PEX 8618 Quick Start Hardware Design Guide – Version 1.2
Copyright © 2009 by PLX Technology, Inc. All rights reserved.
8
2 PCB Layout and Stackup Considerations
PCB layout is of critical importance for PCI Express systems. Numerous form factor specifications (
PCI Express
Base Specification, Revision 2.0
and
PCI Express Card Electromechanical (CEM) Specification, Revisions 1.0a
and 1.1
) exist for providing important implementation guidelines for a given form factor. It is important to
understand the type of system being designed before starting layout.
For example
, the
PCI Express Card
Electromechanical (CEM) Specification
defines two platforms, referred to as
system boards
and
add-in cards
(boards)
. Each platform has its own criteria, in terms of jitter and loss budget, trace lengths and length matching,
and so forth.
2.1
PEX 8618 BGA Routing Escape and De-Coupling Capacitor Placement
One millimeter pitch BGA package routing can typically escape two rows of balls per signal layer. Power and
ground pads typically have small “dog-bone” nets from the pad to a via which will connect it with an internal power
or ground plane. The PEX 8618 places all Transmitter differential pairs on the outer two rows of balls and
Receiver differential pairs on rows three and four. This means it should take two signal layers in a PCB stackup to
escape the differential pairs from the BGA. All Transmitters can escape on the top layer of a PCB, whereas the
Receiver pairs can escape on either the bottom layer or some other internal signal layer.
Each pair is split between two rows on the package; hence, the pairs start off with a 1-mm (39.4-mil) offset. Small
serpentines may be necessary to match the lengths within the pair. If the differential pairs are tightly coupled,
make the serpentines as close to the launching end of the channel as possible, to allow the differential signal to
be tightly coupled as it travels down the channel. If uncoupled differential pairs are used, only the total
propagation delay of each half of the pair needs to be matched. This makes using uncoupled differential pairs
simpler to route. There are two disadvantages to using uncoupled differential pairs; they take more board real
estate to route, and they are not as immune to common mode noise.
The PEX 8618 is a full matrix, 1-mm pitch BGA. Hence, placing de-coupling capacitors underneath the BGA can
be tricky. It is best to use 0201-sized ceramic capacitors under the BGA matrix (bottom layer), so that the
capacitors can be placed as close to the power balls as possible. Refer to section
