Power Planes Ch 6

6.7 A typical 6 Layer stack up

A 4 layer stack up is pretty much straight forward. A 6 layer stack up can have a number of variations. We should know the reasons behind choice of these variations and select the one that suites our requirements. A typical 6 Layer Stack up looks as follows.

Signal1
Ground
Signal2
Signal3
Power
Signal4

In this stack up the traces on signal2 and signal3 should be routed in orthogonal fashion to avoid any potential cross talk. Care must be taken not to route traces in parallel along signal 2 and signal 3.

All 4 signaling layers can refer to at least on power plane. So all the four signaling layers can have controlled impedance traces. One drawback of this arrangement is that the power and ground layers are not adjacent. Adjacent power and ground planes provide inter plane capacitance which is essential for providing low impedance path for the high frequency power supply noise. With this in mind some engineers use the following stackup.

Signal1
Signal2
Ground
Power
Signal3
Signal4


The drawback of this stackup ? The Signal 1 and Signal 4 layers do not refer to the power planes directly. This problem results in inability to provide a controlled impedance on Signal 1 and Signal 4 layers.

So which stack up do you choose? If your design has very few high speed signals that need controlled impedance routing, the second stack up is better choice. You can route all the controlled impedance traces on Signal 2 and Signal 3 layers. The outer layers can be used for normal traces that do not need controlled impedance.

If however, your design has large number of high speed traces that need controlled impedance, the second stack up may not provide enough room. In such case the first stack up is a better choice. If you happen to choose the first stack up, try to fill the unused spaces on signal layers with plane flooding. This will provide extra plane capacitance between the power and ground layers required for the suppressing high frequency power supply noises.

A six layer stack up lacks the number of the power planes. A typical microprocessor based system will need many small power islands, for which a single power plane is not sufficient. In this scenario some of the power islands will have to be on signal2 and/or power plane. If you find the things hard you may use 8 Layer stack up.


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