${\mathrm{BF}}_3\mathrm{and}{\mathrm{NF}}_3$ both molecules are covalent, but ${\mathrm{BF}}_3$ is non-polar and ${\mathrm{NF}}_3$ is polar. Its reason is

The size of boron atom is smaller than nitrogen

$B-F$ bond has no dipole moment whereas $N-F$ bond has dipole moment

In uncombined state boron is metal and nitrogen is gas

${\mathrm{BF}}_3$ is planar whereas ${\mathrm{NF}}_3$ is pyramidal