schematic capture
Schematic capture is the first of the two big steps in designing a board: drawing the circuit diagram inside an EDA (electronic design automation) program. It is exactly like sketching the circuit on paper — symbols for resistors, capacitors, chips, and lines for the wires between them — except the computer is watching and recording every connection as you draw. You are capturing your idea in a form the software understands, hence the name. This step is purely about logic: what connects to what, and which exact parts you will use.
Concretely, you drag symbols from a library onto the sheet, give each a value and a part number (R1 = 10 kohm, U1 = a specific op-amp), and draw wires joining their pins. When two wires touch at a junction, the tool records that they are one electrical node. As you finish, the program distils everything into a netlist — a plain list saying, in effect, node 7 connects pin 2 of U1, pin 1 of R3, and pin 1 of C5. That netlist is the bridge to the next step; the layout tool reads it and refuses to let you forget a single connection.
Why it matters: schematic capture and PCB layout are deliberately separate jobs. The schematic captures intent — it is the readable, reviewable, simulate-able statement of how the circuit should work, with no concern for physical position. Keeping it separate means a reviewer can check the logic without wading through copper, and you can simulate it in SPICE before committing to a board. The honest caveat: a schematic says nothing about physical reality. It cannot show you a long noisy trace or a missing ground plane. A flawless schematic is necessary but never sufficient.
You place an op-amp symbol U1, two resistors R1 and R2, draw wires forming a non-inverting amplifier, and label the supply pins. The tool exports a netlist; the layout program then loads it and shows every part as an unplaced footprint with thin rubber-band lines marking the connections still to be routed.
Draw the logic; the tool records every node into a netlist for layout.
Garbage in, garbage out: the layout faithfully builds whatever the schematic says, including your mistakes. A reversed pin or a wrong value in the schematic becomes a wrong board. Review the schematic carefully before you ever start placing copper.