[SI-LIST] Re: Buried uStrip question
- From: steve weir <weirsp@xxxxxxxxxx>
- To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>,"'MikonCons@xxxxxxx'" <MikonCons@xxxxxxx>,"'leeritchey@xxxxxxxxxxxxx'" <leeritchey@xxxxxxxxxxxxx>
- Date: Mon, 02 Feb 2004 08:14:38 -0800
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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