voids under microstrip boards

What effects do minor voids under a microstrip board design (when screw mounting board material on a metal carriers) have on the line performance. i.e. impendence, loss and input/output interconnects (launchers screwed to the carrier).

Only the dielectric directly under a trace and to a lesser extent one dielectric height on either side affect the impedance. If the void is very small compared to a wavelength it should not have much effect.