SDF containing original delay values of design.
This SDF can generate thro CELTIC tool (cadence)...
i.e crosstalk analysis tool
Here we give SPEF as input, cdB, then library, routed netlist we get the output as SDF format
We can give this SDF and SPEF to PRIMETIME (synopsys) or ETS (cadence) for timing sign off
This step comes after routing bcz once u do routing u have to go for R and C values tht is extraction then after extraction u need to go for delay calculation with the help of elmore delay
Remember one thing spef contains resistance and cap values where as SDF contains T=RC tht is delay values
After extracting r and c values the files wil be saved in SPEF format
And after calculating delay it will be in SDF format
Hope this is clear
SDF just give the timing info from RC of the net and the cell.
when you use NLDM model, it can work well.
but when you want use CCS model to get the more accurate result,
you should switch to SPEF. exclude you use the SPEF + CCS to generate SDF