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First, we need to gather the Requirements & Specifications, then we consider standards that the PCB should meet and select the type of PCB material, copper thickness as well as final PCB finish. Next step we place the critical components and make sure connectors and mechanically critical parts are correctly placed on the board. Then we do the routing. We assumed we already have the Circuit Schematics designed and ready.
PCB Routing is a time consuming process, as all connections which we call them the nets in schematics should be converted to a copper trace which would travel on the PCB and make the connection between two or multiple points. Also these tracks may travel to different layers which they have to pass through vias.