Selective soldering
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Selective soldering is the process of soldering only through-hole electronic components onto a printed circuit board that has surface mount components on the under-side. This is usually done because the surface mounted component is not glued into place, instead solder paste is used with the solder joints being formed using a reflow oven. Using a regular wave solder machine would effectively 'wash off' the components.
Selective soldering is performed by special machines only when there are limited "through hole" components on the board compared to surface mounted ones. When we have more smd components its always preferred to go for a reflow soldering and a wave soldering for the through hole components. But when we have thru hole components soldered on wave soldering machines, the soldered SMD components may fall down in to the waves. To avoid this once the reflow is done for SMD components, any one can go for a selective soldering for the through hole components. Selective soldering machines uses a nozzle with melted solder been pumped from it. The points to be soldered are targeted by nozzles that move to the exact locations to solder the pins individually or by group.
