Prereg is a dielectric material that designers pack in between a copper and a Core or between two Cores in a PCB to provide the required insulation. Prereg is simply an insulation layer. Since it can bind a copper foil and a Core, it is safe to refer to it as a binding material. Users can also modify Prereg to their needs as peculiar dielectric materials. They can also use a chemical process to transform a certain part of a Prereg into a conductive region through the combination of specific catalysts and additives.
What is the PCB Core?
The PCB Core is the glass-reinforced epoxy laminate sheets and copper traces’ FR4 layers. The Prereg binds the layers and PCB Core together when users heat the Core.
The Differences between PCB Core and Prepreg
There are quite some differences and similarities between PCB Prepreg vs. Core. The Core is indeed one or more Prereg laminates that designers cure, harden, and press with heart. They also use copper foil to plate it on each side. According to most manufacturers, the glue that binds core materials together is the Prereg. When they stack two cores on either side of a Prereg laminate, it will expose the stack to heat. As a result, the heat will cause the resin to start bonding to the adjacent layers. Through crosslinking, the hardened resin will begin to cure gradually. Then, the following material properties will start to approach those of the core layers.
Depending on the glass weave, type of resin, and the resin content, there are differences in the dielectric constants in the PCB Prepreg vs. Core materials. When designers are embarking on developing exact impedance matching boards, this situation can be a problem. As such, the dielectric constants of the surrounding materials will determine the effective dielectric constant, as seen by the signal on a track. The fact is there are some disparities in some of the Prepreg vs. Core materials. What’s more? It becomes quite hard to predict exact dielectric constants due to the difference in dielectric constants in Core/Prereg stacks. Thus, its losses in the interconnection.
What holds the layers together is the multi-layered board of Prereg. The user will need to fuse them when they stack all the board’s layers by exposing it to high temperatures. They will then have the similarity between the overall board thickness with the Prereg thickness. When the designers use the Prereg on a multi-layered board, it must have specific characteristics. The new surface must not have excessive resin powder, cracks, foreign matter, defects, stains, or oil. It must have a smooth feel and look.
Engineers will probably have a more complex Prereg if they have a complex PCB. As a result, they can only achieve the needed thickness for the board and Prereg to use different types of Prereg.
Applicable Dielectric Constant for Core and Prereg Materials
Prereg and Core materials have recognizable operational variations. From a signal integrity perspective, it is essential to have a precise value for the loss tangent and dielectric constant. Designers can easily get away with their signal’s low rise time by taking value from a marketing datasheet. As a result, caution is important for the values they quote from datasheets when their analog signals or knee frequencies hit the GHz range. This case is quite essential when using impedance controlled routing or modeling interconnect behavior.
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