On Fri, 15 Jun 2001 15:19:43 -0700, Abdulrahman Lomax wrote:
>Now, what is wrong with this picture? I'm interested in the comments of
>others, but here is my first pass:
>Keeping the planes isolated, such that the return via, for example, only
>connects to the outer planes, is *probably* useless, or maybe worse than
>useless, but not a lot worse. The return currents are going to follow the
>path of least impedance, which will, if the copper exists, be in the ground
>plane paired with the signal layer involved. If the single jumps such that
>another plane becomes the reference plane, the current will flow through
>the nearest via. In my practice, if such a via exists within an inch or so,
>I'd add no special via, but the higher the edge rate involved, the closer
>I'd want it to be.
Well we don't know how many signal layers and where they are, I can only
assume the middle of the board is 4 planes. I also don't see that isolating
the ground planes for signal return vias and decoupling capacitors buys you
anything. Stretch that a bit further and you could argue the extra two
planes are not buying you anything.
>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. 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.
Cheers, Terry.
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