a multilayer PCB
A multilayer PCB is a board with copper wiring on more than just the top and bottom — it has extra copper layers buried inside the FR-4 sandwich, like a multi-storey car park instead of a single ground-floor lot. A simple single-layer board has copper on one face; a double-layer board has copper on both. A multilayer board stacks several thin FR-4 sheets and copper foils, glued together, so signals can run on inner floors and cross over each other without colliding. Four and six layers are everyday; high-end boards reach dozens.
Why add inner layers when you could just route everywhere on two? Because crossings. On two layers, every time two wires must cross you run one on top, one on bottom, and link them with a via — and a busy board quickly runs out of room. Inner layers give more lanes. Even more importantly, multilayer boards let you dedicate whole inner layers to a solid ground plane and a power plane. A typical four-layer stack is signal / ground / power / signal: the two outer signal layers each sit right above a solid copper plane, which gives every trace a clean, short return path directly beneath it.
Why it matters: that buried ground plane is the single biggest reason serious designs go multilayer. It slashes loop area, tames noise, steadies power delivery, and makes controlled-impedance routing possible because each trace has a known, uniform reference plane below it. The honest tradeoff is cost and rigidity: more layers mean more lamination steps, more expense, and harder rework. The art is choosing the fewest layers that give your signals a continuous reference plane and your power a low-impedance path — often four is the sweet spot.
A classic four-layer stack-up is signal-ground-power-signal. Every signal trace on the outer layers has a solid copper plane directly under it, so its return current flows in a tidy path right beneath the trace instead of wandering across the board.
Buried planes give every trace a clean return path — the real payoff of going multilayer.
More layers is not automatically better. A four-layer board with a proper ground plane usually beats a six-layer board whose planes are chopped up by careless routing. Stack-up order and plane continuity matter more than raw layer count.