2.2
Add-In Board Routing
The PEX 8632 Transmitter differential pairs escape on the top layer, but at some point must route to the
bottom layer, to connect to the gold fingers. If a logic analyzer midbus footprint is placed in the routing
path, the layer transition can occur at that point. This works out well, because the midbus footprint will
have a significant number of ground vias, which provide effective ground plane stitching for the differential
signal’s return path. If a midbus footprint is not used, layer changing can occur at the AC-coupling
capacitors. Dedicated ground vias can be placed near the capacitors, close to the signal vias, to provide a
return path. One ground via per pair is ideal; however, one via per every two pairs is acceptable.
Receiver differential pairs must also transition signal layers, at least one time. Receiver differential pairs
start off at the top layer, from the gold fingers, and route into the inner rows of the BGA. The layer
transition should occur at the midbus footprint, if one exists, or close to the gold fingers. Either location
should have plenty of ground vias.
PCI Express add-in boards must be length-matched within 5-mil. AC-coupling capacitors should be
placed close to the gold fingers. Differential pairs for PCI Express Gen2 add-in boards should have
a differential impedance of between 68 to 105 ohms (85 ohms, nominal).
Figure 16. Add-In Board Routing to PCI Express Gold Fingers
2.3
System Board Routing
System board routing is simplified slightly. Transmitter differential pairs can escape on the top layer, from
the BGA, and route to AC-coupling capacitors, placed close to the slot. Similarly, Receiver differential
pairs can escape on the bottom layer and directly route to the slot. Transmitter differential pairs can also
transition to the bottom layer after the AC-coupling capacitors, to minimize the stub effects of a
through-hole PCI Express slot.
PCI Express system boards must be length-matched within 10-mil. AC-coupling capacitors should be
placed close to the slot. Differential pairs for PCI Express Gen2 system boards should have a differential
impedance between 68 to 105 ohms (85 ohms, nominal).
Figure 17. System Board Routing to PCI Express Slot
PEX 8632-AA Quick Start Hardware Design Guide, Version 1.1
© 2007 PLX Technology, Inc. All Rights Reserved.
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