What Are the "Skeleton" and "Glue" of a PCB?

The essential conclusion is that a multilayer PCB is built by alternately laminating core material, or Core, which acts as the skeleton, and prepreg, which acts as the glue. The Core provides structural strength and is already cured. Prepreg is a partially cured resin sheet that melts and bonds adjacent layers during lamination.

Understanding the difference between Core and Prepreg is essential for understanding stackup thickness, controlled impedance, dielectric spacing, and why a symmetrical stackup helps prevent PCB warpage.

1. A Cross-Sectional View of a Multilayer PCB

The cross-section of a multilayer PCB can be understood as a layered sandwich. Core and Prepreg are stacked in a defined sequence and then laminated under high temperature and pressure to form a single rigid board.

• Core is a fully cured rigid material with copper foil on both sides.
• Prepreg is a partially cured resin-and-glass-fiber sheet used to bond adjacent layers.
• Core and Prepreg are alternately stacked and laminated to form the complete multilayer board.

A useful analogy is to think of Core as the biscuit and Prepreg as the filling. The Core provides the rigid framework, while the Prepreg flows during lamination and bonds the structure into one multilayer assembly.

2. The "Skeleton": Core Material

2.1 What Is Core Material?

Core material consists of a fully cured substrate and copper foil on both sides. It is typically made by impregnating glass fiber cloth with epoxy resin and then curing the resin.

• The resin and glass-fiber structure is already cured.
• Copper foil is present on both sides, which is a key difference from Prepreg.
• The Core provides the primary structural support for the PCB.

2.2 Characteristics of Core Material

2.3 Why Core Is the Structural "Skeleton"

The Core determines much of the board's basic stiffness. Because its resin has already been cured, it does not melt during lamination and remains a load-bearing structural element.

The spacing between conductive layers is determined jointly by the thickness of the Core and the final thickness of the Prepreg after lamination. This spacing is important not only for the total board thickness but also for controlled-impedance design.

A Core already includes copper foil, so it is essentially a double-sided copper-clad laminate. After lamination and circuit processing, its copper surfaces can form internal layers or become part of a more complex multilayer structure.

3. The "Glue": Prepreg

3.1 What Is Prepreg?

Prepreg, short for partially cured material, consists of glass-fiber cloth impregnated with epoxy resin that has not yet been fully cured.

• The resin is in the B-stage, meaning it is partially cured and can flow when heated.
• Prepreg does not include copper foil.
• During lamination, heat causes the resin to soften and flow into the surrounding structure.
• After flowing, the resin cures and bonds the adjacent layers together.

3.2 Characteristics of Prepreg

3.3 Why Prepreg Acts as the "Glue"

During lamination, the resin in the Prepreg softens and flows under heat and pressure. It fills the available space between the adjacent materials and bonds the copper foil, Core, and other layers together. Once cured, it becomes a solid dielectric material.

Prepreg therefore performs three functions at the same time:

• It bonds neighboring layers together.
• It provides electrical insulation between conductive layers.
• It helps determine the final dielectric thickness and therefore affects controlled impedance.

Multiple Prepreg sheets can be combined to obtain a different dielectric thickness. The final thickness depends on the material style, the number of sheets, the lamination process, and the surrounding copper pattern.

4. Core and Prepreg Compared

In one sentence: Core contains copper and is already rigid; Prepreg contains no copper and flows during lamination.

5. How Core and Prepreg Form a Multilayer PCB

5.1 Example: A Four-Layer Board

In a typical four-layer construction:

• The copper for the inner layers, layers 2 and 3, comes from the copper foil on the two sides of the Core.
• The outer-layer copper is supplied by separate copper foil.
• Prepreg bonds the outer copper foil to the Core.

5.2 Example: A Six-Layer Board

As the layer count increases, additional Core and Prepreg materials are used to create the required conductor and dielectric layers.

• Core material can provide paired internal copper layers.
• Prepreg provides bonding between adjacent material sections.
• A higher layer count generally requires more Core and Prepreg materials, although the final thickness depends on the selected material thicknesses.

