As simple as it is, a prepreg is nothing but an insulation layer. Prepreg is a dielectric material that is sandwiched between two cores or between a core and a copper foil in a PCB, to provide the required insulation. You can call it as a binding material as well. It either binds two cores or a core and a copper foil.
If you can imagine a hamburger, prepreg is the slice of cheese placed between the bread and the ham. The bread and ham can either be a core or a copper foil. Now I am sure I got your attention.
I would like to tell you something interesting about prepregs. Prepregs are special dielectric materials that can be modified to our need. By the combination of certain additives and catalysts, under a chemical process, a specific section of a prepreg can be transformed into a conductive region. This is called selective conductivity. This phenomenon finds its purpose while drilling holes into the prepregs for connecting the top and the bottom layers. However, the rest of the part of the prepreg remains to be an insulator and protects the circuit board from short circuit which is its basic functionality.
Years of immense research and development have been undergone for the selection of the right prepreg material. Factors like thermal and mechanical compatibility of a material must be considered to accommodate the PCB requirements.
How are prepregs made?
Prepreg, as the name suggests, is a glass fibre weave/cloth impregnated with a resin bonding agent. The glass fibres are interwoven to form a glass fabric. This glass fibre weave is partially dried to form a B-stage material. All prepregs are B-stage materials. During the manufacturing process of prepreg, it is important to follow the grain direction of the material being used since it allows the material to be modified as per requirement. The number of warp and fill counts determine the amount of resin the glass fabric can retain.
There are different types of prepregs that are incorporated in PCBs depending on the thickness and other requirements. Prepregs are available in Standard Resin (SR), Medium Resin (MR), and High Resin (HR) depending on the resin content they hold. The more resin content they hold, the more expensive they get. Here’s a table for your better understanding:
The confusion between prepreg and the core
People often get confused between prepregs and cores due to their similarities. Allow me to clarify it for you. The core is a product of prepreg and lamination. It is more rigid compared to the prepreg. The core is made up of a fiberglass-epoxy laminate which meets the FR4 (Flame Retardant) grade with copper on either side. The core is usually manufactured by a core manufacturer and the company is liable for any faults in the material that might hinder the operation of a PCB. Whereas prepregs, as described earlier, are also made of the same material like the core but are partially dried without lamination and aren’t rigid like the core. Hope this disposes the ambiguity.
Now, the thickness of the PCB depends on the type of prepregs used. It must be noted that the required thickness can be achieved by a combination of prepreg plies. If you are wondering what plies are, they are nothing but thin sheets of prepregs placed on top of one another. This helps in achieving the desired thickness of the PCB.
Other than PCBs, prepregs find their applications in aerospace components, automotive parts, A/C ducting, machinery, tooling, honeycomb and foam panels, and many more.
In conclusion, prepregs are insulators that glue together cores and copper foils and assist in the manufacture of a robust PCB.