Standard PCB Core Thickness: Options and How to Choose

Core thickness is one of the first numbers fixed in a multilayer design and one of the last to be questioned, yet it determines the board total thickness, the impedance of every controlled trace and the mechanical stiffness of the finished assembly. Choosing it from habit rather than from the stackup produces boards that are either unnecessarily thick or unexpectedly flexible.

What the Core Actually Is

The core is the rigid base layer of a multilayer board, a sheet of glass fibre reinforced epoxy with copper foil bonded to both faces. It is not the same as prepreg, which is the partially cured resin sheet used to bond cores together during lamination. The core carries the inner layer copper, while the prepreg fills the space between cores and bonds them.

The finished thickness of a board is the sum of every core, every prepreg sheet, the copper on each layer and the surface finish. That means the PCB core thickness cannot be chosen in isolation: changing one core changes the total thickness, the layer to layer spacing and the impedance of every trace referenced to a plane on the other side of that dielectric.

Standard Core Thickness Options

Fabricators hold a limited range of core thicknesses in stock, and specifying one of them keeps both cost and lead time down. The common values are 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.6 mm and 2.0 mm, with 1.6 mm by far the most widely used for general purpose boards. Thicker cores are available for specialised applications.

Working in the stock range is the single most effective way to avoid a lamination problem. A non-standard core has to be ordered from the material supplier, which adds lead time and a minimum purchase quantity, and it may require the fabricator to qualify a new press cycle. The metric values correspond to familiar imperial sizes such as 8 mil, 16 mil, 24 mil, 32 mil, 40 mil, 48 mil and 63 mil.

Layer Count and Stackup Structure

In a multilayer design the core thickness interacts directly with the layer stackup. A thick core between two signal layers creates a large dielectric separation, which suits a wide trace geometry but makes the layers mechanically rigid. A thin core places the layers closer together, which supports fine traces and tight impedance control but reduces the bending stiffness of the panel.

The stackup has to be balanced as well, with copper distribution and dielectric thickness roughly symmetrical about the centre line, or the board will bow during lamination and during reflow. Core thickness selection is therefore part of the stackup design task rather than a prelude to it, following the options set out in layer stackup from one to eight layers.

Impedance Control and Electrical Performance

For controlled impedance traces, the core thickness sets the distance from the trace to its reference plane, and that distance is one of the three variables that define the characteristic impedance, alongside trace width and dielectric constant. A change of 0.1 mm in dielectric thickness can move a fifty ohm trace by several ohms, which is enough to fail a specification.

High speed designs therefore start from the target impedance and work backwards to a stackup, rather than choosing a stackup and hoping the impedance lands in tolerance. Simulation should confirm the geometry before the artwork is released, and test coupons should be included on the panel so the result can be measured with time domain reflectometry after fabrication. The trace side of the calculation is covered in trace width and current calculation.

Thermal Behaviour

A thicker core spreads heat laterally over a wider cross section and can carry more copper per unit area, which helps in power electronics, LED lighting, amplifiers and automotive assemblies. Where the dissipation is significant, a thicker core combined with heavy copper and thermal vias produces a lower thermal resistance from the component to the far side of the board.

The benefit is not automatic. A thick dielectric also increases the thermal resistance through the board, so the improvement depends on whether heat is being spread laterally or conducted vertically. In high power designs the core thickness decision should follow a thermal management study rather than a rule of thumb, particularly where the board is mounted on a heat spreader and the copper area is doing the spreading.

Mechanical Strength and Durability

Rigid boards used in industrial, aerospace and automotive equipment generally need thicker cores to survive vibration, shock and connector insertion force. The bending stiffness of a laminate rises steeply with thickness, so a small increase produces a disproportionately stiffer board, which is often cheaper than adding mounting hardware.

The counterweight is weight and volume, which matter in portable products and in anything that flies. A thin, stiffened assembly may be a better answer than a uniformly thick board, particularly where the stiffness is needed only at a connector. Where the board also has to remain flat through reflow, balance matters more than absolute thickness.

Manufacturing Limits

Drilling, lamination and etching all constrain what core thickness can be used. A very thin core is difficult to handle through the process without damage, and drilling a high aspect ratio hole through a thick stackup limits how small the drill can be while still plating reliably. Lamination pressure has to be tuned to the resin flow of the prepreg used with each core.

Because those limits are process specific, the stackup should be agreed with the fabricator before the artwork is finalised rather than after. A design that asks for a core and prepreg combination the plant does not run will either be rejected or quoted at a premium, and the response is usually a small change to the stackup rather than a change to the design intent. The principles behind that consultation are described in balanced stackup and odd layer count.

How to Choose

Start from the mechanical requirement, since that is the hardest to change later. Then fix the total board thickness that the product needs, working within the standard range of laminate and prepreg combinations. Then adjust individual core thicknesses to place each signal layer at the right distance from its reference plane for the target impedance.

Finally, confirm the whole stackup against the fabricator process. A design that uses stock cores, balanced copper and standard prepreg flows through production without special handling, and the resulting board meets its impedance target, its mechanical requirement and its cost target at the same time.

FAQ

Is 1.6 mm always the right board thickness? It is the most common default because it is rigid enough for most applications and it fits standard connector and card guide dimensions. It is not automatically correct, and portable or high density designs frequently use thinner boards.

Does core thickness change the impedance by much? It does. Impedance falls as the dielectric gets thinner, so a reduction from 0.2 mm to 0.1 mm moves a typical fifty ohm microstrip by a noticeable margin. Any change to the core requires the impedance calculation to be repeated.

Can I mix core thicknesses in one stackup? Yes, and it is normal practice. Different layers may need different separations to meet different impedance targets, provided the overall stackup stays balanced and the fabricator can press it.