Pick and Place
Once the board has paste applied, the components are placed on top. This can be done by hand or by a machine. Using tweezers is a great way to place components. There is a common fallacy that you need large, expensive machines to make electronics. Not true! A human is quite fast at placing components and the surface tension of liquid metal is such that most components will shift into their correct location during reflow. But humans have a limit to their stamina. After a few hours it becomes harder to place components quickly. The small size of the components also tend to strain eyes.
SparkFun started by placing all the components by hand and we still do! But if we have more than about a hundred units to build we use a pick and place machine to allow us to build lots of boards.
A pick and place (PNP) machine is a robotic assembly device that uses a vacuum to lift a component off of a piece of tape, rotate it to the right orientation, then place it on a circuit board. It takes a few hours to setup a machine to build the assembly, but once everything is running, it is very fast.
In a large factory, a conveyor belt often carries the board directly from an automated paste deposition machine into the pick-and-place machines; at SparkFun, we move our boards manually around the production floor.