Copper Fill Inner Layers - Yes or No?


#1

Hi,

My PCB: 4 layer

I have a GND plane on top and bottom and have enough vias to link any islands. There are 4 voltage levels mostly running on the 2nd or 3rd layer.

I don’t have too much heat to deal with.

Should I or shouldn’t I?

Regards,

Andrew


#2

This is not a Kicad question, but general pcb layouting.

The answer is “it depends on the design”.

I suggest a google search for pcb stack up. There are some readings you will find with 4-layer stack up and why sometimes power planes must be in the inner layers and why sometimes in the outer layers or in both.


#3

In my case I have power tracks in the middle layers as both top and bottom are crammed tight with components. I know the fill may dissipate heat a little better but are there any significant drawbacks?

I haven’t connected the inner layer fills to ground.


#4

That’s certainly an unusual approach.
The whole idea behind inner planes, is to make a large capacitor, with very low inductance.
Powersupply noise is reduced and RFI is reduced too.
If you can’t fill all voltage zones, the general approach is to fill GND the most, as that tends to be the most common, and then fill the other zones as much as practical.

You have paid for the copper, why not use it ? :slight_smile:


#5

So fills on all layers, all connected to ground?


#6

No a typical stackup is that you use one inner layer for a uninterrupted ground plane. The outer layers for signal traces and the second inner layer for the remaining power supply. (Can be done using traces or zones. Depending on your requirements.)


#7

No, that does not make a large capacitor.
For that, you need two parallel plates, one power, one GND.
If you have 4 powers, and one gnd on 2 inner layers, usually you make GND as intact as possible (as above), and then (broadly) split the other layer into 4 fill areas, based on the power destinations.


#8

Interesting, I didn’t consider that approach.

If I were to make an inner layer ground and have no fill on the top and bottom that would require a significant amount of vias connecting the components on the top & bottom layer (considering gnd is the most popular track). Doesn’t the increased use of vias come with it’s own negatives (impedance)?

I have four power lines, all DC. There’s no RF/filter circuitry on the board and EMI is not a concern. There are a few PWM circuits that can cause noise but they’re all to the side of the board.

mid2topmid1bottom

L1: red: mostly component connections, moderate GND usage.
L2: pink (mostly power traces).
L3: mostly MCU tracks where tracks on L4 couldn’t work)
L4: green: mostly component connections and significant GND


#9

How much power is involved,and what decoupling do you have there ?
Usually with PWM, you try to keep lowest Power supply loop impedances, for noise/radiation.

What is the main IC ?
If you have stacked fragmented GND planes it’s common to use stitching vias, to try to make them ‘average more solid’. ie around the IC package, keep under that clear so vias can connect one side to the other on an inner layer.
That also means you try to spread traces to allow all GND vias to connect - on your yellow layer under the main IC, it’s quite good (connects left-right ok), but below that are 2 isolated GND vias, that some simple trace moves can allow to connect better.
Also to the left, a trace can move to give broader GND flows, to other GND areas.


#10

The main purpose of plane layers (ground or power, doesn’t matter) is to minimize current loop area (and thereby radiated emissions and susceptibility) by controlling RF ground return paths. Planes do not add appreciable distributed capacitance unless you use a special PCB process with very small layer spacings and even then the amount of capacitance is small and only significant at high frequencies. Any discontinuities in the plane layers will defeat the purpose of loop area minimization for any tracks in adjacent layers that cross a discontinuity. Copper pours in signal layers (as you show) are generally pretty useless but can reduce in-plane crosstalk between adjacent traces if properly grounded. If not properly ground they can make emissions worse. Vias do have impedance (everything does) but at low frequencies you shouldn’t have to worry about that. The larger the via the lower the impedance, and of course you can use multiple small ones to similar ends.

Using a continuous ground or power plane adjacent to a signal plane is pretty much always good practice and has no downside if your board’s floorplan is well-partitioned. Chopping a plane into sections (as for multiple power supplies) needs a lot of thought in high-frequency design, but if you don’t have anything over a few 10s of kHz then don’t worry about it.


#11

If you have an environmental friendly mind-set . . .

