Eight-Layer Stack-Up; Trace-Impedance Considerations - Emerson COM Express Carrier Design Manual

This figure is an example of a six layer stack-up. Layers L1, L3, L4 and L6 are used for signal-routing.
Layers L2 and L5 are power and ground planes, respectively.
Microstrips on layers 1 and 6 reference solid ground and power planes on layers 2 and 5, respectively.
Inner layers 3 and 4 are asymmetric striplines that are referenced to planes on layers 2 and 5.

21.1.3 Eight-Layer Stack-up

Figure 38 Eight Layer Stack-Up
This figure is an example of an eight layer stack-up. Layers L1, L3, L6 and L8 are used for signal-routing.
Layers L2 and L7 are solid ground planes, while L4 and L5 are used for power. Microstrip layers 1 and 8
reference solid ground planes on layers 2 and 7, respectively. Inner signal layers 3 and 6 are asymmetric
striplines that route differential signals. These signals are referenced to layers 2 and 7 to meet the
characteristic impedance target for these traces. To reduce coupling to layers 4 and 5, specify thicker
prepreg to increase layer separation.

21.2 Trace-Impedance Considerations

Most high-speed interfaces used in a COM Express design for a carrier board are differential pairs that
need a well-defined and consistent differential and single-ended impedance. The differential pairs
should be edge-coupled (i.e., the two lines in the pair are on the same PCB layer, at a consistent
spacing to each other). Broadside coupling (where the two lines in the pair track each other on
different layers) is not recommended for mainstream commercial PCB fabrication.
There are two basic structures used for high-speed differential and single-ended signals. The first is
known as a microstrip. In a microstrip, a trace or trace pair is referenced to a single ground or power
plane.
The outer layers of multi-layer PCBs are microstrips. Figure 39 is a diagram of a microstrip cross section
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Design Guide