Symmetrical molecules are generally non-polar although they have polar bonds. This is because bond dipole of one bond is cancelled by that of the other. $\mathrm{BF}_3, \mathrm{SiF}_4$ and $\mathrm{XeF}_4$ being symmetrical as non-polar. $\mathrm{SF}_4$ is unsymmetrical because of the presence of a lone pair of electrons. Due to which it is a polar molecule.