When laying out a PCB, the general process is to:
- Place all components - Starting with the most important, biggest, and most complex groups, lay out the components in logical groups, approximately where you want them.
- Size Board - Sketch a rough board outline (if not already specified), add mounting holes, etc.
- Route Important Signals - Route the most important signals first. Signals requiring controlled impedance, special routing, differential pairs, etc.
- Route the Rest - Route the rest of the signals, not including power.
- Lay Power and Ground Planes - “Pour” copper to fill in the rest of the PCB with power and ground planes.
- Add Vias for Power and Ground - Finish routing power and ground by placing vias to connect parts to ground, and if more than a two-layer board, place power vias as well.