Reflow soldering is less demanding on pad shape, shadowing, board orientation, temperature profiles (still very important) and more. For surface mount components it is often a very good choice - solder and flux mix are preapplied with a stencil or other automated process, components are placed in position and are often adequately retained by the solder paste. Adhesive may be used in demanding cases. Use with through hole parts is problematic or worse - usally reflow will not be the method of choice for through hole parts.
Where it can be used reflow soldering is used in preference to wave. It is more amenable to small scale manufacture, and generally easier with SMD parts.
Complex and/or high density boards may use a mix of reflow and wave soldering with leaded parts being mounted on one side of the PCB only (call this side A) so they can be wave soldered on side B. Prior to through hole part insertion components can be reflow soldered on side A amidst where TH parts are going to be inserted. Additional SMD parts can then be added to side B to be wave soldered along with the TH parts. Those keen on high-wire acts can try complex mixes with different melting point solders, allowing reflow on side B before or after wave soldering, but that would be very uncommon.
Reflow soldering techniques are used for surface-mount parts. Whilst most surface-mount boards can be assembled manually using a soldering iron and solder wire, the process is slow and the resultant boards can be unreliable. Modern PCB assembly facilities use reflow soldering exclusively for large-scale production, with pick and place machines putting the components onto the boards, which have had solder paste applied to the pads, and the whole process is automated.
Reflow soldering can be performed at home with suitable hot-air soldering equipment, an electric skillet, or a toaster oven. The solder paste is applied with a stencil and a squeegee, the components are placed in position, and the board is heated. I prefer to assemble my boards manually with a soldering iron, as I tend to build them in stages, testing each stage as it is completed.
