User's Manual

Intel

820 Chipset Design Guide 2-23

Layout/Routing Guidelines

2.6.2.7 VIA Compensation

As described in Section 2.8.2, “Strobe Signals” on page 2-44, all signals must have the same

number of vias. As a result, each trace will have 1 via (near the BGA pad) because some of the

RSL signals must be routed on the bottom of the motherboard. Therefore, it is necessary to place a

dummy via on all signals that are routed on the top layer. Because the electrical characteristics of a

dummy via do not match the electrical characteristics of a real via exactly, additional compensation

must be performed on each signal that has a dummy via. Each signal with a dummy via must have

25 mils of additional trace length. That is: a real via = a dummy via + 25 mils of trace length.

This 25 mils of additional trace length must be added to each signal routed on the top layer after

length matching, as documented in Section 2.6.2.6, “Length Matching Methods” on page 2-21.

2.6.2.8 Length Matching & Via Compensation Example

Table 2-5 can be used to ensure that the RSL signals are the correct length.

Note: 2000 mils was chosen as an EXAMPLE Nominal RSL Length.

Figure 2-24. "Dummy" Via vs. Real "Via"

“DUMMY Via”

PCB PCB

“REAL Via”

PCB PCB

Trace

Via Via

Trace