Internal Plane

[mulfycrowh]

Hi everybody,

An internal plane is assigned to a single net name.
How do you set the number of internal planes on a project ?
Thanks a lot.

[robertferanec]

You need to go to stackup manager. Maybe this will help: https://www.fedevel.com/welldoneblog...e-new-old-way/

[mulfycrowh]

Thanks Robert. I already watched this video but I didn't find how you set the number of the internal planes. Let's take an example. We have a schematic with 3 power supplies, digital GND, analog GND. Very often you assign the planes to GND. But in this case, do you assign internal planes to DGND, AGND and the 3 power supplies ?

[Paul van Avesaath]

pretty straight forward.. same in 17 as in 18

[mulfycrowh]

Thanks for reply. Could you please explain a little bit. How many internal planes do you set ? And how do you assign them ?

[Paul van Avesaath]

that all depends on how many you need which can vary between projects.. . just be sure to keep it symmetrical.. let say you need a 6 layer board.. than it could be toplayer/internal plane1/mid layer 1/ midlayer 2/Internal plane 2/ Bottom layer.. or toplayer / midlayer 1/ internal plane 1 / internal plane 2/ midlayer 2 / bottom layer..

to assign a plane go into the pcb editor (to your pcb) select the internal plane.. double click anywhere on you plane and select the net you want it to be.

good luck

[mulfycrowh]

sorry but I gave an example About 3 power supplies with DGND and AGND. Could you please share how to set the internal planes ?

[Paul van Avesaath]

I told you how to add planes, gave an example of a layerstack , and how to set them to a specific net.
you need to give a lot more information before you can ask something like this..

as an example, where do you need the DGND and AGND to be.. maybe they can be on the same internal plane?..
same goes for your 3 power supplies.. maybe they can go on the same internal plane..

so that would give you a simple 4 layer board.. (top / int1 / int2 / bottom)

you do not need to use a full plane for each...you can draw lines (command: place line) on an internal plane, everything you draw on there is inverse.. (so what you draw will be come no copper)
see attached image.

so if you draw a closed box of any shape and double click inside the box you can assign a box to a different net. the highlighted area is now a seperate copper area inside you internal plane

so you can divide your internal plane to be DGND and AGND and you other plane to multiple power supply .

hope this clears something up for you..

[mulfycrowh]

Hello everyone,

Thanks for reply.
Could we just forget Altium a few minutes.
What is the main purpose of the internal planes ? Is it shielding ?

[Paul van Avesaath]

its for distributing power / gnd over a large area.
If you want impedance, then is is used as reference plane.
it has a very low impedance itself so for large currents is is much better than traces.
also in production it gives regidity and stability

[Paul van Avesaath]

also if you look at this from an Signal intergrety point of view.. you need a proper GND plane if you want the current retun path to run underneath the trace.. so when going high speed.. you need planes.. also like you suggest for shielding

[mulfycrowh]

Coming back to you.
Hello everyone !

Well my suggestion for my 4 layers PCB is:

Top Overlay
Top Solder
Top Layer (dedicated to high speed tracks)
Dielectric Prepreg
Inner layer 1 >>> GND (in fact AGND + DGND) used to act as a shielding for high speed tracks
Dielectric Core (FR4)
Inner Layer 2 >>> VCC
Dielectric Prepreg
Bottom Layer (dedicated to low speed tracks)
Bottom Solder
Bottom Overlay

First question:
-------------------

As I already dedicated layers to GND and VCC, do I need additional internal planes ?

Second question
-----------------------

Is the stackup correct ?


Thank you !

[Paul van Avesaath]

seems ok to me..
just keep it symmetrical so same distance between top and interplane1 as for bottom to internal plane2.. use the inner layer between the two internal planes to fill the gap to your pcb thickness.. you do not need any other planes as far as I can see now.. but i dont know what you are designing so it is hard to comment on that..

[mulfycrowh]

Thanks for reply.
The board uses RS485 link. So we use 120 ohm impedance.
So it means that you add internal planes in addition to both inner layers ?

[Paul van Avesaath]

no you do not need this.. for RS485 you can get away with no impedance.. dont worry about it.. if yo were desinging a 10G link then i would be more concerned.. but you can use your stackup with no issues at all! good luck!