Background Theory > PCB Design > What is PCB Prepreg

PCB prepreg, short for "pre-impregnated," is an essential material used in the construction of multilayer printed circuit boards (PCBs). It plays a crucial role in bonding and insulating the different layers of the PCB together during the lamination process. Prepreg is typically a fiberglass cloth that has been pre-impregnated with a partially cured epoxy resin.

The prepreg material is used in conjunction with copper-clad laminates to build multilayer PCBs. Here's how it works:

•Copper-Clad Laminates: Copper-clad laminates are composed of layers of copper foil bonded to either side of a rigid core material, which is often FR-4 (flame-retardant glass-reinforced epoxy laminate). The copper layers serve as the conductive paths for the circuit, and the core material provides mechanical strength to the PCB.

•Prepreg Layers: Prepreg layers are placed between the copper-clad laminates in a multilayer PCB stackup. These prepreg layers are essentially the same fiberglass material used in FR-4, but instead of being fully impregnated with epoxy, they are only partially cured. The epoxy resin in the prepreg is partially cured to a semi-solid state, which allows it to flow and bond during the lamination process.

•Lamination Process: The PCB stackup, comprising alternating layers of copper-clad laminate and prepreg, is subjected to high temperature and pressure in a process called lamination. During lamination, the partially cured epoxy in the prepreg undergoes further curing, becoming fully solid and creating a strong bond between the copper layers and the core material.

•Bonding and Insulation: As the prepreg cures and solidifies, it acts as both a bonding agent and an insulator. It adheres the copper layers to the core material, creating a cohesive structure, and it also provides electrical insulation between adjacent copper layers, preventing short circuits.

•Through-Hole Formation: After lamination, the PCB is drilled to create holes known as vias. These vias allow electrical connections to be made between different layers in the multilayer PCB. The vias pass through the copper layers and prepreg, and they are typically plated with a conductive material to establish the electrical connections.

PCB prepreg ensures the integrity and mechanical stability of the multilayer PCB, enabling it to function as a single, cohesive unit. The number of prepreg layers and their thickness can be adjusted based on the specific design requirements of the multilayer PCB.