Two materials, two jobs
A rigid multilayer board is built from two things: cores and prepreg.
- A core is a fully cured laminate — glass fabric in cured epoxy (C-stage) with copper foil bonded to both faces. It holds the board's thickness and mechanical stability, and it carries the inner-layer circuits that were imaged and etched before lamination.
- Prepreg is glass fabric impregnated with partially cured resin (B-stage), supplied without copper. It has no structural stiffness of its own. Its job is to flow under heat and pressure, fill the spaces around the copper pattern, and bond the cores into one board.
A four-layer board, in the simplest form, is two cores and one prepreg layer — or one core and two prepreg layers, depending on the stack-up.
What happens during lamination
The stack is pressed at temperature. The prepreg resin softens, flows into the etched copper pattern and around the traces, then cures. Two things are decided at that moment:
- Final dielectric thickness. The resin has to fill the copper step, so the dielectric between two layers is a function of the prepreg's resin content and the copper weight, not just its nominal thickness.
- Bond integrity. Too little flow leaves voids; too much squeezes resin out and thins the dielectric, which then shows up as an impedance shift.
Why it matters to a designer
Three consequences follow from the material difference.
Impedance. A controlled-impedance stack-up is calculated from the dielectric between the trace and its reference. That dielectric is usually prepreg. If the stack-up drawing says "0.1 mm dielectric" without specifying the prepreg build, the fabricator has to make an assumption, and the ±10 % tolerance is at risk before the board is even etched.
Thickness. A board's finished thickness is the sum of cores, prepreg after flow and copper. Heavy copper and multiple prepreg plies interact: more copper means more resin needed to fill the gaps, which is why heavy-copper stack-ups are not simply a standard stack with thicker foil.
Symmetry. Cores and prepreg have different expansion behaviour. A stack-up balanced around its centre line bows less during lamination and reflow; an unbalanced one shows up as bow and twist, with 0.7 % as the acceptance limit here.
Reading a stack-up drawing
A usable drawing lists, per layer: the copper weight, the material and the thickness, and whether the dielectric is a core or a prepreg build. The distinction is not academic — it tells the fabricator which plies to buy and the designer what the finished dielectric will be.
Where a design mixes a heavy power layer with a fine-feature signal layer, the stack-up is agreed in DFM rather than assumed. The capability limits are on the capability page and the board options on the PCB manufacturing page.
