Prepreg vs. Core – A Comprehensive Guide and Explanation of Their Differences

Prepreg vs. Core – a comprehensive guide and explanation of their differences

Many questions about the manufacture and even material selection of PCBs have emerged. Often, the battle on Prepreg vs. Core comes up. Many have used these terms – sometimes by swapping their meanings. In this guide, you will understand the differences between core and Prepreg. Then we will go further into the details of both.

Table of Contents

Prepreg – what does it mean?

First, it’s important to note that the reason why designers use Prepreg is for insulation. You may ask how it provides this insulation. Well, it is dielectric in nature, hence, it fits in the middle of a core and copper. Alternatively, it can fit in the middle of two cores in a printed circuit board.

Prepreg has binding properties. Yes, it binds a core and copper foil. As a designer, make adjustments on Prepreg as you wish. In fact, with the help of chemicals/catalysts, adjust a specific point on the Prepreg material to be conductive.

Prepreg

The Prepreg can also help prevent any short circuits in the PCB. The name Prepreg comes from the fact that it has pre-impregnated fabric or fiberglass. Some make it stronger using resin instead.

When choosing a Prepreg material for a multilayer PCB, consider the impedance, layer structure, and thickness.

The PCB core

PCB Core

The PCB core is used in the production of single, double, and even multi-layer PCBs. It forms a robust base for the PCB. It usually has a copper lamination on a single side or even both sides.

A PCB core has several Prepreg laminates pressed together. After undergoing pressing, it is hardened. Afterwards it undergoes curing using heat. Prepreg holds the PCB core in one place.

At high voltage, it is expected that leakage current and creepage will occur to the PCB core. Creepage will specifically occur because of electro migration of copper. Conductive filaments growing can equally lead to creepage.

Prepreg vs. Core – what are the differences?

Prepreg vs. Core – What are the Differences

The basis of this guide was to assess Prepreg vs. Core and try to get the differences between the two. Prepreg binds core materials together. Basically, a typical PCB core is just a single Prepreg panel or several ones crushed. They are later hardened and then undergo curing via heat. On both sides, copper foils cover the core.

Most manufacturers refer to Prepreg as a tape. The basis is that it ties core components. Prepreg vs. Core materials may at times have a distinct refractive index. This is mostly because of the resin variety, amount, and glass fabric.

PCB Prepreg vs. Core – a look into the materials

When analyzing Prepreg vs. Core materials, you will immediately notice that they have differences in how they operate. For instance, the core has a basic composition of fiberglass-epoxy laminate. This makes it capable of resisting flames easily.

On the other hand, Prepreg is dry and doesn’t have the robustness of the PCB core. In addition, the Prepreg dielectric constant tends to change once it has been laminated and even beforehand. But for the core, the dielectric constant doesn’t change.

It is important to know that Prepreg vs. Core materials aren’t always compatible. The number of Prepreg materials used will often determine how thick the PCB is. And this is basically how they achieve a specified thickness of the printed circuit board.

Leakage currents arise when the voltage is high, regardless of what materials are used in the PCB core and Prepreg.

The core forms the base of the PCB material. Nonetheless, the fact that the Prepreg keeps a multilayer PCB in one place is undeniable. The malleability of Prepreg materials ensure that manufacturers or designers can easily put them together in sheets.

Applications of Prepreg vs. Core

Applications of Prepreg vs. Core

Now let’s take a look at some of the applications of Prepreg vs. Core. How can they help in printed circuit boards?

Complex PCBs
Prepreg vs. Core materials are essential in the formation of complex PCBs. Engineers may get a more complex Prepreg when there is a complex PCB. They can simply utilize several varieties of Prepreg to reach the required thickness for the board and Prepreg.

Multilayer PCBs
As we had stated earlier, Prepreg is important in the formation of multilayer PCBs. It actually holds the different layers of the multilayer PCB together. When stacking the board’s layers together, fusing will be necessary. How to do this? It’s simple – just expose them to high temperatures. There will be a similarity between the Prepreg thickness and that of the board.

The board has to possess unique characteristics for a Prepreg to be used on it. For instance, some of these characteristics include: the absence of foreign matter, defects, oil, stains, cracks, and resin powder, among other imperfections. The board needs to be smooth and clean.

Prepreg vs. core materials dielectric constant

There are clear differences in Prepreg vs. Core in terms of operation and functions. From looking at the signal integrity, you need to have a specific figure for the dielectric constant and the loss tangent.  

The dielectric constant is pegged on the layer thickness of the material you use and the trace dimensions too. Therefore, the thickness of the laminate matter is crucial in the Prepreg vs. Core materials.

The laminate’s copper roughness is equally an integral factor that you’d need to have in mind.

How to choose prepreg when designing and fabricating PCBs?

How to choose Prepreg when designing and fabricating PCBs

When you’re selecting the Prepreg during fabrication or design of your printed circuit board, it would be wise to keep in mind the following factors:

  • Layer structure
  • Impedance
  • The thickness of the Prepreg

When you base your search on the content of resin inside the Prepreg material, you will settle on three types of Prepreg:

  • Standard resin
  • Medium resin
  • High resin

How to choose PCB core materials?

How to choose PCB core materials

On the other hand, when selecting PCB core materials, here are some of the factors that you might want to put into consideration:

  • Impedance control – the impedance control should be tight. At least, this is what high-speed circuits require. This will help to maintain the dielectric constant even with changing frequencies.
  • Electrical properties – low dielectric constant is essential for materials acting as insulators to the power planes and traces made of copper.
  • Thermal properties – heat transfer and dissipation is important when selecting materials to use. At high rates, this factor matters because it will help to prevent decomposing, delaminating, and peeling of the layers.
  • Chemical and mechanical properties – low absorption rates for chemicals and moisture matters in the materials for a PCB core. This ensures that the electrical performance of the PCB core is not affected.
  • Signal performance – whenever there’s higher frequencies, the signals shouldn’t go down. The board should have superb signal performance at all times. A low Df can ensure that this is the case. Thus, that’s one thing to consider when selecting materials. 
  • Dimensional tolerance stability – Once exposed to a variety of temperature variations, disturbances, jerks, and electrical spikes, the high-speed PCB should be constructed with materials that offer dimensional integrity.

Prepreg is dielectric in nature and it fits in the middle of a core and copper. Alternatively, it can fit in the middle of two cores in a printed circuit board.

The PCB core is used in the production of single, double, and even multi-layer PCBs. It forms a robust base for the PCB. It usually has a copper lamination on a single side or even both sides.

When you’re selecting the Prepreg during fabrication or design of your printed circuit board, keep in mind the following factors:

  • Layer structure
  • Impedance
  • The thickness of the Prepreg

Final thoughts

As you have seen in this guide, the differences on Prepreg vs. Core are pretty clear and direct. But all in all, they work together to ensure that you’re PCB operates optimally. It is important that you work with the right partner when thinking of Prepreg vs. Core. Both need the craftiness of seasoned professionals. 

At IBE, our main aim is to partner with you to reach your objectives and goals. For more content and information, feel free to check out IBE electronics. We have in-depth knowledge in PCBA that ensures we meet your needs professionally. Feel free to reach out to us and we will ensure the best service.

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