What is the Core of a PCB?

The core of a PCB is the substrate of the PCB with copper traces on one side or both sides. An example would be an FR-4 substrate with copper foil above and below it. The FR4 substrate provides high electrical insulation between the copper layers of the core. The PCB core can be thought of as a solid foundation for the printed circuit board.

Multiple cores are used to create a multi-layer PCB. The cores are bound to each other by using prepreg materials. Multilayer PCBs are formed by PCB stackup - an arrangement of copper layers & insulating layers (prepreg). In a multilayer PCB, the prepreg is a dielectric bonding material (glass fiber weave/cloth impregnated with a resin). The prepreg helps to holds the core of the PCB and the layers together. Thus, the prepreg is an important material in the fabrication of multilayer PCBs.

PCB designers pack the prepreg in between a copper layer and a core or between two cores in a PCB to provide the required insulation and bonding. After the stackup arrangement, the layers are pressed together and heated at a temperature to obtain the required PCB thickness.

For example, in a four-layer PCB, the prepreg is packed in between the core and top & bottom layer. 

Four layer PCB (two single layer (top & bottom) + one double layer (middle)

Similarly, in a six-layer PCB, the prepreg is packed in between a copper layer and a core or between two cores.