Differences Between Prepreg Material vs. PCB Core: What Designers Need to Know

I sometimes get questions from designers that want to know more about the PCB material selection and manufacturing process. Although I’m not a manufacturer, it behooves designers to understand something about the materials they have available when working on a new project. One question I get is on the exact differences between PCB core vs. prepreg materials. The terms are sometimes used interchangeably, including by novice designers; I’ll admit I’ve been guilty of this.

Once the difference between prepreg and core is clear, what exact material should you use for your application? How do the important electrical parameters change during plating, etching, and curing? As more designers must become intimately familiar with working at GHz frequencies, these points become quite important for properly sizing traces on these materials and avoiding complicated signal integrity problems.

What Is The Difference Between Core vs. Prepreg in PCB Design?

PCB cores and laminates are similar and, in some ways, quite different. Your core is effectively one or more prepreg laminates that are pressed, hardened, and cured with heat, and the core is plated with copper foil on each side. The prepreg material is impregnated with a resin, where the resin is hardened but left uncured. The prepreg is the "glue" that holds core materials together; when two cores are stacked on each side of a prepreg laminate, exposing the stack to heat causes the resin in the prepreg to begin bonding to the materials in the adjacent layers. The hardened resin slowly cures through crosslinking, and its resulting material properties start to approach those of the core layers.

The resin material encases a glass weave, and the manufacturing process for this glass weave is very similar to that used to manufacture yarns. The glass weave can be quite tight (e.g. 7628 prepreg) or loose (e.g. 1080 prepreg) when viewed under a microscope, which is controlled with a loom during manufacturing. Any gaps and the overall homogeneity of the yarn will determine the electromagnetic properties, which is then responsible for dispersion, losses, and any fiber weave effects seen by signals in the board.

FR4 PCB core/prepreg weaves and their important material properties. Source: Isola Group.

PCB cores and prepregs use these standardized fiberglass cloth styles, such as those identified above (106, 1080, 2116, and 7628). These designations define the yarn thickness and weave density of the fiberglass fabric. Lighter styles like 106 or 1080 use finer yarns and more open weaves, producing thinner dielectric layers with higher resin content. Heavier styles such as 2116 or 7628 use thicker yarn bundles and tighter weaves, which produce stronger laminates and thicker dielectric layers per ply. Because glass and resin have different dielectric constants, the specific weave style and resin fill influence the effective dielectric constant.

Finally, both core and prepreg can have spread glass, which has become the dominant glass style used in more advanced PCB laminates. The reason for using spread glass in these laminate constructions is to reduce the fiber weave effect, which has helped reduce skew from the fiber weave effect down to ~1 ps/inch, compared to ~10 ps/inch in standard open glass. In most earlier generation computing interfaces (PCIe, DDR, etc.), glass weave skew has become a non-issue, but in high-bandwidth interfaces and SerDes channels it remains a problem as the skew values become more similar to the signal rise times. The skew will take up a significant portion of the safety margin that would normally protect against jitter, ISI, crosstalk, reflections, etc.

Dielectric Constant in Core vs. Prepreg Materials

PCB laminate materials are produced under commercial brand names, and the material vendor will provide core and prepreg materials for purchase. The core and prepreg can have somewhat different dielectric constants, even when they are the same brand name. The differences depend on:

• The resin content
• Type of resin/curing material
• Glass weave style
• Material thickness

Prepreg and core materials from different materials manufacturers have some processing compatibility with each other, which is outlined defined in IPC slash sheets. However, FR4 core/prepreg materials can different dielectric constants and dispersion, even if they are the same brand name.

To see just how different the core and prepreg dielectric constant values can be, we can turn to the material datasheets. Some manufacturers provide a lot of data on their material dielectric constants, which also specifies the glass type, weave type/resin content, and thickness. The dielectric constant may be available over multiple frequency values, but the level of information varies widely by material manufacturer and product. Materials that are marketed for advanced applications tend to have much more detail on the dielectric constant over a larger frequency range.

The tables below show an example of the dielectric constant data designers might find from a manufacturer. In this case, we are looking at the core and prepreg data for IS410, an FR4-grade material from Isola. I like showing Isola material information because they do such a great job of compiling their dielectric constant data into these convenient tables. An excerpt of the core data and the complete prepreg data are shown below.

Effective Dielectric Constant for Different Core and Prepreg Materials

With the obvious structural variations in core and prepreg materials, getting an accurate value for the dielectric constant and loss tangent is important from a signal integrity standpoint. When your signals have low rise time, you can probably get away with taking a value from a marketing datasheet. Once your knee frequencies or analog signals hit the GHz range, you need to be careful with values quoted from datasheets, especially when modeling interconnect behavior and using impedance controlled routing.

The problem with datasheet values is that the actual dielectric constant measured depends on the test method, routing geometry, specific frequencies (especially in the GHz range), resin content, and even material thickness. John Coonrod has discussed this problem extensively in a recent podcast. The weave pattern in different PCB core/prepreg materials makes them highly inhomogeneous and anisotropic, meaning the important material properties vary in space and along different directions. This is the reason we have fiber weave effects, such as skew and fiber cavity resonances.

You might be thinking, why should the thickness of a laminate matter when characterizing material properties? The reason is that the important parameter that characterizes signal behavior is the effective dielectric constant (remember, this is a complex quantity!), which depends on the trace dimensions and layer thickness you use in you material. Take a look at these articles for microstrip and symmetric stripline transmission lines.

Finally, the other important parameter to consider is the copper roughness on a given laminate. The two articles I linked to above provide effective dielectric constant values for microstrip and stripline transmission line geometries while assuming no copper roughness. However, real copper features in PCBs always have some roughness which will impact the impedance and loss. The roughness value is determined by the copper foil manufacturing process and the PCB processing, the latter of which requires roughening the copper prior to constructing the PCB stackup.

There is a simple linear approximation you can use to account for copper roughness:

Effective dielectric constant with copper roughness. Source: B. Simonovich, Demystifying PCB Transmission Line Interconnect Modeling, Signal Integrity Journal.

In this equation, Hsmooth is the thickness of the dielectric, and Rz is the 10-point mean roughness. This value should be specified by the laminate manufacturer. If you’re designing for high speed, and you need impedance controlled routing, then your manufacturer should be able to supply these values for you. For modeling, you’ll need to use the right model to describe roughness; take a look at Bert Simonovich's article in Signal Integrity Journal for more information.

If you’re working at extreme high speeds/high frequencies with low signal levels, and you require highly accurate interconnect characterization, then your best bet is to create a test coupon and use a standard measurement to determine the effective dielectric constant. Your test method should use a geometry that closely matches your intended interconnect geometry. This takes some work on the front end, but accurate test and measurement could save you unnecessary prototyping runs on the back end.

When you’re selecting from a range of different PCB core vs. prepreg materials, the layer stack manager in Altium Designer^® can be a huge help. You’ll have access to a materials library that contains important data on a broad range of standardized materials, or you can specify specific material properties for exotic substrate materials. These features increase your productivity while still allowing you to adapt your design to highly specific applications.

Now you can download a free trial of Altium Designer and learn more about the industry’s best layout, simulation, and production planning tools. Talk to an Altium expert today to learn more.