Prepreg vs Core: The Two Materials Behind Every Multilayer PCB

A multilayer board is not one material but a stack of two. The cores carry the circuits; the prepreg glues them together and forms the dielectric between them. Designers often treat the two as interchangeable because both are made of glass fabric and epoxy, but they behave differently in the press and they contribute differently to the electrical and mechanical result.

What a Core Is

A core is a fully cured laminate with copper foil bonded to both faces. It is rigid, it has a defined thickness and copper weight, and the copper on it will be imaged and etched into a circuit or left as a plane. Because it is cured, a core does not flow during lamination: it keeps its thickness and its dimensions to within the tolerance of the press.

The core is where the electrical function lives. Signal layers, ground planes and power planes are all formed on cores, and their dielectric thickness and dielectric constant are what set the impedance of the finished board.

What Prepreg Is

Prepreg is the same glass fabric impregnated with epoxy, but the resin is only partially cured. It is supplied without copper, feels slightly tacky and flexible, and cannot be bent sharply without cracking. During lamination, heat and pressure make the resin flow, fill the space around the copper features and then cure to a solid, insulating bond.

In other words, prepreg is both the adhesive and the dielectric of the joints between cores. Its properties therefore affect bond strength, resin fill, dielectric spacing and, indirectly, the impedance of any layer that sits above it.

How the Two Are Combined

A stackup alternates cores and prepreg. Two cores with a layer of prepreg between them produces a four layer board, where the outer copper on each core becomes the outer layers of the finished product. Additional cores and prepreg plies add layers in the same way.

Prepreg is usually applied in multiple plies to reach the required dielectric thickness, because a single ply of a given style has a fixed pressed thickness. A 0.1 mm dielectric might require two plies of a thin style or one ply of a thicker style, and the choice affects resin flow, fill and cost.

Resin Flow and Fill

Resin has to move during the press cycle to fill the gaps around copper traces and to eliminate voids. Too little flow leaves voids and poor bond strength; too much flow starves the bond line, thins the dielectric and can leave the copper pattern exposed to the next layer. The resin content of the prepreg and the press profile both control the outcome.

Fill matters most where heavy copper or large copper areas meet a thin dielectric. A dense plane next to a fine signal layer creates a large volume to fill on one side and almost none on the other, which is one reason unbalanced stacks are difficult to press.

Dielectric Thickness and the Stackup

Once pressed, prepreg is thinner than its nominal, unprocessed thickness. The pressed thickness is what appears on the stackup for impedance calculations, and it depends on the ply style, the resin content and the copper weight beneath it.

That is why a stackup drawing lists each ply by material type and style rather than by a single number. A fabricator who is told only that a dielectric must be 0.15 mm has several ways to reach that value, and they will not all have the same dielectric constant or the same pressed thickness under a heavy copper layer. The arrangement of those layers is discussed in the guidance on layer stackup from one to eight layers.

Glass Transition Temperature and Material Choice

Cores and prepreg are usually specified from the same material family, so that the glass transition temperature, the decomposition temperature and the dielectric constant match. Mixing a high performance prepreg with a general purpose core can produce a bond that behaves differently from either material during thermal cycling.

Within a family, prepreg styles differ in glass fabric weight and resin content, which shifts both the pressed thickness and the dielectric constant slightly. A change of style is therefore a change of electrical performance, and it should be reviewed rather than substituted at the press.

Storage and Handling

Prepreg has a limited shelf life and absorbs moisture. It is normally stored refrigerated in sealed bags and conditioned to room temperature before use, because opening a cold bag in a humid room causes condensation on the resin. Using expired or damp prepreg produces voids, delamination and unpredictable flow.

Cores are far more forgiving. They are stored flat, kept dry and handled with gloves to avoid contaminating the copper surface before imaging. Both materials must be clean: a fingerprint on a core becomes an etching defect, and dust on a prepreg ply becomes a void in the finished board. The mechanical consequences of getting this wrong appear as dimensional drift, the subject of the notes on dimensional stability and expansion.

Common Defects and Their Causes

Resin starvation appears as a dry, whitish area where the glass fabric is visible and the bond is weak. Voids appear as small empty pockets, usually where resin could not reach a deep feature. Thickness variation shows up as an impedance shift across the panel, and delamination appears after thermal stress where the bond was never sound.

All four trace back to the same variables: the amount of resin available, the copper distribution it has to fill, and the pressure and temperature profile used to make it flow. The design side of the equation is copper balance and a stackup that does not ask the press to do the impossible; the process side is a controlled lamination cycle with the correct material lot. Layer arrangement decisions that reduce that risk are described in the notes on multilayer design rules.

Cost, Availability and Substitution

Core and prepreg are bought together, but they are not equally available. Common core thicknesses and copper weights are stocked in quantity, while prepreg styles are often supplied in minimum order quantities that exceed the needs of a single prototype run. A stackup built entirely from common styles is cheaper and faster to source than one that needs an unusual ply.

Substitution is a real risk in the other direction as well. A shop that is short of a particular prepreg may be tempted to replace it with a similar style that has a different resin content, which changes the pressed thickness and therefore the impedance. The stackup drawing should name the material by part number so that a substitution is visible rather than silent.

Where a design is expected to run for years, it is worth confirming that the chosen core and prepreg will still be available. A laminate that disappears from the market forces a requalification, and the cost of that exercise usually exceeds any saving made by choosing a marginal material in the first place.

FAQ

Can prepreg be used without a core? Prepreg has no copper and no structural rigidity before pressing, so it cannot carry a circuit by itself. It exists to bond and insulate the cores that do.

Why does the pressed thickness differ from the nominal value? Because resin flows out of the ply and into the copper features beneath it during lamination. The pressed thickness depends on the ply style, the copper weight and the press parameters.

Can different prepreg styles be mixed in one stackup? Yes, and it is common when a non-standard dielectric thickness is required. The styles should be chosen so that the pressed result matches the impedance target, and the choice should be recorded on the stackup drawing.