EMSL’s Windell Oskay plays around with solder paste, the er, soldering medium in paste form used in attaching surface mount components to PCBs. The part that particularly fascinated me was when Windell zoomed in on some paste with his microscope, revealing a multitude of tiny balls of solder suspended in goop:
When you really get down to it, the tiny bits of solder there really do look like the regular wire solder that you might use at your workbench. Only, they’re neatly round, and suspended in a liquid solder flux. After stenciling and placing the components, the circuit boards will be baked in a reflow oven until all those solder balls melt and fuse together, becoming roughly indistinguishable from solder that started out in a wire.