PCB Prepreg vs Core Materials: Engineering Guide to Laminate Selection and Properties

Introduction

In printed circuit board (PCB) manufacturing, the fundamental distinction between prepreg and core materials forms the backbone of multilayer board construction. While these materials share similar base compositions of glass fiber reinforcement and epoxy resin systems, they serve distinctly different functions in PCB architecture. 

Understanding the technical nuances between these materials is essential for engineers, as their properties directly influence the following PCB design parameters in high-performance applications:

•  Impedance control

• Thermal management 

• Signal integrity

Understanding the Differences between Prepreg and PCB Core

Due to the similarities between prepreg and core, they are often confused. But some fundamental differences set the two apart. With their fully cured resin systems and defined electrical properties, core materials provide the stable foundation layers that host copper circuitry.  

On the other hand, the prepreg is partially dried without lamination and is less rigid than the core. Prepreg, containing partially cured resin, is the critical bonding agent that fuses multiple core layers during lamination.  Simply put, the core is created by laminating prepreg, making it significantly more rigid. The core consists of a flame-retardant fiberglass-epoxy laminate with copper layers on both sides. 

Another key difference is their dielectric constant: the core’s dielectric constant is stable, while that of the prepreg changes before and after lamination. This variation in dielectric constants depends on factors such as resin content, resin type, and glass weave.

The selection and implementation of appropriate prepreg and core materials significantly impact manufacturing yields, electrical performance, and long-term reliability of multilayer PCBs, particularly in high-frequency and high-speed digital designs where material properties become increasingly critical.

Suggested Reading: What is a Multilayer PCB?

What are Prepreg Materials?

Prepreg is a conductive material that provides necessary insulation between the PCB’s copper and core. It is a dielectric substance that is placed between two cores or between a core and a copper foil, often referred to as a binding material because it bonds either two cores or a core and copper foil together.

As the name suggests, prepreg is a glass fiber infused with a resin bonding agent. All prepregs are B-stage materials, and they come in various types depending on their thickness and resin content. Prepregs are generally available in three categories based on resin content:

• Standard Resin

• Medium Resin

• High Resin

The typical preparation process for prepreg includes:

• Reinforcement with resins

• Removal of excess resin

• Curing preparation with both high- and low-temperature settings

Prereg Material Types

Various types of prepreg materials are available, each with distinct properties suited to specific applications. Some of the most common prepreg materials include:

• FR-4 Prepreg - The most widely used prepreg in the PCB industry, FR-4 (Flame Retardant 4) is made from woven fiberglass fabric impregnated with an epoxy resin. It offers strong mechanical properties, excellent dimensional stability, and reliable electrical insulation, making it suitable for a broad range of PCB applications.

• High Tg Prepreg - High Tg (Glass Transition Temperature) prepreg materials are engineered to withstand higher temperatures than standard FR-4. They are commonly used in applications that require enhanced thermal stability, such as in automotive, aerospace, and military electronics.

• Low Dk/Df Prepreg - Low Dk/Df (Dielectric Constant and Dissipation Factor) prepregs are designed to reduce signal loss and distortion in high-frequency applications. With a lower dielectric constant and dissipation factor than standard FR-4, these materials are ideal for high-speed digital and RF/microwave applications.

• Flexible Prepreg - This prepreg type provides flexibility and bendability for the final PCB, making it well-suited for applications requiring conformability, such as wearable electronics, flexible displays, and medical devices.

Suggested Reading: Rigid Flex PCB: Revolutionizing Modern Electronics Design

What are Core Materials?

The core material of a circuit board serves as the foundational layer, providing essential structural support and stability for the entire PCB. This core layer is what ultimately gives the board its rigidity and strength, allowing it to withstand mechanical stress and maintain its shape.

Typically, the core material is composed of one or more layers of prepreg, which are subjected to processes like pressing, hardening, and heat curing. These prepreg layers are usually made from FR4 materials, such as fiberglass-epoxy laminates, which meet specific flame-retardant standards, ensuring safety and durability in various applications.

The core is also coated with copper foil on both sides, a feature that leads to its designation as "copper-clad laminate." This copper coating forms the conductive pathways necessary for electrical connections across the PCB layers. In some designs, however, the core may only have copper lamination on one side, depending on the application’s requirements and design specifications.

Key Considerations for Picking PCB Core Materials?

PCB core materials must be carefully selected, and the following are the basic critical considerations for the material selection:

• Impedance Control - For high-speed and high-frequency circuits to maintain a stable dielectric constant as frequencies fluctuate.

• Thermal Properties - To prevent delamination, decomposition, or layer separation issues in temperature-sensitive applications.

