Both ${\mathrm{BF}}_3\mathrm{and}{\mathrm{NF}}_3$ are covalent compounds. ${\mathrm{BF}}_3$ is a non-polar compound but $N{F}_3$ is polar. The reason is that

Boron is a solid and nitrogen is a gas in free state

Boron is a metalloid while nitrogen is a non-metal

$B{F}_3$ is planar but $N{F}_3$ is pyramidal in shape

Atomic size of boron is smaller than that of nitrogen