Multilayer pcb construction is the physical assembly of copper foil, cores, and prepreg that becomes the finished board. A 4-layer board is typically one copper-clad core in the middle with prepreg and outer copper foil on both sides. Higher layer counts add more cores or more prepreg sheets and additional copper foils. The designer's stackup drawing shows the intended dielectric thicknesses and copper weights. The fabricator's construction turns that drawing into actual materials that survive the lamination press, etching, and drilling.
The difference between the drawing and the finished cross-section is usually resin flow and registration.
Signal Layers, Power Planes and Ground Planes
Signal layers carry the etched traces. Outer signal layers start as copper foil that is laminated on and then imaged. Inner signal layers are usually the copper that comes already bonded to a core; that copper is etched before the layers are stacked. Power and ground planes are the same copper, just left mostly solid. Their position in the construction sets the dielectric height to the nearest signal layer and therefore the impedance of any controlled traces. A solid ground plane next to a signal layer keeps the return local. A split power plane under a fast edge forces a detour that the construction itself cannot fix.
Core Material
A core is fully cured laminate with copper foil bonded to one or both sides. It provides the rigid dielectric base and the copper that becomes the inner layers. Common core thicknesses range from about 0.1 mm to 1 mm or more. Because the resin is already cured, core thickness is relatively stable through the press. That stability is why controlled-impedance stripline is often easier to hold when the dielectric is a core rather than a stack of prepreg. The core is imaged, etched, and inspected before it goes into the final lay-up. Multiple cores can be used in higher-layer boards; they are aligned with the prepreg between them.
Copper on the core is usually 0.5 oz or 1 oz. Heavier copper is possible but increases etch undercut and affects the finished dielectric height.
Prepreg and How It Changes During Lamination
Prepreg is glass fabric impregnated with partially cured resin. It has no copper of its own. During the lamination press the resin softens, flows into the etched relief of the copper patterns, and then cures, bonding the cores and foils into one board. The finished thickness of a prepreg layer is not the nominal sheet thickness. Resin flow and the amount of copper it has to fill change the final dielectric height. High resin-content styles (1080, 106) flow more and are used for thinner gaps. Lower resin-content styles (7628) are stiffer and used for thicker separations. Multiple sheets can be stacked to reach a target thickness, but a single very thick prepreg sheet is avoided because the glass can be damaged and resin distribution becomes uneven.
This is why the impedance calculation must use the fabricator's pressed thickness, not the raw prepreg datasheet value.
Copper Foil
Outer layers are almost always added as separate copper foil that is laminated against prepreg. Inner layers use the copper that is already clad on the cores. Typical finished copper is 0.5 oz, 1 oz, or 2 oz. Outer copper receives additional plating in the holes, so the finished thickness on the surface is higher than the starting foil. Heavier copper raises current capacity but increases etch undercut, which narrows the finished trace and raises impedance for a given artwork width. The foil type (standard, reverse-treated, or low-profile) also affects surface roughness and therefore high-frequency loss. The construction notes must specify both the starting weight and the finished weight after plating.
Dielectric Thickness, Dk and Impedance
The dielectric height between a signal layer and its nearest reference plane, together with the Dk of that dielectric, sets the characteristic impedance. Core dielectrics tend to have more repeatable thickness and Dk because they are already cured. Prepreg dielectrics vary more with resin content, glass style, and how much resin flows during the press. A 10 % change in finished dielectric thickness moves impedance several ohms. Dk on standard FR-4 is typically 4.0–4.5 and also varies with frequency and resin-to-glass ratio. Controlled-impedance designs therefore specify the target impedance and the layer, then let the fabricator adjust the construction and artwork width so the finished board meets the target within the agreed tolerance.
Lamination, Bonding and Registration
The cores, prepreg sheets, and outer foils are stacked in the exact order of the construction and pressed under heat and pressure. Typical FR-4 lamination is around 175–185 °C and 250–350 psi for 60–90 minutes. The prepreg resin flows and cures, locking the layers together. Layer-to-layer registration is held by tooling pins or by X-ray targets. Misregistration of even 50–75 µm can reduce annular ring on inner pads or shift the relative position of traces and planes. Higher layer counts and sequential lamination (building sub-assemblies first) tighten the registration budget further. Asymmetric copper distribution or unbalanced prepreg can also cause warpage after the press cools.
Construction Versus the Design Stackup
The designer's stackup is the electrical and mechanical intent: which layers are signals or planes, the target dielectric heights, and the copper weights. The fabricator's construction is the bill of materials and process that produces that intent: specific core thicknesses, prepreg styles and ply counts, foil weights, and press parameters. A common 4-layer construction is one core (layers 2 and 3) plus prepreg and foil on both sides. A 6- or 8-layer board may use two or more cores bonded by prepreg, or a foil build with multiple prepreg gaps. Sequential lamination is used when blind or buried vias require intermediate drills. The finished board only matches the design stackup if the construction accounts for resin flow, etch compensation, and registration. On multilayer pcb construction the materials and the press cycle determine whether the dielectric heights and copper geometry stay inside the window the impedance and reliability requirements need.
