I'm looking into an existing large and complex PCB (500mm x 300mm) with
8 layers.
I will not bother you with all details, but the issue is this:
We want to change the stack-up in a limited region (70x50m) on this PCB.
That means that the original GND plane and the adjacent Signal plane to
switch place.
Planning to introduce a lot of ground vias along the boundaries of the
new GND area. And we are aware that removal of this original GND plane
does not make any problems for the return currents in adjacent signal
plane.
Would you have other concerns or actions to be carried out under such a
switch of layers?
Best regards
Amund
On 2020-02-10 17:58, Ken Wyatt wrote:
Hi Amund/Gert,
I agree with Gert’s advice, that all (especially high speed) signal
layers need to have an adjacent ground return plane in order to
properly capture the electromagnetic wave of the digital signals
(which travel in the dielectric space between layers). The one
exception, in my view, would be that we need the same adjacent return
for power. Here’s my reasoning:
1. With all the digital switching and corresponding transients
occurring, power rails (and planes) are also transmission lines.
2. The electromagnetic waves from these power rail transients travel
through the dielectric space between the power rail and the nearest
metal (adjacent layer).
3. Therefore, to avoid power transient coupling to other signal
layers, there must be an adjacent return plane for every power plane
(or routed / polygon) power.
4. Making the power and return planes adjacent and close together will
also enhance high frequency decoupling.
That is, placing a signal layer between power and power return can
couple switching transients to that signal layer.
I wrote a four-part series on PC board design to minimize EMI that may
help. I discuss in more detail how digital signals move in boards,
good and bad stack-ups, and partitioning of circuits to avoid
coupling. Hope it helps clarify the physics.
https://www.edn.com/design-pcbs-for-emi-part-1-how-signals-move/
Cheers, Ken
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On Feb 10, 2020, at 2:29 AM, Gert Gremmen <[email protected]>
wrote:
It s an extra layer you might allocate to that low frequency signals
, analog signals and or extra power supply.
On 10-2-2020 10:22, Amund Westin wrote:
Thanks Gert
I want to read your advices with great attention.
Just one immediately follow-up èThis Misc layer, is it an extra
layer for signal routing, as a Sbott3?
Mvh Amund
FRA: Gert Gremmen
SENDT: 10. februar 2020 09:57
TIL: Amund Westin <[email protected]>;
[email protected]
EMNE: Re: [PSES] PCB layout technique - multilayer
Just 2 hints of thousands:
If you implement 2 ground planes make sure the ground references on
top and bottom are related to the closest ground layer .
The stack will than be: Stop -- GND --Stop2 -- PWR - Misc - Sbott2
- GND -Sbott ((S=signal))
(basically you route 2 x 3 layer boards critically, and bond them
together with a power plane in between.)
The signal should not cross the board and be tempted to flow on an
opposite ground layer, but
will remain always close to the corresponding ground layer.
Make sure both ground layers are extremely well coupled together,
esp on board edges
If you implement SMPS locally give it a local top layer ground area
to connect the principal
switch elements together. Connect by a dice pattern 5 via to main
GND
Good luck.
Gert Gremmen
On 10-2-2020 8:27, Amund Westin wrote: contact to the nearest ground
layer,
I’m looking for articles about how to do good EMC layout on
multilayer PCB.
Choice of PCB layer stacking (8 or 10 layers) and basic routing
techniques are the issues of most importance right now.
Appreciate if you have some experience about good or bad layer
stacking.
Thanks!
Best regards
Amund
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