[SI-LIST] Re: help with a stack ??

Dear Mike,

To add to James comments, you will also
find your PCB fabricator will be able to achieve
tighter impedance control on inners, as the extra
processing plus resist coating on outers always
makes it more tricky to achieve your desired impedance.

I also agree with referencing the critical traces to an
adjacent plane across core material.

Kind regards
Martyn Gaudion
www.polarinstruments.com

At 15:03 21/02/02 -0500, you wrote:
>
>hello,
>
>for the first time, i've got ecl and ttl on the same
>board.  so i've got two power planes and i figure on
>matching them with a ground plane each, with a narrow
>core between to give me a good switching capacitance.
>here's what i have in mind for an 8-layer stack:
>
>critical traces
>gnd
>-5.2V
>noncritical traces
>noncritical traces
>+5V
>gnd
>critical traces
>
>does this seem reasonable?  if no, then could you 
>suggest an alternative?  if yes, then how should i
>tie the two grounds together.  that makes me very
>nervous.  would a bunch of vias do the trick?  should
>i only have one ground?  should i put one of the non-
>critical trace layers between a power and ground?
>
>and another thing.  the board shop roughly doubles the
>price of the board if i want to do controlled impedance
>and designate the core thickness.  this is just a proto
>board, so what i'm thinking is that if a       NORMAL core 
>minimum is around 6 to 7 mils, then with an 8-layer
>board at a standard 62mil, they can't help but give me
>about a 6 to 7mil core, without me paying extra to spec
>it.  is that reasonable, or faulty thinking?
>
>thank you, miker
>
>
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