• Electrical Properties - To ensure reliable electric performance with a low-dielectric constant, ideal for insulating conductive traces and power planes.

• Chemical Properties - For resistance to moisture and chemical absorption to maintain consistent electrical performance over time.

• Dimensional Stability - To counter temperature changes, electrical surges, mechanical shocks, and physical stress.

• Signal Performance - Different materials absorb signals at varying rates. For high-frequency PCBs, the core should have a low dissipation factor to reduce signal loss and improve overall signal quality.

Material Composition and Structure

Chemical Composition Analysis

The fundamental chemistry of PCB materials centers on epoxy resin systems and glass fiber reinforcement. Prepreg materials utilize B-stage epoxy resins, containing 35-65% partially polymerized resin content. Core materials employ fully cured C-stage resins with cross-linking density >90%. The primary resin systems include:

• Standard FR-4: tetrafunctional epoxy

• High-Tg FR-4: Multifunctional epoxy

• Modified epoxy systems: Phenolic-enhanced structures

Glass fabric specifications vary between materials:

Material Composition Comparison:

• The glass reinforcement consists of E-glass fibres (52-56% SiO₂, 12-16% Al₂O₃, 16-25% CaO) woven into specific styles. 

• The fiber diameter ranges from 5-7μm, with surface treatment using silane coupling agents for optimal resin adhesion.

Physical Structure Characteristics

The cross-sectional structure of prepreg and core materials exhibits distinct layered configurations. Core materials maintain a rigid, fully cured structure with a copper foil bonded to both sides, while prepreg layers possess a more malleable structure before final lamination.

Cross-sectional view of typical PCB layers:

Fig 1: PCB core and Prepreg with external copper layers

The curing process progresses through distinct stages:

• A-stage: Initial liquid resin state

• B-stage (Prepreg): 35-65% cross-linking, tacky consistency

• C-stage (Core): >90% cross-linking, fully cured state

Density variations correlate with glass style and resin content:

• Core density: 1.80-2.00 g/cm³

• Prepreg density (pre-cure): 1.65-1.85 g/cm³

• Post-lamination density: 1.85-2.05 g/cm³

Thickness tolerances maintain strict specifications:

• Core: ±10% of nominal thickness

• Prepreg: ±15% pre-lamination

• Post-lamination: ±5% total thickness

Microscopic examination reveals distinct characteristics:

• Glass fiber diameter: 5-7μm

• Weave patterns: Plain (1x1) or spread weave

• Resin distribution: Uniform in cores, variable in prepreg

• Void content: <1% in cores, 2-5% in prepreg

• Interface thickness: 1-3μm between glass and resin

Electrical Properties

The dielectric properties of prepreg and core materials fundamentally determine PCB performance in high-speed applications. These materials exhibit frequency-dependent characteristics that directly impact signal transmission.

Dielectric Constants (Dk)

The structural differences in core and prepreg materials make it crucial to accurately determine the dielectric constant and loss tangent, especially for signal integrity in high-frequency designs. While for low-speed signals, values from datasheets may suffice, GHz-range signals require more precision. 

This variability occurs because PCB materials are inhomogeneous and anisotropic, with properties affected by the fiber weave pattern, leading to effects like skew and fiber cavity resonances. Material thickness also matters for signal behavior, as the effective dielectric constant—a complex parameter—depends on trace dimensions and layer thickness.

Suggested Reading: Understanding PCB Thickness: A Comprehensive Guide 

Dissipation Factor at Various Frequencies:

Low Dk/Df (Dielectric Constant and Dissipation Factor) prepreg materials are specifically engineered to reduce signal loss and distortion in high-frequency applications. These materials are essential for maintaining signal integrity at high frequencies, where standard materials may fall short.

With a lower dielectric constant and dissipation factor than standard FR-4 prepreg, Low Dk/Df materials are particularly suited for high-speed digital and RF/microwave applications. Their properties make them ideal for use in circuits requiring reliable performance and minimal interference at elevated frequencies.

Impedance Control Parameters

Particularly in high-speed and high-frequency circuits, there is a need for tight impedance control. It ensures that the dielectric constant remains stable with change in frequencies. In terms of impedance control, the manufacturers have more control with the core layers as they are easier to reproduce as compared to prepreg. 

On the other hand, the prepreg material dielectric constant can only be specified for the raw material and not after the assembly. Hence, the true dielectric constant experienced by the electric signal can never be predicted. 

• Dielectric thickness tolerance: ±10%

• Copper roughness factor: 1.5-2.0

• Impedance tolerance: ±10%

Signal Integrity Considerations

• Signal integrity is one of the most critical physical parameters as it directly affects PCB performance. All materials have a different signal absorption rate. In case of a high-frequency PCB, the core is more conducive as it has a low dissipation factor. Hence, the signal losses are greatly minimized. 

