I’m trying to gain a better understanding of multilayer PCB structures. I’m struggling with understanding the roles of “prepreg” and “core.” Although I know they are involved in bonding the layers together, I’m uncertain about why both are necessary and how they differ from each other. Could someone clarify their distinct functions and why they are both necessary?
Example: 8 layer Stack-up
Soldermask
Layer-1 SIG
Prepreg
Layer-2 GND
Prepreg
Layer-3 SIG
Core
Layer-4 GND
Prepreg
Layer-5 GND
Core
Layer-6 SIG
Prepreg
Layer-7 GND
Prepreg
Layer-8 SIG
Soldermask
The key distinction lies in their composition and manufacturing process.
The core consists of a layer of FR4 sandwiched between copper layers, crafted in a specialized core facility. This FR4 layer is formed between smooth copper foils to precise thickness specifications.
On the other hand, prepreg comprises uncured FR4 used by PCB manufacturers to bond etched cores or copper foils to etched cores. As a result, prepreg thickness varies depending on the height of the etched boards it connects.
In applications where the dielectric properties are crucial, such as high-frequency transmission lines and antennae, having signal and ground layers adjacent to a core yields superior repeatability compared to when the fields traverse prepreg.
@BerndKruger That was helpful, but I’m not sure I quite understood correctly. Is the following a fair re-wording?
Core and Pre-Preg as basically the same materials, except that Core has already been baked into a more rigid and permanent shape with consistent composition. Pre-Preg isn’t baked yet, so it will still mold itself around the layer it is attaching to. Because the epoxy/glue will move more than the glass, it won’t be quite as consistent, particularly right near the interface. Because the glass and epoxy have different electrical characteristics, this can matter for extremely precise designs, such as antenna or high speed. If the pre-preg is thick compared to the copper traces, it should be pretty similar after baking.
Is the copper thickness on the other side of a pre-preg also less consistent, either because of lower standards or because the pre-preg itself offers less physical resistance to encroachment?
The biggest problem is just that it flows. As the press heats up the prepreg gets soft and flows anywhere it can. Some mfg’s have problems with air getting trapped for example. Since it can flow, if there are large differences in the amount of copper different areas have that can be a problem. you can add pours or thieving to compensate.
A conventional 8 layer stackup without advanced construction (like buried vias or HDI) will normally have 3 cores with 4 prepreg layers alternating between the cores. An alternate 8 layer structure is to use 4 cores with 3 prepreg layers but note that requires more fabrication steps and generally costs more than a 3 core stucture.
• A core is fully cured while pre-preg is partially cured – referred to as “B” stage- and needs to see more heat to fully cure.
• FR4 is a thermal reactive resin. In the case of pre-preg, as the temperature starts to increase the viscosity drops allowing the resin to encapsulate/fill in around the traces on the inner layers. When the resin hits it’s Tg temperature it hardens and no longer flows. The tricky part in lamination is controlling the temperature rise so the resin flows evenly across the panel without keeping it below the Tg for too long so it runs out and causes resin starvation. Bring the temperature up too fast and it hardens around the edges trapping volatiles towards the center and causes delamination.
• Pre-preg comes in a wide range of glass styles and resin content. This is important for maintaining dielectric thickness as a signal layer requires more resin to fill around the traces than a plane layer and can press out thinner than desired.
• Cores come in a limited range of glass styles so it’s common for us to make cores in-house for special applications.
• Vacuum lamination is pretty much an industry standard as it eliminates air and volatile entrapment. The resin contains solvents/volatiles which out-gas during lamination so a bigger problem than air. The press cycle usually includes a “Bump” where platen pressure is released for a moment and the volatiles are removed.