There are plenty of terminologies in PCB manufacturing, exacerbated by overlap between terms and a mixture of industry jargon and parlance. One area that constantly sees confusion is the stackup materials that build the board’s conductive and insulative layers. PCB core vs. prepreg is less of a comparison between diametrically material structures but more of a comparison of two similar (yet distinct) board stackup materials with some similar functionality indispensable to manufacturing processes and end-performance.
PCB Core vs. Prepreg Comparison
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PCB Core vs. Prepreg – What’s the Difference?
Although the terminologies overlap, there is a clear distinction between PCB cores and prepreg materials. Simply put, a core contains copper foil wrapped around prepreg material used as the building blocks for the layers of a circuit, from a double-sided board to multiple layers. The copper foil acts as the conductor and is etched (or potentially untouched for a whole-layer plane) during the fabrication process to reproduce the artwork found in the attached fabrication data. The prepreg acts as an insulator but has a secondary function during lamination: under heat and pressure, the epoxy resin melts before resetting, fusing the layers of the board.
So, if PCB cores contain copper foil and prepreg, what’s the importance of prepreg on its own? When designing a board stackup to a desired thickness (think for enclosure mounting, edge connectors, etc.), the prepreg provides additional thickness without adding unnecessary electrical layers that would become material and process cost adders. For example, a design may use two sheets of 1080 and one sheet of 106 to achieve the required combination of board thickness and dielectric constant for signal/plane layer insulation. The distance between plane layers also influences the impedance calculations for software field solvers. Similarly, manufacturers offer cores of varying thicknesses so that designers can achieve higher signal/plane layer counts within a standard 1.6 mm / 62 mil board.
Stackup planning will alternate PCB cores and prepregs in one of two approaches:
- Foil build – Prepreg layers sandwiching a core at the center of the board iterate towards either surface before terminating with foil layers on the board’s top and bottom. This build option is the more common method of designing a multilayer stackup. The outermost dielectric layer is prepreg.
- Core build – Cores sandwich prepreg layers at the center of the board and iterate towards either surface; in effect, it is the opposite of the foil construction. The process is also known as a capped build. This option performs better in microwave applications. The outermost dielectric layer is a core material.
Similarly, the prepreg plays an essential role in the core’s shape. Before lamination, copper layers require a submersion into etchant baths that remove copper uncovered by an etch-resist layer (after removing the initial layer of copper, the etchant can attack the lateral surfaces of the covered copper). In this step, the prepreg substrate gives shape and backing to the copper foil, preventing access to the backside of the foil until the etch has burrowed through the copper layer. This copper-prepreg structure also supports rapid, inexpensive wet etching processes better suited to moderate production lots, unlike laser/plasma etching, which is more precise but lacks throughput and cost-effectiveness.
Lamination Changes and Selection Criteria
One crucial factor distinguishing the core and prepreg is the thickness reduction during lamination. Mechanically, the prepreg comprises two features: a glass fiber weave for rigidity and structure and epoxy resin that melts and cures during lamination, fusing the disparate board materials. When liquidized, the epoxy flows around the bare copper on adjacent layers, covering them and filling in any gaps from the etching process, reducing the total board thickness by the thickness of the copper signal layer compared to pre-lamination measurements. While the core materials are open along the perimeter with no copper foil attached, they also experience some loss in pre-lamination thickness, but the effect is negligible. However, the resin extrusion along the edges contributes to a slightly different impedance profile than that towards the center of the board.
Designers must select their stackup materials to optimize board performance and manufacturability simultaneously while meeting all design requirements. In that sense, it’s impossible to give an exact stackup recommendation without knowing the constraints of the design. When choosing between core and prepreg material, consider the following general attributes:
- Signal layers should be adjacent to plane layers as much as possible to reduce crosstalk.
- It’s possible to reduce AC impedance and associated EMI with close-coupled plane layers at high frequencies.
- High-speed layers between plane layers dramatically curb radiation for EMC.
- Reducing the dielectric height between signal/plane layers curbs crosstalk without impacting the board’s routable area.
- Substrates choice should easily accommodate several common impedance values (50/100 digital, 40/80 DDR4, 90 USB, etc.).
Your Contract Manufacturer Gets to the Core of PCB Stackups
PCB core vs. prepreg material selection must balance cost, performance, and manufacturability for the most efficient DFM stackup. Depending on the board’s primary functionality (HDI, RF/microwave, etc.), one of two stackup configurations will be more suitable – a manufacturer can walk designers through rule check setups to synchronize the board development further. At VSE, we’re a team of engineers committed to building electronics for our customers across various industries, and our collaborations start with a thorough review of design documents and board materials. We’ve produced life-changing and life-saving devices with our manufacturing partners for over 40 years.
