Boron trihalides are Lewis acids. The back bonding determines its acidic strength, more is the $\pi$-back bonding, less is the acidity. Thus, due to better overlapping of orbitals it promotes better $\pi$-back bonding, hence $\mathrm{BF}_3$ is least acidic in nature. Thus, order of acidic strength is $\mathrm{BF}_3 < \mathrm{BCl}_3 < \mathrm{BBr}_3$ $ < \mathrm{BI}_3$.