Selective soldering is one of the processes used in the construction of various electronic assemblies, usually circuit boards. Typically, the process involves the soldering of specific electronic components onto a printed circuit board while leaving other areas of the board unaffected. This is in contrast to various reflow-soldering processes that expose the entire board to molten solder. In practice, selective soldering can refer to any soldering method, from hand soldering to specialized soldering equipment, so long as the method is sufficiently precise to apply the solder only on the desired areas.
It is common for a circuit board to undergo several different soldering processes during its construction. As an example, a circuit board may have all of its less sensitive components, such as resistors, installed and then soldered using an oven-reflow process. The board would then undergo a selective soldering process to install its more sensitive components under different or more controlled conditions, such as within a very specific temperature range.
Several different technologies exist to perform the task of selective soldering. These can solder the desired connections either all at once or one at a time. Typically, all-at-once technologies require specialized tooling for every different set of tasks they must perform. While such tooling is usually not needed for one-at-a-time technologies, they do tend to take much longer to accomplish a given task.
The mass selective dip method can solder many connections simultaneously. This method requires the construction of a special tool, which can only be used to solder one set of connections on a specific circuit board design. The tooling for this method has a number of small holes through which molten solder is pumped, creating a series of small puddles. The circuit board is then placed onto the tooling, which dips the desired areas of the board into the solder puddles.
Another all-at-once technology is the selective aperture method. This technology typically uses a special tool that masks off all portions of a circuit board, except where the solder is desired. At those locations, the tooling has an opening, or aperture. The circuit board, with the tooling attached, is then bathed in molten solder, which only reaches the areas exposed by the tooling.
Miniature wave systems for selective soldering are one of the least expensive methods. This technology uses what amounts to a very small bubble of molten solder. The circuit board is moved over the bubble and placed onto it where a solder connection is desired. While this technology requires no special tooling, repeatedly moving the circuit board, making only one connection at a time, is a very slow process.
Laser soldering systems are the fastest and most precise of the one-at-a-time type technologies. With this method, a computer program quickly positions a laser to heat each solder connection individually. This is a very precise method that requires no special tooling; however, it is still much slower than all-at-once systems.
