[SI-LIST] Re: help with a stack ??
- From: James_R_Jones@xxxxxxxx
- To: Robison_M@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 21 Feb 2002 15:00:06 -0600
I might try:
noncritical
gnd
critical
5,-5.2
gnd
critical
gnd
noncritical
It may be a stunt to break up your supply plane, but you will get a C-plane
for these supplies. Also, your critical signals are on striplines, which I
personally prefer. You also get better impedance control this way because
your critical stuff can be referenced with core dielectric instead of
prepreg, especially if you make the criticals closer to the outer grounds
than the inner ground and VCC. Also, you won't have to worry about moat
crossings on criticals if you do this.
Hope this helps,
James J.
-----Original Message-----
From: Robison Michael R CNIN [mailto:Robison_M@xxxxxxxxxxxxxx]
Sent: Thursday, February 21, 2002 2:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] help with a stack ??
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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