Current processes for manufacturing circuit boards start with a layer of foil covering the entire board, on both inner and outer layers. The copper is then chemically etched away except where pads and traces are required. Adding fill zones reduces the amount of copper that must be removed, which in turn reduces the amount of chemicals that must be used and reduces the amount of chemical waste material that must be treated and disposed.

Dale


#12

This might be relevant if you are etching your own boards at home but the topic is about inner layers so that’s not likely the case here. If the fab making your boards is not environmentally friendly then nothing you do on your board is going to make a difference anyway, and if they are then again, nothing you can do on your board is going to make a difference as they already recover both the copper and the chemicals. So this should not be factor in design considerations.


#13

Putting planes on inner layers and signal on the outside does makes cut and strap correction possible.


#14

Lots of good points here, davidsrsb that’s true for prototyping, good point.

I was asked earlier, the MCU is a PIC32, 12-24MHz but the crystal is on the same layer and close by. The fat voltage lines on the pink inner avoid this area (on purpose). The PWM’s are mainly for driving LED’s so aren’t high frequency. There’s a 2.2MHz boost converter but the few components are above each other on the outer layers and at the edge of the board. A bluetooth module sits away from everything else. There’s a USB port and the clock is 48MHz. Hopefully these lines won’t suffer any EMI but that is one area where I could have ran the signals on the exterior plane.


#15

I do not see a ground plane on any of the 4 layers ???
All zones are cut into little pieces by long traces.
I also do not see many “via stitches” as an attempt to connect the different parts of ground planes.
The planes seem to have been added as an afterthought, and not designed in.

If the black cutout on Green and Red is for the bluetooth antenna, then the cutout should be on all layers.


#16

That is correct Paul. Originally I was using GND on the back, a relic of my two layer thinking. The dual sided SMD soon put an end to that. I had filled the inner layers but left them floating.


#17

It did look like some ad-hoc designed pcb that grew into this :slight_smile:
Is this a hobby product or commercial? Does it have to pass EMI tests?

For a single chip uC design the layout is not too critical.
PCB layout becomes difficult if you have multi chip designs with several hundred MHz clock frequency.
(A long time ago) I read some article about decoupling HF digital design.
They placed the decoupling caps as near as possible to the Chips’s as possible, then a short trace and a via to the GND plane. The inductance of that short trace & via kept most of the noise that the IC’s generated out of the GND plane itself

If you make sure that the decoupling is done properly then have a close look at stitching the GND plane parts together. Sometimes you can pretty easily move a few traces from your GND plane to another layer to make it more continous.
If you have signals that switch a lot (PWM, SPI) then try to put a GND plane under them.
Ideally the return current of those signals would be able to follow the same path through the GND plane.

I can not see what all those (parts of) planes are connected to.
As noted before: Close capacitive coupling between GND plane and VCC plane is good for decoupling.
Other nets should (almost ?) never have copper fill.
Copper fill to unconnected nets is not good.


#18

Based upon stackup, its best to keep each step of the construction of the board similar in copper density per side,

Its not seen as much, but a solid plane on 1 side, and a few traces on the other of a long board will lead to it curving,

So if your having a 4 layer board made out of 2x 2 layer pcbs, then your inner and outer pairs should be similar, and if its a 2 layer core, and 2 single layers stacked on, them the inner layers should be even, and the outer ones somewhat close, but far less important in this stackup.


#19

Cheers all.

So lots of ideas. I’ve learned that > 4 layers isn’t necessarily for track placing but for sandwiching GND planes between signal tracks.

As I’m using 4 layer and dual sided SMD, quite condensed, there’s no scope for making layer 2 or 3 fully GND. Even if I did it would still make one of the outer layers susceptible to EMI. However, in my case I believe EMI will be limited plus the board doesn’t need certifying in that regard.

I’m now on prototype #4 after ballsing the PIC32 footprint. I’ll incorporate some of the suggestions;

  • No unconnected islands on any layers
  • More GND vias

It appears most approve of filling all layers with GND copper even if entwined with power tracks (as per the middle layer). If not, speak now. :slight_smile:

Thanks, Andrew


#20

I don’t think copper fills help EMC very much.I like to devote one layer to be a true ground plane and avoid cutting it at all.