PCB Prepreg is simply an insulation layer. This dielectric material is sandwiched in-between two different cores, or between a copper foil and a core in a printed circuit board to deliver the necessary insulation. Also, you can refer to it as a binding material. It serves a very significant role in the designing and manufacturing of printed circuit boards.
Prepregs are unique dielectric materials, which you can modify to suit your needs. By combining certain catalysts and additives, under chemical process, you can transform a section of the prepreg into a unique conductive region. We refer to this as selective conductivity.
To understand this topic better, we will be going into details on what prepreg material is, why it is important. We will also discuss what PCB core material is, and the differences between prepreg and PCB core.
What is Prepreg?
The PCB prepreg material is conductive and useful in offering the right insulation between the copper foil layer or copper layer and the PCB core. This dielectric material is sandwiched in-between two different cores, or between copper foils and a cores in a printed circuit boards.
From its name, it is known as a glass fiber that is impregnated with resin bonding agent. Note that all prepreg are classified as B-stage materials. Different prepregs are available.
This depends on their resin content as well as their thickness. There are three main formats for prepregs. This depends on the quantity of resin in them. These include high resin, medium resin, and standard resin.
The typical preparation process of a prepreg involves:
- Reinforcement using resins
- Removing excess resins
- Preparing curing using low and high-temperature settings
Why is Prepreg Important?
Concerning the process of PCB manufacturing, prepreg is known as an important component, taking the following into account.
- You can apply it and work with it easily. You can get accustomed to it easily and it doesn’t need excessive technical expertise
- Different prepregs are available and they will work with different requirements
- In contrast to the wet style of layups, prepreg isn’t messy. This means that the waste management all through the process of pcb manufacturing is efficient
- It is useful in reducing a device’s weight
What is PCB Core?
PCB core is a rigid material that is laminated using copper. Also, it is useful in the manufacturing of double or single-sided boards or multilayer board.
Furthermore, when heat is brought to the core, the prepreg helps in binding the PCB core and the layers together. For multilayer PCB, you will find these components: copper foil – prepreg materials – copper foil – prepreg materials – copper foil.
What are the Differences Between Prepreg and PCB Core?
Due to the similarity found between the PCB core and prepreg, some people are confused about the two. The core layers are known as the product of the lamination and the prepreg. Therefore, in contrast to prepreg, it is much more rigid.
The core materials are composed of fiberglass-epoxy laminate. This is flame retardant. Also, it features copper on any of its sides. On the other hand, the prepreg is dried partially without any lamination and it is not as rigid compared to the core.
Also, another significant difference between core and prepreg PCB is the dielectric constant. Though the core’s dielectric constant doesn’t vary, the exact dielectric constants of prepreg vary after and before lamination. The difference in dielectric constant has to do with the resin content, glass weave, and resin type.
This could become a problem when you require impedance matching. Furthermore, as the core materials and all prepregs aren’t compatible with themselves, and then the prepreg or core stacks having different dielectric constants, predicting the actual dielectric constant and losses in interconnect becomes difficult.
Furthermore, the PCB thickness has a link with the type of prepregs used. You can achieve the necessary thickness by combining prepreg plies. For example, thin prepreg sheets that are placed over the other.
When there is high voltage, whether a prepreg material or PCB core materials are used, leakage currents and creepage can become an issue. Also, creepage specifications meant for FR4 materials are necessary for copper electromigration as well as the growth of the conductive filaments.
Applications of Prepregs
Multi layered boards
Multiple layers or multilayer sheets of prepregs hold the layers together. Therefore, the user has to fuse them anytime they are stacking all the layers of the board by exposure to a very high temperature. Then there will be a similarity between the general board thickness and the thickness of the prepreg.
When designers utilize this prepreg on multilayered boards, it has to possess specific characteristics and features. Furthermore, cracks, excess resin powder, defects, oil, stains, or foreign matter must not be present in this new surface. Also, this surface must possess a smooth look and feel.
These are also applicable in complex printed circuit boards. Engineers having complex pcbs will surely have a prepreg that is more complex. This is one reason why they will only be able to achieve the required board thickness as well as prepreg to make use of the different prepregs.
What is the Applicable Dielectric Constant for Prepreg and Core Materials?
Core and prepreg materials have well-known operational variations. Talking from the perspective of signal integrity, it is very important that you have an accurate value for dielectric constant and loss tangent or dissipation factor.
It is easy for designers to escape with a low rise time in signal by removing value from the marketing datasheet. This is why caution is necessary when it comes to these values quoted from datasheets whenever their knee frequencies and analog signals reach the GHz range.
Both core and prepreg materials are important components of printed circuit boards. While this core is a PCB material, the multi layered board is held firmly together with the prepreg materials. There are differences between them. However, the major one is that, for the core, it is cured, while for the prepreg materials, they are malleable, meaning forming into sheets is easy. This binding material melts when it is subjected to any high temperature, this helps to fuse all the layers. The dielectric constants are also important.