LIBRARYPCB stackups: layers, materials, impedance.

A stackup is the sandwich of copper and insulation the board is built from. Two layers is the cheap default. Four or more lets you dedicate internal layers to power and ground, which makes a quieter board. The fabricator sets the exact thicknesses, and you design to their stackup.

Copper, prepreg, and core

A board is layers of copper foil bonded to an insulating substrate. The rigid inner insulator is the core; the sheets that bond layers together under heat and pressure are prepreg. The insulator itself is usually FR-4, a woven glass-epoxy laminate that is cheap, strong, and stable enough for almost everything you will build.

Two layers versus four

A two-layer stackup is copper, insulator, copper: signals and ground share both sides. A four-layer stackup is signal, ground, power, signal, so the two inner planes give every outer signal a return plane a fraction of a millimetre away. Tighter spacing between a signal and its return plane means lower loop inductance, which is the whole reason to add layers.

Controlled impedance, and when it matters

For fast signals like USB or high-speed data, a trace and its reference plane behave like a transmission line with a characteristic impedance set by the trace width, the distance to the plane, and the insulator. When a signal is fast enough that this matters, you ask the fab for a controlled-impedance stackup and route those traces to the width they specify. For low-speed hobby work it rarely comes up, and knowing the term tells you when to reach for it.

The fab sets the stackup

You do not invent the thicknesses. Each fabricator publishes a stackup table, the exact copper weights and insulator heights they build, and you design to it. A four-layer order from a low-cost fab comes with a standard stackup you can look up before you route, so your impedance and via choices match what they will actually make.