Re: [SI-LIST] : Distance between Power and Ground planes

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Mon Jan 24 2000 - 11:22:53 PST


The short answers are:

1. It depends if the board was originally constructed
   to suppress noise with the smaller separation.

2. If you're having to deal with a lot noise anyway.

By first approximation, increasing the separation
by a factor of 7 will lower the effective interplanar
capacitance by a factor of approx. 1/7 assuming same
area in both cases. Thus, the resonance of the board
*may* shift to a frequency that may or may not have
been an issue before. Or, it may introduce nodes
of resonances at various points around the board
rendering some decoupling caps ineffective if that
had been an issue prior. Regards, Doug McKean

david gil donate wrote:
>
> Hi,
> We have some problems with signal impedances between
> two PCB boards. We want to rebuild the board with
> different layer heights to fit impedances. I have to
> preserve the board height. Then when I try to reduce
> the distance between the sig int 1 and power plane I
> add distance between ground and power plane.
> My question is: Can I have some SI or EMI problems if
> I change the distance between Power and Ground planes
> to 1.42 mm from 0.2 mm?
>
> Current PCB New PCB
> sig top --------------- ---------------
> 0.36 0.2
> sig int 1 --------------- ---------------
> 0.8 0.35
> grd --------------- ---------------
> 0.2 1.42
> pwr --------------- ---------------
> 0.8 0.35
> sig int 2 --------------- ---------------
> 0.36 0.2
> sig bottom --------------- ---------------

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