This isn't directly related to your question, but relates to my
experience with PCBs with more than 1 layer:

We almost NEVER use the "power" layer as just as VCC plane. While we
keep GND as a plane layer, we convert the "power plane" layer to a POWER
CONNECTIONS layer, because most of our PCBs have several VCCs (3.3V, 5V,
12V, etc etc) and it caused too many problems. So, the "power" layer
uses large traces carrying the misc voltages as needed to the right places.

Keeping power inside has good consequences: the GND layer is very close,
pressure on normal signals routing is reduced. Signal traces are then
always available to be cut and pasted on the top or bottom, not locked
away inside.

--Bob


tony@ELROYNET.COM wrote:
> Hello all,
>
> I have only done 1 and 2 layer boards, and I now have a project that
> requires 4 layers.
>
> It will be a fairly standard 4 layer board, 2 signal layers on the outter,
> and power and ground planes on the inner.
>
> My question is, when putting the caps in for the IC's on the board I have
> 2 options - as far as I can imagine they would both do the same thing, but
> I wasn't sure if there was some benefit of doing it one way over another
> as I have never done a 4 layer board before.
>
> Normally, one would have a decoupling cap for each IC connected between
> the Vcc and Vss pins.  My question is - do I need to connect this cap
> directly to the vcc and vss pins of the given IC, or is placing the cap
> near the IC and connecting it to the power and ground planes sufficient?
> Technically it will be connected to the power and ground planes if it were
> just tied to the pins, but I was wondering if there was some difference
> I'm just not aware of by having it connected to the pins rather then to
> the planes?
>
> I hope I was clear on that.  I realize it may be a silly question, but I
> am really not sure on which route would be the best since I really don't
> see a difference, but I was hoping someone with more experience on 4 layer
> boards might be able to clarify whether I am correct or wrong and possibly
> why (if I'm wrong of course :).
>
> -Tony
>
> --
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>
>


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