Schematic

The schematic records what is connected to what: every resistor, chip, connector and net, with part values and reference designators (R12, U3). It is drawn in an electronics design (EDA) tool, which then drives the PCB layout and generates the bill of materials.

Schematic review is one of the cheapest places to catch expensive mistakes: a swapped pin, a missing pull-up, an undersized regulator, an unprotected external connector, missing test points. An error found after boards are built usually costs a respin and weeks of schedule.

A good schematic is also documentation. Clear sheet organization, named nets, notes on design intent and part numbers tied to approved sources make later changes and failure analysis much faster.