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 — controlled impedance, signal integrity, current capacity — and its mechanical and thermal character. Two boards with identical schematics but different stackups are, in practice, different products.

Cores, prepreg, and dielectric

A stackup is built from cores — 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 rather than leaving it to the fabricator. The workhorse substrate is FR-4, with a dielectric constant in the range of roughly 4.2 to 4.5. Stackups are normally kept symmetric about the centre line; an asymmetric build stresses unevenly during lamination and the board warps — a real problem when you later need a perfectly flat surface to reflow a large BGA.

Copper weight and the power problem

Copper thickness is part of the stackup too, specified in ounces per square foot. Ordinary logic boards use 1 oz copper; boards that move serious current use 2 oz or heavier, or stack multiple planes in parallel. An ASIC hashboard is an extreme case: it must deliver hundreds of amps at low voltage down a chain of hash domains, so its stackup dedicates generous copper to power distribution — wide copper pours and internal planes carrying the domain rails, with a solid ground plane as the return path. Arrays of vias and via stitching tie those planes together vertically, sharing current and pulling heat down through the board. The stackup is as much a thermal design as an electrical one.

Why repairers care

Knowing the stackup tells a technician which layers carry which nets and how deep a given via reaches — essential information when tracing a fault that disappears into an inner layer. On a dense hashboard, a rail that reads open between two domains may be broken on a buried plane you can never probe directly; without the stackup or a known-good reference board, diagnosing an inner-layer break is largely guesswork. The stackup also explains the board's behaviour under the hot-air station: buried copper planes are heat sinks, wicking energy away from the joint you are trying to reflow. That is why a ground-connected pad needs noticeably more preheat and dwell than an isolated signal pad, and why underheating dense boards produces the cold joints that come back to the bench a month later.

Reading a board without documentation

Manufacturers rarely publish hashboard stackups, so repairers reverse-engineer what they need: counting layers on a sanded board edge or a failed sacrificial board, mapping planes with continuity testing between via fields, and comparing against a known-good unit. The annular rings visible at each via, the via drill sizes, and the pattern of stitching fields all hint at what the inner layers are doing. It is detective work — but it is the difference between replacing components at random and actually understanding the failure. If a board has you beat, that is exactly what our repair bench is for.