Circuitwise has released a video demonstrating the benefits of selective soldering in a nitrogen-rich atmosphere. The video shows how oxidisation causes a skin, or soupy layer, to form on the solder wave in a normal atmosphere but is smooth in a 99.999% nitrogen atmosphere. The use of nitrogen results in better wetting and clean, shiny joints, with less flux and a lower temperature.
In a normal process, a lot of flux is used to combat the effects of oxidation, which results in a dull surface and left-over residue and may necessitate washing or brushing of the PCB. With nitrogen, the result looks like leaded solder is used, even with lead-free solder.
Selective soldering is used to automate the manual placement of through-hole components for low volume projects, typically in the prototyping stage of product development when there are frequent changes to board design.
The board is normally already populated with SMT components top and bottom. Through-hole components can’t be soldered using a conventional wave soldering machine as the bottom side SMT components would fall off in the high-temperature solder wave.
Selective soldering also provides more options than wave soldering. Different sized nozzles can be used depending on the clearance from the through hole pin to the SMT parts around it. On bigger thermal mass locations, you can set the nozzle on the part longer than sensitive parts. For example, the nozzle can be programmed to sit for around five seconds on bigger pins, down to about one second for smaller pins.
Selective soldering normally refers to a standalone machine with a soldering nozzle, but an alternative “selective” process is the use of a custom mask (called a pallet) in the wave soldering process. This option is used in high volume producing where the cost of fabricating the pallet from high-temperature resistant materials is justified. The pallet is fabricated to protect the SMT components as the board passes over the wave, exposing only the selected components to be soldered.
Circuitwise has an EBSO SPA 400 automatic selective soldering machine which can accommodate a 400mm x 400mm solder area. It is a dual pot system to accommodate both leaded and lead-free solder.