PCB Core vs. Prepreg Materials: What's the Difference
PCB manufacturing is a complex matter, so some confusion between one or another material used is frequent. But still, the production of circuit boards is barely possible without understanding the specificities of cores and prepreg as main PCB layers.
Let’s learn more about the PCB manufacturing process. You’ll figure out about cores and prepregs are and the crucial differences between them, so manage it to the end of the post.
What is PCB Core Material?
PCB core is a rigid material laminated with copper layers. You may think of it as metal, fiberglass reinforced, or other PCB material coated with a thin layer of copper. PCB core layers are included in multilayer boards.
The core is also called a substrate. It has copper PCB traces serving as signal layers on one or both sides. Standard thicknesses of cores without copper vary in the range of 0,1 – 1,0 mm. Coated with copper and the standard resin epoxy, cores become thicker. The most common single-layer board thickness is about 1.57 mm. or 0.062 in., most of which is core.
The interesting thing is that end board characteristics depend much on core material properties. From dielectric constants and loss tangent characteristics to flexibility, durability, and rigidity — most of the properties are defined by the PCB core chosen.
You may take a look at what the PCB core (which may be a single-layer board) looks like in the image below.
What is PCB Prepreg Material?
In its turn, the prepreg is a conductive substance with insulation between the copper and the PCB core. Prepreg means a reinforced glass fiber impregnated with a resin bonding agent. They are not cured until laminated with cores into a multilayer stack-up. Prepregs belong to B-stage material.
This laminate material is sandwiched between cores or between a core and a copper foil. You may have heard of it as a binding material between cores or a core and a layer of copper.
You may have a look at what PCB prepregs look like in the image below:
You should be aware that there are three prepreg formats ranging by the resin content:
- Standard resin.
- Medium resin.
- High resin.
Resin content parameters affect electrical and thermal insulation properties of the prepregs, dielectric constants, and the overall prepreg thickness.
What's the Difference Between Them?
let’s review crucial aspects that differentiate cores and prepregs from each other:
- Purpose. Cores are substrate materials that are obligatory in any circuit board. They define most of the essential characteristics of end PCBs. In their turn, prepregs are like duct tapes that tie the PCB layers together. Once heated, they let resin inside all the layers. After curing, it makes all the PCB layers bond together.
- Reflective index. If you need to make a PCB with an exact impedance, you should pay particular attention to the absorption coefficients observed in your materials. The idea here is that not all prepregs and cores and compatible. Some stacks may have varying dielectric contracts, making predictions of the exact value slightly problematic.
- Capacity. Prepregs simply are not capacitive, meaning that they don’t carry electricity. At the same time, cores have circuit patterns etched in copper that allow them to transfer signals.
- Production necessity. It’s absolutely impossible to fabricate a PCB without a single core. At the same time, single or double-sided boards are made without prepregs at all.
- Observable properties. A core is a thick, rigid layer of glass fiber. At the same time, the prepreg is a thin layer of glass fiber impregnated with resin and laminated onto the core.
Truth be told, it’s never the choice between cores and prepregs, as you may see. After all, these two components are complementary and not mutually exclusive.
Summary
PCB cores are substrates — rigid materials forming the base of any board. They made be made of metal or fiberglass material and coated with copper foil and resin. Prepregs are thin, laminated sheets pre-impregnated with resin. They bond the PCB layers together.
Prepregs and cores are different in an array of aspects, from purpose to their observable properties. Your manufacturer requires both to produce multilayer PCB. Or you may go without them if you stick to single or double-sided board fabrication only.
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