The order of acidic strength of boron trihalides

The strength of acidic character of boron trihalides depends upon $p \pi - p \pi$ back bonding.

In boron trihalides, $p \pi - p \pi$ -back bonding occurs due to empty orbital of boron and filled orbitals of halogens.

The $p \pi - p \pi$ 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 size of halogen increases. The order of size of halogens are $F < Cl < Br < I$. Thus, acidic nature is in order

${BF}_{3} < {BCl}_{3} < {BBr}_{3} < {BI}_{3}$