Here’s a partial schematic we’ll use to illustrate the advantage of a
ground plane. The idea is that an output pin on the microprocessor
is driving an external gate. The logic gate, U2, will have some input
capacitance, such that a transient return current must flow in the
ground connection. (ignore the pin numbers - it’s just an illustration)
Here’s a simple first attempt at making the connections.
This layout provides a return-current path with a large loop area.
Inductance is a complicated function of geometry, but in general it
increases with increasing loop area.
Our schematic effectively looks like this. A step change in voltage on
the processor pin will resulting in ringing of the parasitic LC circuit,
possibly resulting in unintended transisitions of the inverter output.
Using a ground plane on the back side (solder side) of the board
reduces the loop area. But why? Where will the return current flow?
.
If the return current takes the shortest path - the path of least
resistance – the loop area is roughly the same as before.
But the transient current will take the path of least impedance, which
is to say, least inductance, and therefore the one with the minimum
loop area. The ground plane provides a return path directly
underneath the trace, no matter how the trace is routed.
Now let’s suppose we need to route a signal which must cross the previous
signal. The crossover trace on the back side only removes a small amount of
copper, but completely disrupts the return current path. If crossovers like this
can’t be avoided, then it’s time to go to a 4-layer board where an entire inner
layer can be committed to the ground plane.
4-layer PCBs from ExpressPCB
1.75 mils, 1.25oz Cu
12 mils, k=4.6
1.4 mils, 1oz Cu
28 mils, k=4.7
1.4 mils, 1oz Cu
12mils, k=4.6
1.75 mils, 1.25oz Cu
(Via to inner layer)