5.3 Calculating the Stackup Thickness

The basic stackup calculation is:

Total board thickness = sum of all Core thicknesses + sum of all Prepreg thicknesses + sum of all copper thicknesses

• Copper thickness: 1 oz is approximately 35 μm.
• Core thickness: determined by the selected Core specification.
• Prepreg thickness: based on the final thickness of one sheet multiplied by the number of sheets, subject to the lamination process and copper distribution.

Stackup design is therefore largely an exercise in arranging Core and Prepreg materials to obtain the target total thickness and dielectric spacing. The selected dielectric spacing also affects the impedance of traces referenced to adjacent planes.

6. Why a Symmetrical Stackup Matters

6.1 The Warpage Problem

During lamination, different layers may have different resin shrinkage behavior and different copper distributions. These differences generate internal mechanical stress.

• Asymmetric stackup → unbalanced stress → PCB warpage, including bow or twist.
• Warped boards can cause problems with component placement, soldering, and final assembly.

Warpage is not determined only by the nominal layer count. The arrangement and thickness of the Core and Prepreg, as well as the copper distribution on each layer, all contribute to the mechanical balance of the finished board.

6.2 Symmetry Guidelines

From the top surface to the bottom surface, the distribution of Core and Prepreg should be as symmetrical as practical around the centerline of the board.

• Copper distribution should also be balanced. One side should not contain dense copper while the opposite side is largely empty.
• The material thickness on both sides of the centerline should be similar.
• The overall dielectric and conductive structure should be mechanically balanced.

An asymmetric stackup is one of the most common causes of avoidable PCB warpage during multilayer fabrication.

7. Other PCB Materials

Common Prepreg glass-cloth styles include the following:

• 1080: relatively thin.
• 2116: medium thickness.
• 7628: relatively thick.

Different glass-cloth styles and sheet counts can be combined to obtain different dielectric thicknesses. The exact final thickness must be considered together with the selected laminate construction and lamination conditions.

8. Why Core and Prepreg Matter in PCB Design

1. Stackup thickness: Core and Prepreg determine the total board thickness and the spacing between conductive layers.
2. Impedance control: The dielectric thickness between a trace and its reference plane directly affects the characteristic impedance, Z₀.
3. Warpage prevention: A symmetrical material and copper distribution helps reduce mechanical imbalance.
4. Cost: The layer count, Core specifications, Prepreg quantity, and material selection all affect manufacturing cost.
5. Material selection: The dielectric properties, including Dk and Df, affect the electrical loss and signal behavior of the finished board.

The practical meaning is that stackup design is not simply a matter of counting layers. It is the controlled arrangement of cured Core material, flowing Prepreg, copper foil, and reference structures.

9. Ten Common Misconceptions

1. "Core and Prepreg are the same." Core includes copper and is fully cured; Prepreg has no copper and is only partially cured.
2. "Prepreg includes copper foil." Prepreg consists of resin and glass fiber; copper foil is supplied separately.
3. "Core melts during lamination." Core is already cured and remains a rigid structural element.
4. "Stackup symmetry is not important." An asymmetric stackup can produce unbalanced stress and PCB warpage.
5. "Thickness can be selected arbitrarily." Material thickness affects both total board thickness and impedance.
6. "Prepreg only acts as glue." After curing, it also serves as the dielectric between conductive layers.
7. "Copper distribution does not matter." Uneven copper distribution can contribute to mechanical imbalance and warpage.
8. "More layers always mean a thicker board." The final thickness depends on the thickness of the selected Core and Prepreg materials.
9. "All Core materials have the same thickness." Core is available in multiple thickness specifications.
10. "Stackup design is unrelated to impedance." The dielectric spacing created by the Core and Prepreg directly affects controlled impedance.

10. Quick Reference

11. The Core Concept

A multilayer PCB is formed by alternately laminating Core, the fully cured copper-clad structural material, and Prepreg, the partially cured, copper-free bonding material. Core provides mechanical strength and internal copper layers. Prepreg bonds the layers, provides insulation, and determines part of the dielectric thickness.

Stackup design is essentially the controlled arrangement of Core and Prepreg. That arrangement determines the board's total thickness, dielectric spacing, impedance behavior, mechanical balance, and resistance to warpage.

Remember: Core contains copper and is already rigid; Prepreg contains no copper and flows during lamination.