Re: [SI-LIST] : some board layout issues

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From: e (evillaf@home.com)
Date: Tue Apr 17 2001 - 22:06:06 PDT


Bernard,

Thanks for the feedback. I am not at liberty to change the stack up at this
point (I inherited this project), although I had voiced my opinion on the issues
I outlined in my original email. I was just wondering if anyone else felt the
same way I did.

Ellis

Bernard Voss wrote:

> e,
>
> I would use the following alternative stackup to reduce the impedance
> between all power delivery signals and their returns and avoid the impedance
> discontinuities signals routed adjacent to split reference planes
> experience:
>
> signal1
> gnd2
> signal3
> signal4
> gnd5
> power6
> power7
> gnd8
> signal9
> signal10
> gnd11
> signal12
>
> Power7 would be split to provide multiple, fully isolated from routing,
> voltages, and the dielectric thickness between layers 6 and 7 would be
> increased to meet the target finished thickness, (and yes, Of course I would
> also have to show you how I would fit your 8 layer routing solution into a 6
> layer low coupled routing solution but only if you insist.)
>
> Bernard
>
> e wrote:
>
> > To all the experts, I would like to get some feedback on a couple of
> > issues:
> >
> > Board dimension is about 4"X8"
> > signal1
> > gnd2
> > signal3
> > signal4
> > power5
> > signal6
> > signal7
> > power8
> > signal9
> > signal10
> > gnd11
> > signal12
> >
> > 1) With the stack up above, the plane power5 is used as reference for
> > signal4 and signal6 (same is true or power8). To accommodate a
> > different (third) voltage, a two square inch cut is made in power5. The
> > cut is 50-100 mil. For signals (of different rise times 2-5 ns) on
> > layers signal4 and signal 6 that cross that boundaries between the two
> > voltage partitions on power5:
> >
> > a) Is it possible to to control the impedance, now that the
> > reference plane is broken? How much of problem does this broken
> > reference pose on controlling the impedance of that trace? Is it
> > comparable to the discontinuity presented by a via as a trace changes
> > layers? Will this broken reference cause significant reflections?
> >
> > b) To the concept of return current that normally follows that path
> > of least inductance, which is the reference plane directly below or
> > above the trace, this current will now seek a different path back to the
> > driver, possibly through a cap to a ground plane and back to the
> > source. Does this less than optimum return path necessarily increase
> > radiation, because the loop area is now larger?
> >
> > 2) A fourth voltage is routed with 100-200 mil trace on signal3 and
> > signal4 from a signal DC-DC supply to two adjacent pins on a
> > board-to-board connector, where two independent paths are used for each
> > pin, thereby forming a loop with an area of about one square inch. Does
> > this loop add to radiation at the frequencies of interest for FCC
> > compliance?
> >
> > Ellis
> >
> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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>
> --
> Bernard Voss
> Principal Interconnect Specialist
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
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