What is prepreg in PCB?

 

The prepreg acts as an insulating layer in a printed circuit board and is also a dielectric material. In order to provide insulation, the prepreg is located between a core and a copper foil or between two cores. The type of prepreg used determines the thickness of PCB, so the required thickness can be realized by a combination of prepreg stacks. Through the combination of additives and catalysts, the selective part of the prepreg can be transformed into conductive areas during the chemical process. This process is called selective conductivity. This phenomenon comes into play when drilling holes in the prepreg to connect the top and bottom layers. While the remaining section of the prepreg is still acting as an insulator to prevent the circuit board from short circuits.

 

Figure 1: Prepreg in PCB

So, what is prepreg? A glass fiber cloth impregnated with a resin binder will finally become a prepreg. During this process, the interwoven glass fibers will form a glass fabric which will be partially dried to form a B-stage material. It is necessary to follow the grain direction of the used material during manufacturing in order to adjust materials as required. Moreover, the amount of retained resin of glass fabric will be determined by the number of warp and fill counts.

In the PCB design, they are various types of prepregs according to requirements such as thickness. Also, prepregs can be classified into standard resin(SR), medium resin (MR) and high resin(HR) based on the resin content they hold. With the increase of resin content, the price of the prepregs is increasing. Why is resin content important in the prepreg selection and lamination process? Because resin content determines the thickness of a laminate during pressing. In addition, resin content also has significant impacts on such properties as dielectric constant, coefficient of thermal expansion (CTE), and drilling and etching quality. Resin flow is another important principle in the lamination process. The reason is that the actual flow of the resin is crucial to lamination operation and can be greatly affected by the lamination conditions. Such characteristics as interlayer adhesion, adhesion to oxide inner copper foil, the overall effect of the prepreg as a bond sheet, and adhesion of laminates to ED copper foils used in laminate manufacturing will be affected by the amount of actual resin flow that exists.

The core and prepreg are made of the same fiberglass-epoxy material. But the core has copper on either side of the laminate, which is not suitable for the prepreg. Compared to prepreg, the core is stiffer since the core is laminated and not partially dried. In the stack-up, the dielectric constants (Dk) of the prepreg are different before and after lamination, whereas the dielectric constant of the core does not change.

 

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Reference:

https://www.protoexpress.com/blog/prepreg-the-slice-of-cheese-in-your-pcb/