As soon as parts of your schematic begin to solidify you can start laying out your PCB.
The PCB has a dependency on the logical design and schematics, so it will always be finished after the schematics are.
This dependency can be a limiting factor depending on how serial your design process is. The more serial you operate, the more it will limit; the less serial and more collaborative your process, the more likely your layout will finish within days or even hours of your schematics.
When you begin laying out your PCB you will start by making sure all of the logical parts you have used have footprints. This is normally just a sanity check but in some cases you will need to add or update footprints for parts that don’t have them.
As soon as all of your parts have footprints you can begin to place the parts and get a feel for how big your PCB will be. Move and place the largest parts in the design so that they are as adjacent as possible to the parts they connect to. As you place more and more parts it will make sense to begin rotating and shuffling all of the parts in the design.
Eventually most of your parts will have a location, most of them will be oriented the right way, and you can begin to hook up pins with traces. Trace routing is a lot like painting, or vector art. You start at a pin and using mostly straight lines and 45 degree angles, work your way to the destination pin. It’s a bit of an art form, so don’t worry too much about doing it perfectly. Expect it to take a few tries and for things to get ripped up and redone before it is finished.
Your Workflow:
Make sure all of your logical parts also have physical footprints
Begin spacing out the circuit board and placing the largest parts
Start shuffling and rotating parts to optimize the rats nest
Begin hooking up the lowest risk nets with traces
Continue clearing rats nests until the design is complete
Check & double-check
Once your PCB layout has been completed, and your schematic requires no further changes you are ready to move on to the design review stage of verification.
