Our common computer boards are basically epoxy glass cloth-based double-sided PCB boards, one of which is a plug-in component and the other side is a component foot welding surface.

It can be seen that the solder joints are very regular. We call it the PCB board pad for the discrete soldering surface of the component feet of these solder joints. Why are other copper wire patterns not tinned?

Because in addition to the pads that need to be soldered, the rest of the surface has a solder mask that is resistant to wave soldering. Most of the external solder mask is green, and a small amount of yellow, black, blue, etc. is used, so the solder mask oil is often called green oil in the PCB circuit board industry. The effect is to prevent bridging during wave soldering, improve soldering quality and save solder.