What is prepreg PCB

What exactly is prepreg and core in a circuit board?


I am trying to get my head around the structure of a multilayer circuit board and while I can understand many things, I am unable to grasp the concept of "prepreg" and "core". What exactly make? I've attached a reference stackup below.

I just understand that they are used to glue the layers together. But why both, why not just "Prepreg" or "Core"? How do they differ from each other?

Could you please demystify these things for me?

Any good clue to understand this and how the layer structure is determined is also appreciated.





Reply:


The important difference is this.

The core is a layer of FR4 with copper on both sides that is manufactured in a core factory. The layer of FR4 is formed between two smooth copper foils with a certain thickness.

Pre-Preg is a layer of uncured FR4 that PCB manufacturers use to glue etched cores or copper foil to an etched core. This means that the thickness of the prepreg varies with the height of the etched plates on either side of it.

For applications where the physical properties of the dielectric are important (such as radio frequency transmission lines and antennas), you get much better signal and ground repeatability on either side of a core than if the fields go over pre-preg.

Choosing which layers to fabricate and how can have an impact on processing steps, and therefore cost, when creating a board with buried vias. It is easy to drill holes through cores to create buried vias, but this limits which layers can be connected to which.





A core is a thick, stiffer layer of fiberglass while a Prepreg is a thin fiberglass / copper layer laminated to a core. In the past there was always only one thick core, so the distinction made much more sense than it is today when it is roughly the same thickness.

There is still a difference in the way vias are treated, but you'd better be referring to the entire stackup in question rather than making assumptions about how vias will relate for '' core '' and '' prepreg '' distinguish.


Via this link:

Prepreg, an abbreviation for pre-impregnated, is a fiber fabric impregnated with a resin binder. It is used to glue the core layers together. The core layers are FR4 with copper traces. The stack of layers is pressed together to the desired thickness of the top layer at the same temperature. Prepreg is available in different strengths.

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