The way that you rework an assembly depends on the type of problem.
If the fault relates to poorly wetted joints, or joints with insufficient
solder, the procedure involves re-heating the connection (having
applied flux) and adding solder. However, faults due to misplaced
components, or requiring the component to be replaced, or with excess
solder, require the solder to be removed first (with the component
if necessary) and the pad cleaned and re-soldered. This is a much
more extended process, and needs the use of solder braid or vacuum
suction to remove the solder.
When reworking a board, one has to remember that, by confining the heat to a small area, one inevitably experiences a substantial heat sinking effect from the remainder of the board. Depending on the quality of these conducting paths, it may be necessary to preheat the board. One should never try to compensate for heat loss simply by ‘turning up the wick’ on the hot air jet or soldering iron, since excessive heating will damage both board and components.
Typically one uses soldering irons and similar direct sources of heat to remove components, because heat can be applied quickly and the component removed. For remaking the joint, however, it is advantageous to try as far as possible to simulate the original process by which the board was assembled, so a non-contact process may give better results.
In addition to equipment, one has to bear in mind the need for using both a flux and solder in appropriate forms. Typically fine wire would be easier to use than solder paste, but the flux core must be compatible with the flux residues already in the circuit.
Cleaning is a particular problem with rework, since inevitably the reworked area is visible because of its poor cosmetic quality. If this is unacceptable to the end customer, even though care is chosen to use no-clean materials, then local cleaning may be necessary.
There are two reasons why it is better to try and avoid the need for rework: the process is extended, with the result that it is expensive, there is always the possibility of damaging the circuit during the rework process, and a reworked joint has in any case been shown to be less reliable than one that has been soldered only once.