The boiling points of halogens increase down the group due to the increasing strength of intermolecular attraction as the size and relative atomic mass of the atoms increase. The halogens all exist as diatomic molecules in nature. In other words, they exist as two joined atoms of the element such as F2, Cl2, Br2, I2 . The forces that hold the molecules of halogens together are called London dispersion forces. These are the forces of intermolecular attraction that must be overcome for liquid halogens to reach their boiling points. Electrons move in a random fashion around the nucleus of an atom. At any one time, there may be more electrons on one side of a molecule, creating a temporary negative charge on that side and a temporary positive charge on the other side -- an instantaneous dipole. The temporary negative and positive poles of different molecules attract each other, and the sum of the temporary forces results in a weak intermolecular force.
Therefore, the boiling point affects by intermolecular attraction not by strength of the bond.