• Maximum crosstalk: -30dB @1GHz

• Insertion loss: -0.5dB/inch @1GHz

• Return loss: -20dB minimum

• Propagation delay: 180ps/inch ±10%

• Rise time degradation: <10% for 1ns rise time

The frequency-dependent behavior affects:

• Signal attenuation: 0.1-0.5 dB/inch/GHz

• Phase stability: ±5° maximum variation

• Group delay: <±5ps variation

• Bandwidth limitations: Effective up to 20GHz

Mechanical Properties

The mechanical characteristics of PCB materials directly influence board reliability and manufacturing yield. Tensile strength measurements conducted according to IPC-TM-650 2.4.19 demonstrate significant variations between prepreg and core materials.

Suggested Reading: IPC Class 2 vs Class 3: Understanding the Critical Differences in Electronics Manufacturing Standards

Tensile Strength Specifications

• Core Materials (Machine Direction): 380-420 MPa

• Core Materials (Cross Direction): 340-380 MPa

• Prepreg (Pre-cure, Machine Direction): 280-320 MPa

• Prepreg (Post-cure, Machine Direction): 360-400 MPa

Coefficient of Thermal Expansion (CTE)

Flexural Strength Measurements

Dimensional Stability Parameters

• Length/Width Change: ±0.10% maximum

• Thickness Change: ±5% maximum

• Bow and Twist: 0.75% maximum

• Post-etching Shrinkage: <0.15%

• Thermal Stress Resistance: >10 seconds at 288°C

Comprehensive Mechanical Properties

Manufacturing Process Integration

Processing Parameters

Temperature and pressure requirements for PCB lamination processes demand precise control throughout the manufacturing cycle. The lamination press operates under specific parameters shown in the following table:

The lamination process flow can be seen in the following figure: 

Fig 2: The lamination process flow

 The Critical Control Points during the lamination process flow are described in the following table:

Quality Assurance

PCB material quality assurance requires systematic inspection protocols and precise measurement techniques following IPC-4101 specifications. Inspection methods employ both destructive and non-destructive testing approaches to ensure material integrity.

Primary Inspection Methods

1. Visual Inspection

   • Surface defects examination

   • Color uniformity assessment

   • Foreign material detection

   • Registration mark alignment

2. Microsection Analysis

   • Glass/resin distribution

   • Void content measurement

   • Interface examination

   • Layer thickness verification

Suggested Reading: Mastering PCB Testing: Techniques, Methods, and Best Practices Unveiled

Quality Metrics and Acceptance Criteria

Troubleshooting Guidelines

1. Delamination Issues

   • Check moisture content

   • Verify cure temperature profile

   • Examine pressure distribution

   • Review material storage conditions

2. Registration Problems

   • Verify material acclimation

   • Check tooling hole accuracy

   • Assess dimensional stability

   • Monitor pressure application

3. Thickness Variations

   • Review pressure settings

   • Check material distribution

   • Verify stack-up calculations

   • Examine press plate flatness

Fig 3: A quality control engineer performs a visual inspection for a PCB

QC Checkpoint Checklist

To enhance proper quality check, it’s important to ensure multiple checkpoints. Here are some points to consider:

• Incoming Material Inspection - This step ensures the quality of materials received for PCB production. Key checks include verifying the moisture content of materials, conducting a thorough visual examination for defects, reviewing compliance documentation, and confirming that storage conditions meet specified requirements.

• Pre-lamination Checks - Before lamination, additional inspections ensure materials are prepared properly. This includes monitoring the staging time of materials, checking that environmental conditions are controlled, ensuring tools are clean, and confirming the correct orientation of materials for the lamination process.

• Process Monitoring - During the lamination process, critical parameters are tracked to maintain quality standards. These include monitoring the temperature profile, pressure readings, and vacuum levels, along with verifying that the cycle time meets specified requirements to ensure consistency and reliability in the final product.

Common Defect Analysis Methods

The following table summarises the multiple defect analysis methods for high-quality PCB production:

Design Implementation Guidelines

Stack-up Design

Stack-up design rules for PCB manufacturing require precise material selection and layer arrangement to achieve optimal electrical and mechanical performance. The fundamental principles govern signal integrity and impedance control across multiple layers.

