Selective soldering

Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or Through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.
Processes.
Assembly processes used in selective soldering include:
Less-common selective soldering processes include:
Other selective soldering applications are non-electronic, such as lead-frame attachment to ceramic substrates, coil-lead attachment, SMT attachment (such as LEDs to PCBs) and fire sprinklers (where the fuse is low-temperature solder alloys).
Regardless of the selective soldering equipment used, there are two types of selective flux applicators: spray and dropjet fluxers. The spray fluxer applies atomized flux to a specific area, while the dropjet fluxer is more precise; the choice depends on the circumstances surrounding the soldering application.
Miniature wave selective solder fountain.
The miniature wave selective solder fountain type is widely used, yielding good results if the PCB design and manufacturing process are optimized. Key requirements for selective fountain type soldering are:
Thermal Profiling.
The thermal profile of the selective process is critical as with other common automated soldering techniques.
Topside temperature measurements within the pre-heat stage must be verified as with conventional flow solder machine, additionally flux activation must be verified as sufficient.
As number of miniature profiling dataloggers are now available to make the process more simple such as the Solderstar Pro units.
Selective Solder Optimization.
A number of fixtures are available to allow daily checking of the selective solder process, these instruments allow the verification of machine parameters to be performed on a periodic basis.
Parameters such as contact time, X/Y speeds, nozzle wave height and profile temperature can all be measured.
Use of Nitrogen Atmosphere.
Selective soldering is normally undertaken in a nitrogen atmosphere. This prevents oxidation of the fountain surface and results in better wetting. Less flux is needed with less left-over residue. The use of nitrogen results in clean, shiny joints without the need for PCB cleaning or brushing.