Power Planes Ch 6

6.8 A typical 8 Layer stack up

A four layer or a 6 six layer stack have one or other shortcoming. An 8 layer is the minimum number of layer that sufficiently addresses all the signal integrity requirements. It does not mean that 4 layers and 6 layers can not be used. The 4 layer and 8 layer can be and should be used for lower cost. But they will not fulfill all the signal integrity and EMI requirements is a dense PCB routing environment.

A typical 8 Layer Stackup looks as follows

Signal1
Ground
Signal2
Power
Ground
Signal3
Power
Signal4

The 8 Layer stack up provides ample routing space for multiple power islands. In the above typical stack up all signaling layers have at least one referencing power plane. The power and ground layer in the center provides good inter plane capacitance. If your system has many power islands and these power islands are next to the bottom layer, they do not have enough power to ground capacitance. Also the return path for the high speed signals in bottom layer (signal4) refer to the power plane which is not adjacent to a ground plane. For these reasons, some engineers use an alternate stackup as shown below.

Signal1
Ground
Power
Signal2
Signal3
Ground
Power
Signal4

By keeping the separation between signal2 and signal3 large we can eliminate any potential crosstalk between signal2 and signal3.

A 4 layer stackup 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.


Previous - 6 Layer Stackup                                Next - Symmetry and Board Warping