a trace

A trace is one of the flat copper roads on a PCB — the printed-on-the-board equivalent of a wire. Instead of a round insulated wire you could pick up, a trace is a thin, flat strip of copper bonded to the board, running from one pad to another. Every connection your schematic asked for becomes, on the finished board, a trace (or a chain of traces hopping between layers through vias). When people talk about routing a board, they mean drawing these copper roads.

A trace is not a perfect zero-resistance wire; it is a real conductor with width, thickness, and length, and those set its behaviour. Its resistance follows the same rule as any conductor: longer and thinner means more resistance, wider and thicker means less. Its width sets how much current it can safely carry before it overheats — a rough rule of thumb on standard 1 oz copper is about 10 mm of width per 10 amps for a modest temperature rise, so a 0.25 mm signal trace handles a fraction of an amp easily, while a power trace might be several millimetres wide. Width also lets a trace run cool: a fat trace spreads its heat and drops less voltage.

Why it matters: choosing trace width is a real design decision, not a default. Too thin for the current and the trace warms up, drops voltage, and in extremes acts like a fuse and burns open. Beyond resistance, a trace also has inductance, and a fast-changing current through that inductance makes voltage spikes — which is why power and ground traces want to be short and wide, and why on fast signals the trace's geometry over the ground plane (its impedance) suddenly matters. A trace is humble, but it is a genuine circuit element.