4 layer stackup / ground pour question

[difflvl]

I have two stackups in mind.


L1 - Signal / Power
L2 - GND Plane
L3 - Signal
L4 - GND Pour only no signals


L1 - Signal
L2 - GND Plane
L3 - Power Plane
L4 - Signal

Which is better?

Also is it bad to add GND pours on all the layers besides planes even though I already have a GND plane? Is there such thing as over doing the ground pours?

Thanks

 

[Rame]

Hi,

Second layer stack up seems to be fine and there is no such thing that if you have a plane you need not to have copper pour.....if you have different grounds then how you will go about it?

Regards

Ramesh

 

[difflvl]

Thanks Ramesh,

Is it possible to overdue the copper pour ?

I ask because I have a pcb using the second layer set, with ground pour on top and bottom and I seem to have emc problems. The pcb is a module with headers, which mates to a motherboard with sockets/recepticles.

Now it works on one motherboard, but doesn't work on the other one (maybe 1 out of every 20 attempts it actually works).

I have a pcb module from another company with the same pinout and it works on both motherboards.

Thanks

 

[Rame]

Hi,

Remotely guiding on this matter is difficult,there can be many reason for noise generation,check the flow of your circuit completely tested,then try to isolate that noisy circuit from other functional block.

sometimes noise will be picked from the connector, from one board ,which in turn mated with the other board and carry away there,which causes more problem and it will be very difficult for you to figure it out the cause and from where it is generated.

Go through the articles from Douglas brooks on that topic,also check this site out,

http://www.hottconsultants.com/pdf_files/pcb_guide.pdf

http://www.atmel.com/dyn/resources/prod_documents/doc4279.pdf


**broken link removed**


Hope this helps you in progressing......


Regards

Ramesh

 

[House_Cat]

Is it possible to overdue the copper pour ?

The answer is YES - it is possible to overdo copper pour.

Think of it this way - a copper pour is one plate of a capacitor; your signal traces are the other plate(s). You are coupling the signals on the layer with the pour to one another through a common capacitor plate. To minimize the effect, it's necessary to connect the pour to power or ground at as many places as possible, and/or keep the spacing between the pour and signals as wide as possible. You are also affecting the impedance of the traces by the presence of the pour. Additionally, the extra pour provides potential paths for undesired signal return loops. It is even possible to form resonant loops in the odd shapes that result in a surface pour. Those resonant loops become sources for emi, as well as distorting signals on the board itself.

Your stackup with internal solid planes for power and ground, allows impedance control as well as providing a direct signal return path. Additional external pours are unnecessary and undesirable.

 

[difflvl]

Thanks Ramesh and HouseCat