For greater producibility, there should be as few metallization
layers as possible. For digital boards with up to eight metallization layers,
two typical layer configurations are shown in Figure 4-3. Configuration A shows
the power and ground planes furthest removed from the component mounting layer,
with Vcc (+5v supply voltage) next to the ceramic blank and GND (ground) between
the signal layers and Vcc. This configuration allows less noise from the Vcc
plane to be coupled with the signals and also less capacitance. Having the Vcc
and GND planes switched could result in possible noise coupling from the Vcc
with the signal layers.
The advantages of configuration B in Figure 4-3, with the ground
plane sandwiched between signal layers, are that there is less cross-coupling
between signals on the same layer and also that shielding is provided. One
disadvantage is that there is more capacitance to ground on the signal layers
which leads to increased signal rise and fall times.
For analog boards, Figure 4-4 shows two of many possible
configurations. The analog ground is sandwiched between signal layers and the
analog voltage plane. The digital voltage and ground are shielded from the
sensitive analog signals by the analog power/ground planes.