We start at the entrance, where Gigabyte has an actual model of its headquarters made from CPU trays, heatsinks, and other components that it sells.
The first step in motherboard design is figuring out what platform to design for. After this is decided, engineering teams work with chipset makers like AMD and Intel to come up with a product scheme. Once settled, reference designs are provided, and Gigabyte then goes through its initial work, defining actual board specifications and looking into possible third-party components to add value. Features like onboard RAID, sound, connectivity, and other extras are picked at this point.
Once the specs are finalized, the engineering team begins its layout job in software simulation. This allows for bug checking and other changes to be made before the layout goes to "the wire." During this process, engineers take everything into consideration--even the actual length of traces on a board.
Once done, layers are defined. Typically, you will see four-layer PCBs consisting of the following:
Layer 1: Top layer. This layer carries signals
Layer 2: Copper. Power distribution
Layer 3: Copper. Ground
Layer 4: Bottom layer. Signaling
Signals can be passed through layers by what's known as a "via."
Components, routing, and layout are done on specialized layout software. Often times, several revisions are made to either tune, fix bugs, or change a component.
Of course, this is an oversimplification of the entire process, but this is essentially the major steps before a board enters production.