Intel 820E Benutzerhandbuch

52  

Design 

Guide 

It is necessary to compensate for the slight difference in electrical characteristics between a dummy via 
and a real via. Refer to the following section for more information on via compensation. 

As described in Section 2.7.2.1, all signals must have the same number of vias. As a result, each trace 
will have one via (near the BGA pad) because some 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 exactly match the electrical 
characteristics of a real via, additional compensation must be performed for each signal that has a dummy 
via. Each signal with a dummy via must have 25 mils of additional trace length. That is: 

Real via = 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 2.7.2.6. 

dum_vias_vs_real.v

 

Table 8 can be used to ensure that the RSL signals are the correct length.  

Note:  2000 mils was chosen as an example nominal RSL length.