> > >Thanks, Lee, for including me on your distribution.
> > >
> > >I absolutely agree that high-speed boards can be designed with many traces
> >on
> > >the PCB surfaces IF there is a high integrity enclosure.
> > >
> > >The tool you mentioned was likely of thick, planar construction with many
> > >separate pickup coils in a grid. I have seen several test labs with this
> > >tool. As
> > >you indicated, the lead frames DO radiate substantially. Assuming buried
> >PCB
> > >traces, some of this radiation can be cancelled by routing the lead pad
> >back
> > >under the package before dropping the via. Aside from that, the tool 
> pickup
> > >coils detect near fields; hence, the lead frames (because of their closer
> > >proximity to the pickup coils) indicate a disproportionately high field
> >level
> > >relative to any PCB surface (microstrip) traces. The field strength is
> > >comprised of
> > >first-order, second-order, and third-order rolloff terms. Therefore, the
> > >field
> > >signature would show much less relative differences (even with very small
> > >spacing displacements) because of an exponential rolloff in the coupled
> > >signal;
> > >hence, some test results may be misleading. Please be aware that I am in
> > >agreement with you on the benefits of low-profile lead packages. Any
> > >disbeliever
> > >should measure the fields from a small daughter board connected via
> > >standoffs of
> > >1/4 to 3/8 inch length, as the results will scare you.
> > >
> > >Re: your statement, "None of the papers did such an experiment in a way
> >that
> > >could be used to bet money on."
> > >
> > >As (many months) before, I disagree. I demonstrated the relative radiation
> > >and crosstalk performance of microstrip (50 & 100 Ohm lines), guarded
> > >microstrip
> > >(50 Ohms), stripline (50 Ohms), and guarded stripline (50 Ohms) as part of
> > >the (now ancient) Hewlett-Packard High-Speed Digital Design Seminar Series
> > >(that
> > >included such respected pros as Ed Sayre, Eric Bogatin, and Henri 
> Merkelo).
> > >The results were clear that either containment (via an enclosure) or the
> > >use of
> > >stripline was needed for FCC or CISPR Class B compliance. The data and
> > >knowledge gained from these findings led to the redesign of scores (yes,
> > >hundreds)
> > >of EMI-deficient PCBs over the following decade that achieved a typical 20
> >dB
> > >(some as high as 40 dB) reductions in radiated emissions. No one has 
> yet to
> > >find that the HP spectrum analyzers, HP near-field probes, EMCO 
> (far-field)
> > >antennas and near-field probes that I used (and still use) were 
> technically
> > >deficient in any way. Additionally, my software predictions support the
> > >empirical
> > >test data as well.
> > >
> > >My concern (and the only reason I'm sending these comments) is that less
> > >experienced designers than yourself will think surface traces are "no
> > >problem" and
> > >NOT employ the many other routing techniques that you and I have learned
> >over
> > >the years to be beneficial (or mandatory) for a successful design.
> >Obviously,
> > >non-enclosed/shielded designs will suffer most from this oversight.
> > >
> > >Good engineering to all,
> > >
> > >Mike
> > >
> > >Michael L. Conn
> > >Owner/Principal Consultant
> > >Mikon Consulting
> > >
> > >*** Serving Your Needs with Technical Excellence ***
> > >
> > >
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