The Lewis acid character of boron tri halides follows the order:

The strength of the acidic character of boron trihalides depends upon $\pi - \pi$ back bonding. In boron trihalides, $\pi - \pi$ -back bonding occurs due to empty orbital of boron and filled orbitals of halogens.

The $\pi - \pi$ back bonding is shown maximum by${BF}_{3}$, as the size of B and F are small and comparatively same. Due to this effect tendency of accepting lone-pair of electrons of boron decreases as the size of halogen increases. The order of size of halogens are $F < Cl < Br < I$ . Thus, acidic nature is in order ${BF}_{3} < {BCl}_{3} < {BBr}_{3} < {Bl}_{3}$.