Soldering
The tank of molten solder has a pattern of standing waves (or, in some cases, intermittent waves) on its surface. When the PCB is moved over this tank, the solder waves contact the bottom of the board, and stick to the solder pads and component leads via surface tension. Precise control of wave height is required to ensure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is sometimes performed in an inert nitrogen (N2) atmosphere to increase the quality of the joints. The presence of N2 also reduces oxidization known as solder dross.
Solder dross, its reduction and elimination, is a growing industry concern as lead soldering is being replaced by lead-free alternatives at significantly higher cost. Dross eliminators are entering the market and may hold some solutions for this concern.
Read more about this topic: Wave Soldering