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What is the GND Copper Plane for in MultiLayer Board?

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mkbs

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GND Plane

I have seen some MultiLayer Board ( 6-8Layer with 1or2 GND plane ), with on top and bottom a GND Plane Copper.
Is It necessary for what?... shieding board or other ?
And if necessary where I have to join this Copper Plane? All GND Plane (External & Internal) together with via ?

Many Tanks
mkbs :)
 

pisoiu

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Re: GND Plane

Hi,
Usually, there are more than one gnd plane in mixed analog digital boards. There is at least one analog ground and one digital ground, as well as for several separated supply voltages analog and digital. Of course, planes or gnd areas can be used for shielding or for traces impedance control in microstrip or coplanar waveguide configuration (mose used). Usually, ground planes are connected in one single point, in order to avoid ground loops. If you need more information on this look at e-book upload/download section. Search for signal integrity, high speed pcb design, etc.
 

ewabed

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Re: GND Plane

For high speed digital design it is recommended to connect the multiple ground layers together everythere the signal jumps through via from one layer to another which is referenced to another ground layer to allow the return current follow the least impedance path. If the grounds are connected in one point only it doesn't improve the way for return currents. One point connection of multiple ground planes is good only for mixead analog/digital designs or to make the shield for electrical fields.
 

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Re: GND Plane

For the same purpose, at 4 layers pcb, when only one gnd plane and one power plane are used, and the trace is routed on both routing layers above the gnd and power plane, the return path can be closed with a cap placed near the via, between power planes. From AC pov, power plane is identical with gnd plane.
 

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