PCB Prepregs: Crucial Dielectric Materials in PCB Design

Prepreg is the insulation layer sandwiched between a core and a copper foil or between two cores in a multi-layered PCB. The prepreg also functions as a binding material to hold the two cores together or to bind the copper foil to the core—it plays a crucial role in PCB design and manufacturing. Prepregs are special dielectric materials and the manufacturer can modify them by adding certain catalysts and additives.

For the selection of the right prepreg material, it is necessary to consider factors like mechanical and thermal compatibility, while looking at its properties to accommodate the requirements of the board.

What is Prepreg

As its name suggests, a prepreg is a woven glass fiber cloth impregnated with an agent such as a bonding resin. The manufacturer interweaves the glass fibers to form a cloth fabric. After impregnating with the bonding resin, the manufacturer lets the cloth dry partially so that it forms a B-stage material. The manufacturer follows the grain direction of the material during the prepreg preparation, as this allows modifications according to requirements. The weave of the cloth has a certain number of fill and warp counts that help to determine the amount of resin the cloth can retain.


Choosing a Prepreg

Depending on the PCB thickness and other requirements there can be various types of prepregs. For instance, prepregs are available in High Resin (HR), Medium Resin (MR), and Standard Resin (SR) formats, based on their resin content. The higher the resin content, the more expensive the prepreg will be. To measure the resin content or resin percentage, it is necessary to dissolve the resin in acid and measure the difference in weight of the PCB before and after dissolving.

Resin content is important as it decides the thickness of the laminate when pressed while fabricating the PCB. Other parameters depending on the resin content are the coefficient of thermal expansion or CTE, the dielectric constant, and the drilling and etching quality.

Prepreg Resin Flow

During PCB lamination, the fabricator applies heat and pressure. This melts the resin and it flows. The amount of flow depends on the pressure applied and the rate of application of heat. The resin only flows freely for a limited period after which it gets over. The lamination process must be completed before the resin hardens.

Therefore, the process of the resin flowing is critical to the operation of lamination, as it can significantly affect the lamination quality. The actual flow affects the properties of the inter-laminar bond, bonding to the laminate to the ED copper foil, and bonding to the inner oxide layer of the copper foil. The resin flow also affects the effectiveness of the prepreg as the bonding material.

Difference Between Core and Prepreg

Due to the similarities in the properties of core and prepreg, it is easy to confuse the two. The core also has a fiberglass cloth with epoxy impregnation. It has to meet the Flame Retardant characteristic (FR4) and has copper on both sides. The major difference between the core and prepreg is the epoxy in the core is fully cured, whereas in the prepreg, the epoxy is semi-cured. The process of lamination cures the epoxy in the prepreg.

The other difference between the two lies in the dielectric constant. While the dielectric constant is constant for the core, it varies for the prepreg before and after lamination.


Prepregs are essential to manufacturing multi-layered PCB boards. Rush PCB Ltd has extensive knowledge and experience in PCB fabrication using prepregs. We have mastered the science of prepregs, and we can create boards that will function to any specifications.