[SI-LIST] differential impedance

There are many pcb trace geometries that give 100R differential impedance. 
However, the even mode impedance will vary, depending on the amount of coupling 
between the traces. Consider a situation where two signal boards are connected 
together by being plugged into a backplane. Is there any advantage in using the 
same pcb trace geometry on the backplane as on the signal boards, or does it 
not matter (as long as both have 100R differential impedance)?
My reasoning is that it would make no difference in a perfect world. However, 
in practice there will be common mode components in the signal (for example, 
because of crosstalk and drivers not being perfectly symmetrical). Therefore, 
changes in even mode impedance may be significant in the real world, but how 
significant? Is this just a second-order effect?

Why might a designer consider different geometries? Signal boards may require 
very thin traces with relatively high coupling (for example, to route signals 
between the balls of a BGA). By comparison, a backplane may be better designed 
with wider traces (as these are more easily manufactured and less subject to 
impedance variation with manufacturing tolerances).

Any insights will be appreciated.

Stephen Greenhalgh

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