Propagation Delay - Tpd

Lately I have been routing DDR3 designs and I'm keenly aware of Tpd. Why ?

Tpd = 1/(11.8/(SQRT(Keff)))*1000

Where Keff = the effective dielectric constant of the material surrounding the trace.

Let's say you have routed a portion of a DDR Data signal as a 50 Ohm 1 inch track on an outer layer, note Tpd ~ 139pSec per inch for a 50 Ohm microstrip.

And for the same net you have routed another 1 inch of track as 50 Ohm stripline on an inner layer, note Tpd = ~ 170pSec per inch for a 50 Ohm stripline.

Not counting for the Via length, the two inches of net routed as described above are 139pS + 170pS, which equals 309pSec.

As you can see if we are routing high speed signals with matched trace lengths then it matters what layer the traces are on.

Example:
2 inches of 50 Ohm microstrip = 2 x 139pS = 278pS
2 inches of 50 Ohm Stripline = 2 x 170pS = 340pS

That's big timing difference in terms of pSecs.  340pS - 278pS = 62pS

62pS would equal ~372mils of trace length.

Knowing Tpd and the length of your traces is critical for high speed designs.

Because signal velocity is FASTER FOR OUTER LAYER TRACES THAN FOR INNER LAYER TRACES, propagation delay is smaller for the outer layer.

There are fewer Tpd calculators available for differential pairs.  Differential pairs are slightly faster than single trace stripline conductors.  For a typical 100 Ohm differential pair Tpd ~ 155pSec per inch.

Clock skew needs to be considered in DDR3 length matching to the data lanes, therefore knowing the Tpd for the differential clock pairs is important.

Matched Timing:

To avoid timing mismatches designers often route data lanes on the same layers using equal trace lengths and geometry on each layer that the signals traverse.

Use the same number of vias in each signal routed. 

You may need to adjust the trace length for the differential pairs to match timing to the single-ended microstrip and stripline traces.

Tpd is propagation delay and V (velocity) is the reciprocal of Tpd. 

Example: if Tpd = 139pS/inch then V = 1/139 = 0.00719 inches per pSec.
Example: if Tpd = 170pS/inch then V = 1/170 = 0.00588 inches per pSec.

Rule of thumb: 
Typical FR4 material 6mils of trace is ~ 1 pSec.

That's it.