[SI-LIST] Re: EMI fixed by flooding?
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>,"Bill Dempsey" <BDempsey@xxxxxxxxxxxx>
- Date: Mon, 3 Feb 2003 09:49:13 -0800
Bill,
I think that the concept that you are referring to is one I teach in my
high speed class. I call it signal plane fill and it works very well to
help contain EMI at high frequencies. If a signal layer has unused space,
I look at the plane layer it is next to and then fill the unused space with
copper that gets attached to the opposite power rail. This increases the
interplane capacitance that is needed to supply the very fast switching
currents needed to drive transmission lines. So, its not just ground fill.
This is a very good way to help low layer count PCBs that cannot bear the
cost or added thickness associated with adding a plane layer set, such as
ZBC.
And, not is does not violate the BC patents as might be claimed by
Sanmina/Hadco,Zycon.
Lee
Lee Ritchey
leeritchey@xxxxxxxxxxxxx
Why Wait? Move to EarthLink.
> [Original Message]
> From: Bill Dempsey <BDempsey@xxxxxxxxxxxx>
> To: Si-List (E-mail) <si-list@xxxxxxxxxxxxx>
> Date: 1/30/2003 2:19:22 PM
> Subject: [SI-LIST] EMI fixed by flooding?
>
>
> Last summer there was a good thread going around about ground
> floods and their effect on EMI. I am interested in
> finding out the opinions of those who care regarding the
> following problem.
>
> First some sarcasm. We all know that good design is verboten when
> the band-aid box is full. Given this I'm been given a severly failing
> design, a box of band-aids, and a set of rules I'm supposed to implement.
> (By severely failing I mean the product did not pass FCC part A emissions
> because of large emissions at the system clock frequency.)
>
> Back to the rules. The rule that's got me uncomfortable is I've been told
to
> flood
> the outer layers of a backplane (the culprit) with Frame gnd. This will
fix
> the EMI failure. Ok... does it?
>
> Some background:
>
> The box that this backplane is going in is metal on several sides but
> has openings for cards in the front and cables in the back.
>
> The box is tied to Frame (i.e. Earth).
>
> Digital GND (just a name to imply not Frame GND) is used for signal
> return on *some* layers of the backplane. Low voltage power is used
> for signal return on other layers. Not all signals that transit the
> backplane
> use the same common reference. Hmmm...
>
> Currently the outer layers are microstrip (soldermask coated).
> If I flood the outer surfaces of the backplane with FGND and I have
> a dielectric thickness of around 6 mils to DGND haven't I just
> introduced a great AC path to FGND that would not have existed if the
> box metal was allowed to float?
>
> And by doing so doesn't this help increase chances of emissions? And if a
> "farday cage"
> is open on two ends does this concept help?
> It seems to me that the cause of failure has to do more with the fact that
> they
> have a gazillion (power of ten higher than yotta) cables going out of this
> chassis and
> the return energy for the clock has found a lower impedance path across
the
> cables and
> therefore has created a loop which is radiating.
>
> Thanks,
> Bill
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