appropriate boundary condition for PCB in CST

Dear All,
i need to simulate a pcb in CST ,but i dont know whats appropriate boundary condition !
can anyone help me ?

you can set the boundary condition to default and set your frequency, select a solver and simulate...

please explain more ...
i want to analyse scattering parameters ,but i don't know what is default boundary condition !

dont need to care about boundary conditions at all meaning dont need to click boundary conditions icon. just set the freq, select a solver and run the simulation.