PCB Stackup: Core, Prepreg & Copper Foil

What is PCB Core? (The Foundation)

The Core is the fundamental "backbone" of your PCB. In technical terms, a core is a fully cured (C-Stage) fiberglass-epoxy laminate with copper bonded to one or both sides. Think of it as a "double-sided PCB" in its simplest raw form.

Common Core Specifications & Dielectric Constants (Dk)

In modern designs, the Dielectric Constant (Dk) and Dissipation Factor (Df) of the core are vital for calculating characteristic impedance and ensuring lower insertion loss at high frequencies. Below are the standard core specifications used at NextPCB:

Note: Copper thickness values listed above are nominal. Final copper thickness may vary after plating and etching processes.

For high-performance applications where insertion loss is a critical constraint, standard FR-4 cores might be replaced with Low-Df materials like Rogers or Isola to maintain signal integrity at multi-gigahertz speeds.

What is Prepreg? (The Binding Agent)

Prepreg (short for "Pre-impregnated") is the insulation layer that binds cores and copper foils together. Unlike the core, Prepreg is "B-Stage" material. This means the fiberglass is impregnated with resin but is only partially cured.

Prepreg Types, Resin Content, and Dielectric Constant (Dk)

Professional engineers select prepreg not just by thickness, but by "Glass Weave Style" and "Resin Content (RC)." This directly affects the Dielectric Constant (Dk), which is crucial for PCBA signal integrity.

Note: Dk values are typical at 1GHz frequency. For 2026 high-frequency designs, Dk and Df (Dissipation Factor) are key metrics for achieving lower insertion loss.

Core vs. Prepreg: Key Engineering Differences

While both are composed of similar base materials (Fiberglass + Resin), their function in the PCBA (PCB Assembly) ecosystem is distinct:

• Rigidity: Core is rigid; Prepreg is pliable until laminated.
• Curing State: Core is C-Stage (Cured); Prepreg is B-Stage (Semi-cured).
• Function: Core provides structural integrity; Prepreg acts as the adhesive and dielectric spacer.
• Dielectric Constant (Dk): Because Prepreg flows during lamination, its final thickness and Dk can vary slightly based on the copper distribution (copper balance) of adjacent layers. Cores have a more stable Dk.

Copper Foil: Types and Surface Roughness

The copper foil acts as the conductive pathway for your signals. In modern quick turn PCB fabrication, we typically encounter two main forms:

1. - HTE (High Temperature Elongation) / Electro-deposited (ED): The standard for most rigid PCBs. It has a rougher surface tooth, which provides excellent peel strength (adhesion) to the resin.
2. - RA (Rolled Annealed): Smoother and more ductile. Often used in Flexible PCBs (FPC) or high-frequency applications where the "Skin Effect" is critical.

Pro Tip for 2026 Designs: For high-speed signals (10Gbps+), the surface roughness of standard copper can cause signal loss. Engineers should request VLP (Very Low Profile) or HVLP (Hyper Very Low Profile) copper foil to minimize insertion loss and improve high-speed data integrity.

Available thicknesses at NextPCB include:

• 1/3 oz (12 μm) - For HDI / fine pitch
• 1/2 oz (18 μm) - Standard internal layers
• 1 oz (35 μm) - Standard external layers
• 2 oz (70 μm) and up - For power supplies and heavy copper boards
• For more customized material options or specific copper thickness requirements, please consult our sales managers for expert guidance.

Note: All copper weights are nominal; final thicknesses may fluctuate slightly due to the etching and plating cycles required for circuit definition.

DFM & Stackup Design Tips for Cost Reduction

To ensure your low cost PCB stays within budget without sacrificing quality, follow these Design for Manufacturing (DFM) rules:

1. Balance Your Stackup

Always maintain symmetry in your stackup (e.g., layer 2 matches layer 5 in a 6-layer board) regarding copper thickness and Prepreg type. Asymmetrical stackups cause warping (bow and twist) during the high temperatures of PCB assembly reflow, leading to defective solder joints.

2. Minimize "Hybrid" Stackups Unless Necessary

Mixing materials (e.g., FR4 Core with Rogers Prepreg) is possible but requires specialized lamination cycles. This increases cost and lead time. Consult NextPCB engineers early if you need high-frequency materials for lower insertion loss at high frequencies.

3. Use Standard Prepregs

Designing a stackup that relies on obscure prepreg thicknesses can delay your quick turn PCB order. Stick to standard weaves (1080, 2116, 7628) whenever possible. Using standard stackups also improves yield consistency in mass production.

NextPCB Manufacturing Capabilities

As a leading reliable manufacturer, NextPCB provides comprehensive solutions from prototype to mass production. Our 2026 capabilities feature:

• Layer Count: 1 to 32 Layers (Standard), up to 64 Layers (Advanced).
• Material Inventory: Full stock of Shengyi S1000-2 (Tg170), Isola 370HR, Rogers 4350B/4003C, and Aluminum metal cores.
• Turnkey Services: We handle everything from PCB Fabrication, Components Procurement, to PCB Assembly (SMT/DIP).
• Speed: 24-hour manufacturing for standard 2-layer boards.

If you are unsure about your stackup, use our Free Stackup Builder tool or upload your Gerber files for a complimentary DFM check by our engineering team.

Get Instant Quote & Free DFM Check  Get Instant Quote & Free DFM Check

Frequently Asked Questions (FAQ)

Q1: Can I use two Prepregs stacked together?

A: Yes, this is common. For example, stacking two sheets of 1080 prepreg to achieve a specific thickness for impedance control. However, we generally avoid stacking more than 3 sheets to reduce the risk of measling, resin voids, and thickness inconsistency.

Q2: What is the difference between FR4 and CEM-1?

A: FR4 is a woven glass epoxy (suitable for multilayer boards). CEM-1 is a composite of paper core and glass surface (cheaper, but usually limited to single-sided PCBs). For professional electronics, NextPCB recommends FR4.

Q3: Does Prepreg thickness change after pressing?

A: Yes. We call this the "pressed thickness." The resin flows into the copper gaps of the adjacent layers. NextPCB’s engineering software calculates this automatically to ensure your 50Ω impedance traces are accurate.

Q4: Why is my turnkey PCB quote higher with specific stackups?

A: Custom stackups may require non-standard material stock or extra lamination cycles. Using our "Standard Stackup" options is the best way to achieve a low cost PCB and maintain high production yields.

Ready to start your next project? Trust NextPCB for reliable, high-quality manufacturing.

About the Author

Lolly Zheng- Sales Account Manager at NextPCB.com

Four years of proven sales experience across electronic components and PCBA industries, with strong expertise in key account acquisition, customer relationship management, and contract negotiations. Focused on driving revenue growth through strategic client development and solution-based selling. Experienced in expanding high-value accounts, securing long-term partnerships, and consistently exceeding sales targets in competitive markets.

LinkedIn