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What is the best layer for a groundplane?
436 1 Jul 19.2019, 10:31:39

I am facing a serious problem choosing what layer of my multilayer PCB is the best to put a groundplane. Can someone advise me?

I often put the groundlayer on top, but yesterday I had a EMC-course, and the people overthere have another insight.
They say it is best to put it on an inner layer. They say, because it must be "unbroken"
Also they say NOT to use power planes, but just to route each trace needed for a power pin.
This is why (according to them): The power plane and the groud plane are exactly the same, for HF signals.
But, they need to be stitched needly together(for HF reasons) But you can't ( for DC reasons)
So it is better NOT to work with a powerplane.
Now I see the following advantages / disadvantages:
Groundlayer on top:
-Each smd pin for ground directly connected (adv)
-Ground layer broken by placing parts (dis)
-coupling of the return path in x-y direction uses less copper area (dis)
Groundlayer inner:
- Groundlayer more continious (adv)
- Not broken by placing parts (ground layer is running under the parts) (adv)
- every smd pin that needs a grn-level needs a via to connect to it (dis)
- coupling of the return path in z direction uses whole trace area (adv)

What do you think of these arguments? What is, in your experience, the best option?
I use smd parts and a micro running at 72 MHz.

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Jul 23.2019, 09:33:05

If you are going >2 layer, definitely inner layer to avoid breaks and  wasting placement space. You  don't want to be running signals in inner layers if you can avoid it as it makes track-hacking impossible. 
There are always situations which need something different, but inner power and ground is the normal way to do it. 
 I can't see any disadvantage in using a power plane in most circumstances - if nothing else it makes layout quicker. 
There sometimes can be some very slight EMC and layout density advantages to putting power and ground on outer layers and signals inside but the fact that you very rarely see it done suggests that the downsides outweigh any advantages. 
The only time I've ever seen it was on a large dense board with a lot of through-hole, as the smaller TH pads on inner layers allow more routing area.

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