Basic Stack-up Rules:

• Maintain symmetrical construction

• Alternate signal and plane layers

• Balance copper distribution

• Include ground reference planes

• Maintain minimum dielectric spacing

Impedance Calculation Guidelines:

The following equations may be used for calculating the characteristic impedance of PCB materials:

Where:

Z₀ = Characteristic impedance

εᵣ = Dielectric constant

H  = Dielectric height

W  = Trace width

T  = Copper thickness

Thickness Planning Matrix:

Layer Thickness Calculation:

1. Start with the finished board thickness

2. Subtract outer copper layers

3. Calculate core thicknesses

4. Determine prepreg requirements

5. Verify total stack-up height

Common Design Pitfalls:

• Asymmetrical construction leading to warpage

• Insufficient spacing between power/ground planes

• Improper prepreg selection for impedance control

• Mismatched CTE between materials

• Poor reference plane allocation

• Incorrect dielectric thickness for impedance

• Insufficient consideration of material properties

• Overlooking manufacturing tolerances

• Improper signal layer pairing

• Poor copper balance across layers

• Ignoring a design review during the initial stages.

Suggested Reading: The Five Biggest Design Mistakes for PCB Assemblies

Material Selection Framework

Material selection for PCB manufacturing requires systematic evaluation of multiple parameters to achieve optimal performance within specified constraints. The decision matrix encompasses electrical, mechanical, and thermal properties alongside manufacturing considerations.

Material Selection Decision Matrix:

Application-Specific Requirements:

Environmental Considerations:

• RoHS Compliance

  • Lead-free compatibility

  • Halogen-free options

  • Bromine content <900 ppm

  • Chlorine content <900 ppm

• Environmental Impact

  • Energy consumption in manufacturing

  • Recyclability potential

  • Waste reduction capability

  • End-of-life disposal

• Sustainability Metrics

  • Carbon footprint

  • Water usage

  • Chemical emissions

  • Resource depletion

Conclusion

The selection and implementation of PCB prepreg and core materials fundamentally influence the performance, reliability, and manufacturability of multilayer printed circuit boards. Understanding the distinct characteristics, processing requirements, and quality control measures ensures optimal material selection and successful PCB manufacturing outcomes.

Frequently Asked Questions

1. What are the key differences in dielectric properties between prepreg and core materials? 

Core materials typically exhibit more stable dielectric constants (Dk) due to their fully cured state, with variations of ±5% compared to prepreg's ±10%. At 1GHz, standard FR-4 cores maintain Dk values of 4.0-4.2, while prepregs range from 3.8-4.4. This difference stems from the B-stage nature of prepreg materials and their varying resin content (35-65% versus core's 40-45%). Reference: IPC-4101E specifications.

2: How do processing temperatures affect prepreg flow characteristics? 

Prepreg materials exhibit distinct flow characteristics across three temperature ranges:

• Initial Flow (85-105°C): Resin viscosity begins decreasing

• Gel Point (130-140°C): Optimal flow window for void-free lamination

• Final Cure (175-185°C): Cross-linking completion Processing must maintain 250-300 PSI pressure during the flow window to achieve proper resin distribution. Reference: IPC-4101E/126 processing guidelines.

3. What are the essential quality control parameters for incoming material inspection? 

Key QC parameters include:

• Moisture content: <0.2% for prepreg, <0.1% for core

• Glass transition temperature (Tg): ±5°C from specification

• Resin content: Within ±3% of nominal value

• Thickness tolerance: ±10% for core, ±15% for prepreg

• Surface quality: No scratches >0.1mm depth Reference: IPC-4101E/126 acceptance criteria.

4. How can delamination issues be effectively diagnosed and prevented? 

Delamination prevention requires:

• Moisture content verification: <0.2% maximum

• Proper storage conditions: 20-23°C, 45-55% RH

• Optimal pressure application: 350-400 PSI during cure

• Adequate prepreg staging time: <4 hours at room temperature

• Correct cure temperature profile: ±2°C tolerance Reference: IPC-6012E, Class 3 requirements.

5. What are the critical factors for achieving reliable copper-to-dielectric adhesion? A6: Essential factors include:

• Surface preparation: Minimum 1μm roughness

• Oxide treatment type: Reduced oxide preferred

• Lamination pressure: Minimum 250 PSI

• Temperature ramp rate: 1.5-2.5°C/minute

• Post-cure cooling rate: Maximum 3°C/minute Reference: IPC-4101E adhesion requirements.

Reference:

1. Difference between PCB Core and Prepreg - Technotronix 

2. Prepreg vs. Core: What Are Their Role in PCB Manufacturing – PCB COPY

3. https://pcbinsider.com/core-vs-prepreg/ 

4. Adhesion of Epoxy Resin with Hexagonal Boron Nitride and Graphite - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Chemical-structure-of-a-epoxy-resin-and-b-its-fragment-models_fig2_331461924 [accessed 12 Nov 2024]

5. https://resources.altium.com/p/pcb-core-vs-prepreg-material-what-designers-need-to-know