What are the factors affecting Boiling Point in Ions?

I understand that for molecular substances, intermolecular forces such as van der Waals forces and hydrogen bonding are the major factors effecting the boiling point. 


However, I am not sure about what factors effect the boiling point in Ionic substances. I assume it is to do with the strength of the bonds between ionic units, but then what effects this strength?


Thanks for your time!


