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Why large area of ground in PCB design can improve the performance of the PCB?

We can also analyze it in terms of E/H fields .


As illustrated above, both microstrip line and stripline radiate E-field outside.
Thus, we need ground plane to absorb their E fields.
And larger the ground plane,
lower the ground impedance, more the E-fields that ground plane absorbs.

If ground plane is not large enough, both microstrip and stripline have EMI emission risk.


Besides, some PCB layout guides tell you that don’t lay your PCB layout trace
(especially high speed digital signal , or power trace) in the board edge.
It’s the reason as well. Otherwise, there will be Fringing Effect.



RF, or High speed digital currents fringing between the trace and ground planes at the edge of the
board can result in RF emissions.
This is fringing effect.
Consequently, the ground plane should exceed the trace by 20H,
where H is the total thickness between the power and ground planes.
This is so-called 20-H rule.
20-H provides for approximately a 70% reduction of the fringing flux.

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