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I am trying to wrap my head around the structure of a multilayer PCB, and while I can understand many things, I am not able to grasp the concept of "prepreg" and "core". What do they exactly do? I have attached a reference stackup below.

Only thing I understand about them is that they are used to glue the layers together. But why both, why not only "prepreg" or "core"? How do they differentiate from each other?

Could you please demystify these things for me?

Any good reference to understand this and how the layer stackup is determined is also appreciated.

An 8-layer stackup example with prepreg and core (source: pcbcart.com

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    \$\begingroup\$ "prepreg" means "pre-impregnated with resin" i.e. glass weave that is pre-glued, ready to be hardened (which in PCB processing, requires heat, though other fibreglass technologies may use UV light or added compounds as hardener/activator/catalyst). \$\endgroup\$
    – user16324
    Feb 14, 2018 at 11:44
  • \$\begingroup\$ This Google search turns up some apparently very good answers in the first few hits. Specifically addressed by Cadence here and by Altium here \$\endgroup\$
    – Russell McMahon ♦
    Aug 31, 2022 at 11:30
  • \$\begingroup\$ Note my Google search text - this sort of detail and 'direction' helps (even with an extra r at the end by mistake :-) . ||| Search string: what are prepreg and core in pcb manufacturer \$\endgroup\$
    – Russell McMahon ♦
    Aug 31, 2022 at 11:33

3 Answers 3

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The important difference is this.

Core is a layer of FR4 with copper either side, that's made in a core factory. The layer of FR4 is formed between two smooth foils of copper, to a specified thickness.

Pre-preg is a layer of uncured FR4, that's used by PCB manufacturers to glue together etched cores, or a copper foil to an etched core. This means the thickness of the prepreg varies with the height of the etched boards either side of it.

For applications where the dielectric's physical properties are important (as in high-frequency transmission lines and antennae) you get much better repeatability with signal and ground either side of a core, than if the fields go across pre-preg.

Choosing which layers are made in which way can affect the processing steps and so costs if you are building a board with buried vias. It's easy to drill holes through cores to get buried vias, but this restricts which layers can connect to which.

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    \$\begingroup\$ Excellent answer, thanks a lot. So basically, both are FR4 dielectric, but named differently due to the way they are formed/used? Pre-preg also glues together two copper layers/foils as seen in the figure above, right? \$\endgroup\$
    – LoveEnigma
    Feb 15, 2018 at 11:32
  • \$\begingroup\$ thanks for the answer! What do you mean with better repeatability by high frequencies? And what is the reason for this? I have never heard this before. I usually go for a build-up (when using a 4 layer board) Signal/GND/VCC/Signal. \$\endgroup\$
    – J. Joly
    Jun 18, 2019 at 8:53
  • \$\begingroup\$ Repeatabe dimensions mean repeatable RF properties. Reread my first and second paragraphs for why dimensions across a core are more repeatable than those across a pre-preg. \$\endgroup\$
    – Neil_UK
    Jun 18, 2019 at 10:05
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A core is a thick, more rigid layer of glass fiber while a prepreg is a thin layer of glass fiber/copper laminated onto a core. In the past, there only ever was one thick core, so the distinction made a lot more sense than today, when they are roughly the same thickness.

There's still a difference in how vias are handled but you better refer to the complete stackup in question instead of making assumptions about how ''core'' and ''prepreg'' vias differ.

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  • \$\begingroup\$ I like first paragraph of your answer, it's useful. I'm not sure what you tried to explain in second paragraph. \$\endgroup\$
    – msimunic
    May 27, 2021 at 10:17
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    \$\begingroup\$ Vias can only go through a core plus any number of prepregs, or they can go through a single prepreg. You cannot have a via go through two or more prepregs but not through a core. \$\endgroup\$
    – Janka
    May 27, 2021 at 13:47
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From this link:

Prepreg, which is an abbreviation for pre impregnated, is a fibre weave impregnated with a resin bonding agent. It is used to stick the core layers together. The core layers being FR4 with copper traces. The layer stack is pressed together at temperature to the required board finish thickness. Prepreg comes in different thicknesses.

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