PCB Stackup

A PCB stackup is the layer-by-layer arrangement of copper foils and insulating dielectric materials that make up a multilayer printed circuit board. It specifies how many copper layers there are, the order of signal, power, and ground layers, and the thickness and material of the dielectric between them. The stackup defines both the electrical behaviour of the board, such as controlled impedance and signal integrity, and its mechanical stability.

Cores, prepreg, and dielectric

A stackup is built from cores, which are rigid copper-clad laminate panels, bonded together by layers of prepreg, a fibreglass weave partially impregnated with resin that melts and cures under heat and pressure during lamination. The dielectric constant and thickness of these layers set the impedance of the traces routed on them, which is why high-speed designs specify the stackup precisely. A common substrate is FR-4, with a dielectric constant in the range of roughly 4.2 to 4.5. Stackups are usually kept symmetric about the centre to prevent the board from warping after lamination.

Why repairers care

Knowing the stackup tells a technician which layers carry which nets and how deep a given via reaches, information that is essential when tracing a fault that disappears into an inner layer. It also explains the board's thermal behaviour, since buried copper planes pull heat away from the surface and make some joints harder to reflow. On dense ASIC hashboards a high layer count packs many power and ground planes between signal layers, so without the stackup or a known-good reference, diagnosing an inner-layer break is largely guesswork. The stackup works hand in hand with the copper pour planes and the annular rings that anchor each via.