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What Is The Difference Between Prepreg Vs Core?

Introduction

As a manufacturer with over 20 years of PCB manufacturing experience. many of our customers want to know more about PCB material. selection and the manufacturing process. Many people confused by the question prepreg vs core. once the difference between core vs prepreg understood. which material should you use for your application? Follow this post to completely comprehend the distinction between prepreg and core.

prepreg material vs core

What is a Prepreg in PCB Manufacturing?

In PCB manufacturing, a prepreg refers to a layer of fiberglass cloth. impregnated with a partially cured epoxy resin. It used as a core material in multilayer PCBs. To provide insulation and bonding between the inner copper layers.To create a multilayer PCB, the prepreg layers placed between the inner copper layers. The stack-up typically consists of a copper layer, a layer of prepreg, another copper layer, and so on. The stack is then subjected to heat and pressure in a lamination process. which activates the resin in the prepreg layers and causes them to fully cure. The cured resin bonds the copper layers together, forming a solid and rigid structure.

The prepreg layers provide insulation between the inner copper layers. preventing electrical short circuits. They also help maintain the desired spacing and alignment. between the copper layers during the lamination process. The number and thickness of prepreg layers used in a PCB depend on the design requirements. such as the desired dielectric properties, controlled impedance, and overall board thickness.

What Types of Resin Used in Prepregs?

Prepregs in PCB manufacturing predominantly use epoxy resin as the matrix material. Epoxy resins offer excellent electrical insulation properties. high mechanical strength, and good adhesion to copper and fiberglass materials. However, there are other resin systems used in specific applications. Here are a few examples:

Epoxy Resin:

Epoxy resin is the most commonly used resin in prepregs for PCB manufacturing. It provides high strength, good dimensional stability, and reliable electrical insulation. Epoxy-based prepregs are widely available and offer a wide range of options. in terms of resin formulations and performance characteristics.

Polyimide Resin:

Polyimide resin used in high-temperature applications. where excellent thermal stability and reliability need required. Polyimide-based prepregs known for their superior thermal resistance. low moisture absorption, and excellent mechanical properties.

Cyanate Ester Resin:

Cyanate ester resin used in prepregs when high-temperature performance. low outgassing, and low dielectric constant are required. And Cyanate ester-based prepregs exhibit good thermal stability. low moisture absorption, and excellent electrical properties at elevated temperatures.

PTFE (Polytetrafluoroethylene) Resin:

PTFE-based resin used in prepregs for specialized applications. that require exceptional electrical properties. such as low dielectric constant and low loss tangent. PTFE-based prepregs are commonly used in high-frequency PCBs. such as microwave and RF circuits.

These are just a few examples of the resin systems used in prepregs. Other resin types, such as phenolic, bismaleimide (BMI), and silicone. may also used in specific applications based on their unique properties and requirements. The choice of resin depends on factors such as the desired electrical performance. thermal stability, mechanical strength. and the specific application or industry where the prepreg used.

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What is Core in PCB?

In PCB manufacturing, the core refers to the central layer(s) of a multilayer PCB. It typically composed of a solid laminate material. that provides mechanical strength, rigidity, and electrical insulation to the PCB.

The core material typically made of a fiberglass-reinforced epoxy resin. similar to the prepreg material used in PCBs. However, the core material is fully cured . and does not contain copper layers like the outer layers of the PCB.

Important Functions of the core in PCB

Mechanical Support: The core provides structural integrity and stiffness to the PCB. It helps prevent bending, warping. and flexing of the board, ensuring its dimensional stability.

Insulation:

The core material offers electrical insulation between the inner copper layers. It prevents short circuits and ensures proper electrical . isolation between different circuit traces and components.

Thickness Control:

The core material helps maintain the desired overall thickness of the PCB. It determines the spacing between the inner copper layers . and contributes to controlled impedance characteristics.

During PCB fabrication, the core layers sandwiched. between the prepreg layers and the outer copper layers. The stack-up typically consists of a copper layer. a layer of prepreg, a core layer, another layer of prepreg, and so on. The entire stack is then subjected to heat and pressure during the lamination process. which activates the resin in the prepreg layers. fully cures the core material, and bonds the copper layers together.

The number and thickness of core layers in a PCB depend on the design requirements. such as the desired board thickness, mechanical strength, thermal management needs. and electrical performance considerations. Multilayer PCBs may have multiple core layers. to accommodate the desired circuit density and functionality.

Overall, the core layer is a critical component in multilayer PCBs. providing structural support, insulation, and dimensional stability to the board. while enabling the interconnection of various circuit traces and components.

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What is the difference between Prepreg Vs Core?

PCB cores and prepreg materials’ dielectric constants may vary slightly. based on resin quantity, resin type, and glass weave. Since the effective dielectric constant observed by the signal on the track. depends on the dielectric constant of the surrounding material. That a problem for building boards. that require highly exact impedance matching. Not all prepregs vs core materials are compatible with one another. and core/prepreg stacks with widely varying dielectric constants . make precise dielectric constants and connection losses difficult to anticipate.

pcb core vs prepreg

Prepreg Vs Core

ItemPrepregCore
CompositionPrepreg refers to a layer of fiberglass cloth impregnated with a partially cured resin, typically epoxy. The resin in a partially cured or B-stage state, remaining tacky and flowable when heat and pressure are applied.The core refers to the central layer(s) of a multilayer PCB. It  made of a solid laminate material that provides mechanical strength, rigidity, and electrical insulation. The core material is fully cured and does not contain copper layers.
Purposeprovide insulation and bonding between the inner copper layers. They act as an adhesive material that ensures proper adhesion and electrical isolation between the copper layers.The core serves as a stable foundation for the PCB, offering structural support, dimensional stability, and electrical insulation. It helps maintain the desired overall thickness of the PCB and provides spacing between the inner copper layers.
CharacteristicsPrepregs offer flexibility, allowing for easy handling, cutting, and stacking during PCB manufacturing. They provide consistent resin content, ensuring uniformity and predictable material properties. Prepregs are available in different thicknesses and resin formulations to meet specific design requirements.

Core materials are typically composed of fiberglass-reinforced epoxy resin, similar to prepreg material but fully cured. They offer high mechanical strength, rigidity, and excellent electrical insulation properties. Core thicknesses generally uniform and predetermined, contributing to controlled impedance in the PCB.

At high voltages, creepage and leakage currents are a problem for any PCB core or prepreg material. One of the reasons for the creepage specification of FR4 materials. That is the electromigration of copper and subsequent formation of conductive filaments. This issue, as well as the desire to raise the glass transition and decomposition temperatures. motivated the use of non-dicyandiamide (non-DICY) resins in FR4 cores and laminates. When fully cured, phenolic resins have higher decomposition. and glass transition temperatures than DICY resins. as well as greater insulating resistance.

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Dielectric Constant Effective for Core and Prepreg Materials

Because of the substantial structural differences between core and prepreg materials. exact estimates of dielectric constant and loss angle tangent are critical for signal integrity. That able to extract a value from the marketing data sheet if your signal has a short rise time. When your inflection frequency or analogue signal approaches the GHz range. you must exercise caution when referencing values in the data sheet. especially when modelling interconnect behaviour and employing impedance-controlled cabling.

When multilayer utilised, prereg becomes an essential requirement. not just a necessary part of the PCB production process. Manufacturers will find themselves unable to hold the numerous layers together without Prereg. The other two components of the PCB are the Core and Prereg. As a material FR4 prepreg, this Core contains traces of copper. Meanwhile, the PCB stays in place by the Core through Prereg.

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