How long routing on a PCB needs time 6.55 nS?

Hi,

I have an S-Parameters in Matlab. I find that the delay time is 6.55 nS after fitting the S-Parameters to a sum of rational components. On web, I find that 1 nS can pass 1 foot in free space. Can I use this to estimate PCB routing time? I try and get the PCB length is 6.55 * 1=6.55 foot=6.55*12 inch=78 inch. This seems too long from this back-plane on a serial link. Am I wrong?

Please correct me if you could.

Thanks,

Due to the dielectric material of the PCB, the wave is slower for PCB lines. If the field would be completely in the PCB material, it would be slower by sqrt(epsilon_r) where epsilon_r is the dielectric materials permittivity. For FR4 material, epsilon_r is around 4.2 ... 4.9.

So if we assume that all the electric field is in FR4 material, 1ns delay would correspond to approximately 0.47 ft

(This estimate is not exact. For a microstrip line, where some of the electric field is in air, the effective permittivity is smaller than the dielectric materials permittivity. The exact value depends on line width etc. )