At 05:42 PM 6/16/01 +0100, Terry Harris wrote:
>On Fri, 15 Jun 2001 15:19:43 -0700, Abdulrahman Lomax wrote:
>
> >If I were working for the client here, I'd suggest that they have a board
> >fabbed both ways, one as described and the other without manipulation: the
> >vias connect to all layers, as do the bypass caps and other parts. One
> >experiment is worth a thousand engineers theorizing until their heads hurt.
>
>Only if the new way obviously fails. Otherwise you are sitting there with a
>very expensive scope looking at all the possibly critical signals on both
>samples and making signal quality comparisons.
No oscilloscope is as expensive as is my time, and this would be true for
most PC designers.
>If you find there is not
>much difference it only saves your layout guy doing some manipulations -
>probably less cost than doing the tests. If you tried one without the two
>planes that might save enough production costs to be worthwhile.
I've responded to this issue in another post, but I'll add here that, yes,
trying the board without the extra planes would also be interesting,
perhaps, but there are other variables involved and the experiment becomes
less controlled. What's the stackup? What is the interplane capacitance?
Testing only the effect of keeping the vias isolated as described is a
single variable, and thus the results will be much easier to interpret.
There are other combinations to try. For example, I've done
1 signal
2 power
3 ground
4 power
5 ground
6 signal.
This stackup addresses the power distribution problem mentioned in another
post, by doubling up both power planes. It also increases the interplane
capacitance (if the plane separations were equal, it would triple it, but
they would probably not be equal, though that depends on the board
thickness and target impedance).
My favorite stackup is still:
1 signal (fat traces)
2 signal
3 power
4 ground
5 signal
6 signal
A variation on this is that layers 1 and 6 are poured with ground/power
copper, and/or 2 and 5, but this can get complicated.
[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433
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