What Is Prepreg?

May 10, 2022 Cadence PCB Solutions

Key Takeaways

  • Prepregs are the dielectric material between the adjacent cores or the core and a layer.

  • The best prepreg material for a given multilayer PCB depends on the thickness, layer structure, and impedance.

  • The three types of prepreg available based on the content of resin present are high resin (HR), medium resin (MR), and standard resin (SR).

 

In  multilayer PCBs, prepreg is a vital part that holds the PCB core and layers together

A higher density of assembly is always recommended in electronic circuit board design. Multilayer boards are encouraged due to their high assembly density, reduction in multiple separate PCBs, light weight, and compact construction. An optimal multilayer circuit board design adopts the merits of verticality offered by PCB stackups.

A PCB stackup arrangement includes layers, planes, cores, base, substrate, laminate, and prepreg. While some of these terms might be familiar, you might be wondering, what is prepreg? In multilayer PCBs, prepreg is a vital part that holds the PCB core and layers together. This article discusses multilayer boards and PCB stackups as well as what prepreg is and the role it plays.    

Multilayer Printed Circuit Boards

In the ever-growing electronics industry, there is a high demand for compact products. To achieve compact circuits, designing them as multilayer PCBs is the best solution. Multilayer boards offer high capacity in a smaller footprint, thereby enhancing circuit compactness. The verticality offered by PCB stackups in multilayer boards increases the assembly density and the count of separate PCBs required. As the number of separate PCBs is reduced, it brings down the number of connectors as well. 

Multiple layers play a significant role in improving energy distribution within a board. It is an excellent idea to lay circuits handling high-speed signals in multilayer boards, as this ensures less cross-interference and electromagnetic interference. Multilayer boards rely on PCB stackups. The PCB stackup arranges layers of copper and insulators vertically to incorporate multiple electronic circuits to form a single PCB. 

PCB Stackups

PCB stackups have gained a lot of attention in recent years due to the production of complex, yet compact, circuits in multilayer boards. A PCB stackup contributes to producing efficient and low-cost PCBs. A good PCB stackup is effective in minimizing the circuit’s vulnerability to external noise and radiation, thereby improving the electromagnetic compatibility of the system being designed. Good layering techniques in a PCB stackup can decrease the impedance and cross-talk problems in high-frequency signal boards.

A PCB stackup involves the process of arranging alternate layers of different materials into a single PCB unit by utilizing high pressure and temperature. A single PCB constructed this way ensures uniform encapsulation of conductors without any air trapped between the layers, which are bonded together with adhesives. 

PCB Layers

In a PCB stackup, there are three general layers: the inner signal layer, ground plane, and power plane. Between these layers, there can either be core or prepreg. PCB cores are glass-reinforced epoxy laminate sheets with copper traces. Prepregs are the dielectric material between adjacent cores or a core and a layer. There is often confusion revolving around cores and prepregs. Let’s get a better understanding of what prepreg is in the upcoming section. 

What Is Prepreg? 

So, what is prepreg, really? Prepreg is one of the main constituents of multilayer PCBs. It is the insulation material that binds the adjacent cores, or a core and a layer, in a PCB stackup. The basic functionalities of prepregs are to bind a core to another core, bind a core to a layer, provide insulation, and protect a multilayer board from short-circuiting.

Prepregs are composed of fiberglass or fabric that is pre-impregnated or reinforced with resin. Usually, epoxy-based materials or partially cured polyimides are used for pre-impregnating the dielectric material. There are three main types of prepreg available based on the content of resin present: high resin (HR), medium resin (MR), and standard resin (SR). The best prepreg material for a given multilayer PCB depends on its thickness, layer structure, and impedance. The thickness of the prepreg used in a multilayer board varies in accordance with the overall thickness of the board.

To be used as prepreg, fiberglass is impregnated with resin before it gets laid on a PCB stackup. Once all the layers are arranged, the stackup is exposed to high temperature and pressure and pressed together to form a multilayer board. When applied with heat and pressure, the prepreg softens and flows through the PCB. Once the stackup cools down, the prepreg gets solidified and binds everything. Prepreg is the one thing in a PCB stackup that holds the layers together within the structure.

At this point, hopefully, you have a better understanding of what prepreg is and the role it plays in multilayer PCBs. If you are designing a multilayer PCB, the selection of prepreg can be made simpler with the PCB design and analysis tools available from Cadence.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.

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