Tuesday, September 2, 2008

PCB Stackup and Reference Planes PCI Express

PCI Express requires no new technology. Generally desktop system boards are designed with 4-layer stackup, whereas server, workstation, and mobile system boards use 6-layer stackup or greater. Add-in cards may use either 4-layer or 6-layer stackup. ½ oz copper plated microstrip and 1 oz copper stripline are used.

An add-in card is required to have overall board thickness of 0.062 inches. A mobile platform can have a thickness of 0.062 inches or 0.050 inches.

To minimize loss and jitter, the most important considerations are to design to a target impedance and to keep tolerances small. Thicker dielectrics and wider traces will minimize loss. Microstrip differential traces produce greater impedance variation than stripline traces.
A signal pair should avoid discontinuities in the reference plane, such as splits and voids. When a signal changes layers, the ground stitching vias should be placed close to the signal vias. A minimum of 1 to 3 stitching vias per pair of signals is recommended. Never route a trace so that it straddles a plane split.

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