7.7.2.3.3.3 PCB Stackup
The minimum stackup for routing the DDR3 interface is a four-layer stack up as shown in Table 7-61. Additional layers may be added to the PCB stackup to accommodate other circuitry, enhance signal integrity and electromagnetic interference performance, or to reduce the size of the PCB footprint.
Table 7-61 Minimum PCB Stackup(1)
LAYER |
TYPE |
DESCRIPTION |
1 |
Signal |
Top signal routing |
2 |
Plane |
Ground |
3 |
Plane |
Split Power Plane |
4 |
Signal |
Bottom signal routing |
- All signals that have critical signal integrity requirements should be routed first on layer 1. It may not be possible to route all of these signals on layer 1, therefore requiring routing of some signals on layer 4. When this is done, the signal routes on layer 4 must not cross splits in the power plane.
Table 7-62 PCB Stackup Specifications(1)
NO. |
PARAMETER |
MIN |
TYP |
MAX |
UNIT |
1 |
PCB routing and plane layers |
4 |
|
|
|
2 |
Signal routing layers |
2 |
|
|
|
3 |
Full ground reference layers under DDR3 routing region(2) |
1 |
|
|
|
4 |
Full VDDS_DDR power reference layers under the DDR3 routing region(2) |
1 |
|
|
|
5 |
Number of reference plane cuts allowed within DDR3 routing region(3) |
|
|
0 |
|
6 |
Number of layers between DDR3 routing layer and reference plane(4) |
|
|
0 |
|
7 |
PCB routing feature size |
|
4 |
|
mils |
8 |
PCB trace width, w |
|
4 |
|
mils |
9 |
PCB BGA escape via pad size(5) |
|
18 |
20 |
mils |
10 |
PCB BGA escape via hole size |
|
10 |
|
mils |
11 |
Single-ended impedance, Zo(6) |
|
50 |
75 |
Ω |
12 |
Impedance control(7)(8) |
Zo-5 |
Zo |
Zo+5 |
Ω |
- For the DDR3 device BGA pad size, see the DDR3 device manufacturer documentation.
- Ground reference layers are preferred over power reference layers. Be sure to include bypass capacitors to accommodate reference layer return current as the trace routes switch routing layers.
- No traces should cross reference plane cuts within the DDR3 routing region. High-speed signal traces crossing reference plane cuts create large return current paths which can lead to excessive crosstalk and EMI radiation.
- Reference planes are to be directly adjacent to the signal plane to minimize the size of the return current loop.
- An 18-mil pad assumes Via Channel is the most economical BGA escape. A 20-mil pad may be used if additional layers are available for power routing. An 18-mil pad is required for minimum layer count escape.
- Zo is the nominal singled-ended impedance selected for the PCB.
- This parameter specifies the AC characteristic impedance tolerance for each segment of a PCB signal trace relative to the chosen Zo defined by the single-ended impedance parameter.
- Tighter impedance control is required to ensure flight time skew is minimal.