Prepregs, Cores, and Copper Foils: The Building Blocks of PCB Stackup

What is Prepregs?

Prepreg (derived from "pre-impregnated") is the insulating material placed between two cores or between a core and a copper foil. Often referred to as "PP sheets," these are the critical bonding agents in multilayer PCB production.

• Composition: Most prepregs consist of a fiberglass fabric impregnated with a resin system (typically epoxy-based).
• Resin Categories: Based on the resin content, prepregs are classified into:
  • Standard Resin (SR)
  • Medium Resin (MR)
  • High Resin (HR)
  • Note: Higher resin content generally improves gap filling between traces but may increase the material cost.
• Quality Standards: High-quality prepreg must have a smooth surface, free from oil, stains, defects, cracks, or excessive resin powder.

How Does Prepregs Work?

Prepreg acts as the "glue" that bonds the entire multilayer stack-up together. Its unique property lies in its B-stage state:

1. The Fluid State: Before lamination, the prepreg is dry but not fully cured. When the PCB stack-up is placed in a heated press, the resin liquefies and flows to fill the gaps between etched copper traces.
2. The Bonding Process: Under heat and high pressure, the resin undergoes a chemical reaction, impregnating the fiberglass and bonding the adjacent layers.
3. Solidification: As the board cools, the resin solidifies (reaching the C-stage), resulting in a rigid, unified board with the precise desired thickness.

What is the Core?

While Prepreg and Core are both dielectric materials, they serve different purposes. The Core is a factory-cured, rigid substrate with copper cladding on one or both sides. It serves as the stable foundation for the inner circuitry.

YSX offers a variety of core substrates to meet diverse application needs:

• Standard Substrates: FR-4, CEM-1, CEM-3.
• High-Performance Materials: PTFE (for RF/High-speed), Polyimide, and Polyester.
• Metal Cores (MCPCB): Aluminum or Copper bases, ideal for high-power LED and power conversion applications requiring superior heat dissipation.

What is Copper Foil?

Copper foil is the conductive material applied to the inner and outer layers to form the electrical paths (traces).

• Application: It can be pre-bonded to the core by laminate manufacturers or added as standalone foil sheets during the multilayer pressing process.
• Manufacturing Methods: Produced via electroplating or rolling to ensure uniform thickness and high conductivity.
• Available Weights: YSX provides various weights, with 1oz (35 μm) and 2oz (70 μm) being the most common standards for standard and power-heavy designs.

Comprehensive Solutions at YSX

Whether you are designing a simple double-sided board or a complex high-density multilayer stack-up, selecting the right material combination is key to your project's success.

• Customized Requirements: Choose from our wide range of materials directly in our online quote system.
• Expert Support: Not sure about the best stack-up for your high-speed design? Our technical team is available 24/7 to provide DFM (Design for Manufacturing) support and material recommendations.

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