PCB stack-up

A PCB is a sandwich of copper layers separated by fiberglass-epoxy core and prepreg. The stack-up specifies how many layers there are, their thicknesses, the dielectric material, and which layers carry signals, ground or power. Two-layer boards are cheapest; four layers make room for solid ground and power planes, which usually improves noise and emissions considerably; six or more are common for dense, high-speed or processor-based designs.

The stack-up determines trace impedance, how well signals are shielded, how heat spreads and the overall board thickness. It is normally agreed with the fabricator, who can adjust it to their standard materials.

Where it bites: saving a little unit cost by dropping layers often costs much more in EMC failures and redesign, and changing the stack-up mid-project changes every impedance-controlled trace.