Depending on the complexity of the board this is done in various steps and even the CPU socket and the chipset is mounted by machines. However, things like the capacitors, slots, I/O ports and pretty much anything that isn’t surface mounted is done by hand. Once all these things have been fitted onto the PCB it goes through yet another oven to solder all these components into place and then the motherboard is ready to have the BIOS fitted. Once this has been done it goes onto the testing stage where every single port and connector on the board is tested and the board is booted up and several different software tests are run to make sure the board works correctly.
Read more at Tweaktown.