In a PCB, prepreg is a semi-cured fiberglass material pre-impregnated with resin that acts as both an adhesive and an insulator to bond layers together during the lamination process. It binds cores and copper foils, provides electrical insulation between layers, and helps form the rigid structure of a multilayer PCB when heat and pressure are applied, fully curing the resin. 


Key Functions

• Binds Layers: It acts as a glue, sticking multiple layers of a PCB stackup together, such as a core to another core or a core to copper foil.
• Provides Insulation: Prepreg is a dielectric material, meaning it electrically insulates the conductive traces on different layers, maintaining signal integrity.
• Fills Gaps: When heated, the resin softens and flows to fill any gaps between layers, creating a solid and uniform structure. 

Composition

• Reinforcement: Typically a woven glass fabric, providing structural strength.
• Resin: A partially cured (B-stage) resin, most commonly epoxy, that acts as the bonding agent. 

How it's Used

1. Stackup: Prepreg sheets are placed between rigid PCB cores and copper foils.
2. Lamination: Under high heat and pressure, the resin in the prepreg flows, bonds the layers, and fully cures into a solid, insulating laminate. 

Prepreg vs. Core

• Prepreg: Is a semi-cured dielectric, used to bond layers together.
• Core: Is a fully cured laminate (like FR4) with copper on both sides, serving as the rigid backbone of the PCB. 

Types of Prepreg
Prepregs are often classified by their resin content, which affects the PCB's characteristics: 

• Standard Resin (SR): A balanced resin content.
• Medium Resin (MR): Higher resin content than SR.
• High Resin (HR): The highest resin content, often used for thicker boards or specific electrical properties.