Why do these chips have heat sinks??? To remove heat to prolong chip life.
And you want to apply heat from a hot air gun heating chips past their rated thermal limit to the point where solder re-flows (approx. 400 degrees C) Most chips have an upward thermal limit of around 120 degrees C.
Take it to a professional do not do this yourself. Remember that the chips have internal solder connections as well so whilst your heating up the chips to reflow the solder you also run the risk of detaching the internal connections thus forever ruining the chip.