Final concentration of primers 100pM for a PCR? Is that some kind of weird PCR that uses 1000x lower final concentration in reaction, than the usual range? (0.1 - 0.5 µM)
Or is that pmol/µl or just something different molar concentration than per liter (so 100µM primers are equal to 100pmol/µl primers).
In any case, you don't want to dilute your primers to the final concentration, but to a working concentration 10-20-50x or so higher than the target one, you need to add other things in reaction too. The volume of the primers used must be suitably low, but not too low, to be pipetted accurately.
Stock solution of each primer is usually 100µM (100pmol/µl).
You make a working solution of 10µM out of that (dilute 10x).
For a 20µl PCR reaction, you use 1 µl of working solution of each primer (and so have a space to add other reagents) to achieve final concentration in reaction 0.5µM of each.