Core vs. Prepreg: How These Insulation Materials Differ

Want to know the differences between core vs. prepregs

When making multilayer circuit boards, you’ll often come across the terms core and prepreg. While both are insulators, each has at least one secondary function. 

So we will compare core vs. prepregs to define their roles, differences, and how to select each one. Let’s get right into it!

What Is a PCB Core?

A circuit board’s core material is the base layer that holds the other layers and gives the board its structural integrity. 

Usually, it consists of one or more prepreg layers that undergo pressing, hardening, and heat curing.

These prepreg layers are FR4 elements, such as fiberglass-epoxy laminates, that meet the required flame-retarding requirements.

And it has copper foil platings on both sides, which explains why some refer to it as the copper-clad laminate. 

But it can have copper lamination on one side.

Core and prepreg layers in a 4-layer and 6-layer PCB

Core and prepreg layers in a 4-layer and 6-layer PCB

The core might form the entire board stackup in single and double-sided circuit boards. But multilayer boards feature core and prepreg materials pressed together.

How To Choose Core Materials

Consider the following factors when selecting a circuit board’s core material.

  • Impedance control: High-speed and high-frequency circuits need tight impedance control to maintain the dielectric constant as the frequencies change.
  • Thermal properties: The core material should have heat dissipation properties that match the application. The goal is to prevent delamination, decomposition, and layer peeling.
  • Electrical properties: The material should have a low dielectric constant to insulate the conductive patterns and power planes.
  • Chemical properties: PCB cores should have low moisture and chemical absorption rates to maintain dimensional integrity and electrical performance.
  • Dimensional stability: The core should have dimensional tolerance when exposed to temperature variations, electrical spikes, jerks, physical disturbances, etc.
  • Signal performance: Materials absorb signals at different rates. If building a high-frequency PCB, ensure the core has a low dissipation factor. This property will minimize signal losses along the way.
The side profile of a PCB (note the multiple layers)

The side profile of a PCB (note the multiple layers)

What Is a PCB Prepreg?

Abbreviated as PP, prepreg (pre-impregnated) is a fabric or fiberglass sheet material impregnated with resin. 

So it is similar to the core layer but with one key difference. This resin (polyimide or epoxy) undergoes partial heat curing.

A thin fiberglass mat

A thin fiberglass mat

So although it contains the same materials as the core, a prepreg is not as hard. And it does not have copper foil laminates.

 Manufacturers use this material before PCB lamination to provide adhesiveness and insulation.

When placed between two copper layers, the prepreg insulates the two. 

And when the PCB stack undergoes lamination, the outer layer presses the prepreg, squeezing the semi-cured epoxy. 

This gluey substance flows to all crevices and solidifies, bonding the multiple layers together.

Epoxy resin bottles

Epoxy resin bottles

So the FR4 prepreg holds multilayer boards together while isolating and insulating the copper traces.

How To Choose Prepreg Materials

Prepregs have two variables to consider. There is the resin and FR4 material. With the former, check whether it is a standard, medium, or high resin. And regarding the FR4 material, check the impedance, layer structure, and thickness.

Applications of Prepreg Materials

Prepreg laminates are usually not necessary in single or double-sided boards. But they are a must-have in complex and multilayer PCBs.

Complex Circuit Boards

You can use prepregs in complex PCBs to achieve the required board thickness for different applications.

Multilayer PCBs

Prepregs are responsible for holding all layers together in multilayer circuit boards. 

After creating and aligning the PCB stackup, the board undergoes lamination.

 This process involves exposing the board to high pressure and temperatures.

A circuit board in the production process

A circuit board in the production process

The heat softens the half-baked resin while the pressure pushes it to fill empty spaces. So when the PCB cools, the prepreg solidifies and holds everything together.

However, this insulation layer must have the following characteristics to hold multilayer boards together.

  • Be free of defects, stains, oils, excess resin powder, cracks, or foreign matter
  • Have a smooth cloth or fiberglass surface

Core vs. Prepreg: Structure

PCB cores consist of fully cured substrate materials. 

They can have one or more prepregs that undergo lamination (heat and pressure treatment) to press, harden, and solidify the structure. 

Core layers usually also have copper laminations on one or both sides.

 So overall, this layer is solid.

On the other hand, prepregs consist of uncured or partially cured materials. 

They undergo complete curing after application when the stackup undergoes lamination.

 So before lamination, the prepreg is not as solid as the core material.

 It resembles cardboard. But after heat pressing, it hardens between the cores or conductive regions.

Core vs. Prepreg: Usage

The primary purpose of core layers is to provide structural integrity to the circuit board. So they are critical layers in all PCBs. 

But they have a secondary function; to insulate the inner conduction materials.

Prepregs are insulators, as well, because they separate the copper layers in multilayer PCBs on either side of the core material.

Conductive tracks on a circuit board

Conductive tracks on a circuit board

Another critical function of the prepreg is to stick the layers together. 

When heated, the unbaked or half-baked resin melts and fills all crevices to join the layers. 

After cooling, the epoxy solidifies and attaches the traces while the FR4 insulates them.

Core vs. Prepreg: Location in the Stackup

Core materials form the inner layers of the circuit board. They usually separate copper layers or routing and power planes. 

But prepregs can sit between two core layers or the core and copper foil in multilayer circuit boards.

Core vs. Prepreg: Dielectric Constant

The core’s dielectric constant does not change because the material retains its structure before and after lamination. 

But this property changes in the prepreg layers because the material becomes more solid and thinner after lamination.

But it is vital to note that the dielectric constant also depends on the resin type, glass weave, and resin content.

The changing dielectric constant can be an issue if you want to achieve precise impedance matching. 

Also, some core and prepreg materials don’t have matching dielectric constants. So it can be challenging to predict the resulting K after laminating the stackup.

Wrap Up

You might have been interchanging these PCB terminologies before, but this article must have clarified them. 

And we hope it has been insightful. Comment below to let us know your thoughts and sentiments or if we left something out. We appreciate your feedback.