Hello everyone and thank you in advance.

I'm working on my first 4 layers board (SMT). My layer stack is like this:
TOP - Signal (most important signals)
Ground plane
Power tracks
Bottom - Signals

The board is for a mixed system including sensitive analog electronics, a few digital ICs, a high current Amp (1A) and some ports for a display and buttons.

After long and careful reading of many books and online documents, I've reach the conclusion that the best way to make it work properly is to let a big ground plane for everything, providing that buttons and screens will be rarely used. However, I have read contradictory pieces of information about letting a second ground plane on the top layer. On one hand, making a "plane" in the copper-free space in the sensitive analog electronics could make it more robust against EMI noise. It can improve also crosstalk as there will be a ground track between adjacent noisy tracks (there aren't high-speed signals here, so don't expect any). On the other hand, it can lead to ground loops if I attach the polygon to the net. There is a possibility to connect the two planes only at one point, but it would worsen the ground loops.

To sum up, there are some alternatives and don't know which one is the best:

1- Let the TOP layer without a ground copper pour

2- Make the copper pour and connect it to the ground plane in only a point (not connect it to the ground pins of the components)

3- Make the copper pour and connect it to every ground pin and to every via to the ground plane

I'm thinking of a ground copper-pour only in the analog area.

Thank you again.

 

I'm not the most experienced guy here with layout so I'm sure you'll get better advice, but I use signals on top and bottom with a power and ground plane on layers 2 and 3. Read the datasheets for the individual components, often there will be notes about how those parts need to be routed, including notes about ground planes and pours. For the signal layers, I try to use one for exclusively vertical tracks and the other for horizontal. There are always small exceptions, but generally speaking this reduces vias and makes your life easier. I would try to keep your analog and digital lines as far from each other as possible, and keep your capacitors as close to their respective parts as possible. If you need to route analog and digital lines over or next to each other, perhaps a ground plane in-between them would help keep noise down. I'm not knowledgeable enough to say when a star ground topology is appropriate, but maybe that's worth reading on.

 

Thank you, Mr Software,

I have taken into account the tips you mention, every noisy line is as far as possible to the sensitive circuits, I have different tracks for digital and analog power, capacitors as close as possible and with a ground via for every connection. The only thing I'm not at least 60% sure about is the redundant plane in the top layer.

 

I've built many a PCB with both sensitive RF circuitry and noisy digital on the same board. To minimize crosstalk, you need very low impedance grounds, so ground flood was used EVERYWHERE and heavily stitched together with corresponding ground vias. Phase lock loops needed to be protected from "ground waves", so they had "directing slots" cut around their circuits so that these waves wouldn't go directly under the PLLs (a quasi star pattern). When the slots were used, they were staggered on the different layers, because when stacked on top of each other, they became slot radiators (antennas). If you show us your work so far, we can give you specific guidance as to your routing.

 

Thank you SLK001!

So the correct answer is number 3: I should put copper everywhere in the top layer and connect it to GND pins and GND vias at every connection. Is it right?

 

Thank you SLK001! So the correct answer is number 3: I should put copper everywhere in the top layer and connect it to GND pins and GND vias at every connection. Is it right?

Yes, as much as possible. This can vary from board to board - you have to be smart when routing. Avoid orphans, thin ground lines, etc.

 

Yes, as much as possible. This can vary from board to board - you have to be smart when routing. Avoid orphans, thin ground lines, etc.

Can I change the smart thing for hard and harmful trial and error? My wife says to me every day that I'm a complete dumb.

Thank you so much, I think that the board is not that demanding but as I don't have much experience I'm never completely sure. Now I'm confident that the board is going to work.

Just to sum up, and for helping others in the future:

If you can, make a cooper pour in the top layer and connect it to every via to the ground plane.

 

Post your PCB result for feedback.