Selective soldering is a method used to construct printed circuit boards in a specialized way. This process usually involves soldering specific electronic components to a circuit board without affecting other sections of the board, unlike many reflow-soldering techniques, which subject the entire board to molten solder. Any type of soldering can be considered selective soldering as long as it is precise enough to apply solder only to intended areas.
A variety of soldering techniques are employed during circuit board construction. For example, you may install and solder more rugged components, such as resistors, using an over-reflow procedure, and then you might selectively solder the sensitive components in a more controlled manner (e.g., at a specific temperature).
You can selectively solder connections one at a time or all at once, but comprehensive soldering (i.e. all at once) generally require specialized tools, depending on what must be done, and this process requires a considerable time investment.
You can solder a number of components at the same time with the mass selective dip method. This technique requires a special tool that masks off the parts of the circuit board you do not wish to solder — the areas you do want to solder have an opening (i.e. aperture) to expose them to the molten solder without reaching other areas of the circuit board.
Miniature wave systems, which use a tiny bubble of molten solder, are an economical way to selectively solder. You move the circuit board over the bubble where you need a connection. Although this process needs no special tools, you can only make one connection at a time and you must continually move the board, which makes for rather slow soldering.
The fastest and most precise means of selective soldering one component at a time is accomplished with a laser soldering system. Laser soldering systems employ a computer program to swiftly position the laser to heat each connection by itslef. This process is especially precise and you do not need any special tools, but it is significantly slower than your typical all-at-once selective soldering system.