[SI-LIST] Re: ground planes at top / bottom layer
- From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
- To: r_jungert@xxxxxxxxxxx
- Date: Sat, 3 Jan 2009 00:15:00 GMT
Richard:
You are absolutely correct for the majority of cases. Of course, consultants
are often brought in after the fact, by a customer frantic about schedule.
There are two things to do in that situation, (1) explain that you should be
brought in at the beginning of projects, and (2) unless they are willing to
take the schedule hit for a proper design, drop in such beads, resistors, et al
as make sense, and in the worst case, add outside grounds. Sometimes the
customer takes the latter advice too much to heart and decides to address the
symptom rather than the design flow problem, by adding outside grounds to
future designs as a matter of course. They see it as cheap insurance for small
production runs. It's not so much a matter of covering up bad designs as not
having to notice if there are any. That is, until a really nasty problem comes
along.
Obviously, the constraints are entirely different for high volume consumer
electronics.
On another note, in the right circumstances dropping series resistors on signal
lines works well, but not all the world is digital, (and not all digital lines
are amenable to this fix). It is but one tool in the armamentarium. I would
not, for instance, try it in the power path of a switching power supply, to
cite one obvious example.
Orin
Earl
If one designs them right then you won't need a blanket to cover up the
mistakes. I have
done over 180 of them and not once did we add extra copper on the outer layers
to
control EMI. I still think that extra copper is something to cover mistakes.
Richard Jungert
> Date: Fri, 2 Jan 2009 12:42:49 -0800
> From: earlalbin@xxxxxxxxx
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: ground planes at top / bottom layer
>
> Lee:
> The Sun rises in the East and sets in the West. "ground layers on the
> outside of PCBs to control EMI", East-West thing here.
>
> "Xbox didn't do it and it passes," ok, yes. Obviously their four layer
board
> had enough margin. Likely as well they started with a six layer solution
> (safe), added up the cost (consumer vs profit) made appropriate changes and
> they pass with a four layer. Six layers in not the only tool in your tool
> box, many ways to skin the cat, etc...
>
> If you don't own an Xbox and you haven't torn it apart, why, if your goal is
> similar. I hope you have the time to gain a critical understanding of what
> Xbox did, mostly though why, they did what they did.
>
------------------------------------------------------------------------------------------
----------------------------------------
> To all of those who recommend ground layers on the outside of PCBs to
> control EMI, take a look at a PC mother board or the motherboard in the
> Xbox. They are 4 layer PCBs that have signals on both outside layers and
> the products in which they are shipped pass EMI standards.
>
> Where did the notion that outer layers had to be ground in order to pass
> EMI come from? My guess is it is an RE.
>
> Lee Ritchey
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