PCB FR-4 Materials Are Not All The Same

In the PCB industry, “FR-4”.is a common designation for laminate materials. To a certain extent, FR-4 as a specific type of laminate is one of the many myths promulgated throughout the industry. This blog will address the history the term FR-4, what it really means, the various quantifiers associated with it, and the characteristic issues of concern when selecting design-specific laminates.

The Origins of FR-4

If you research FR-4 as a term, nearly every reference defines it as being a glass-reinforced epoxy laminate. Essentially, the Wikipedia definition is:

FR-4 is a NEMA (National Electrical Manufacturers Association) grade designation for glass-reinforced epoxy laminate material. FR-4 is a composite material composed of woven fiberglass cloth with an epoxy-based resin binder that is flame resistant (self-extinguishing).

This definition of FR-4 is so ubiquitous within the industry that many people involved in PCB product development have come to use FR-4 as a laminate designation in this manner.

The reality is that FR-4 is not and has never been a laminate material. Rather, it is a UL rating that means “flame retardant class 4.” If you go back far enough into the history of PCBs, there were only two laminate choices: polyimide and epoxy-based materials. If you were designing an aerospace product you used polyimide because the resin system could withstand higher temperatures. But, it comes with a lot of negatives—it is expensive, difficult to manufacture, and at the top of the list of problems, it soaks up moisture. This means you must bake a board made out of polyimide dry and then have a conformal coating put on it to prevent severe leakage problems.

The other “original” material was epoxy. Epoxy was less expensive, easier to manufacture, and was the most commonly used resins. But, the material cannot withstand high temperatures. So, it becomes soft during the soldering process, making it prone to warping. Also, the term “epoxy” does not denote any one particular resin, but to a class of laminates.

So, based on these two original laminate choices, FR-4 came to mean “not polyimide”, but, by no means, are all FR-4 boards the same. In fact, it’s possible to make 50 different boards, all of which satisfy the“FR-4” designation, but differ in their appearance, performance, and where they fall within the cost spectrum.

Even when the industry moved beyond purely epoxy-based materials, the FR-4 designation remained because the newer resin systems weren’t polyimide. For instance, to address the shortcomings of epoxy-based systems, we now have several epoxy blends with a variety of temperature characteristics. One of these is known as “High-Tg FR-4” addresses the low Tg (glass transition temperature) problems associated with epoxy-based boards.

What Really Matters

Along with copper foils and woven glass reinforcement, resin systems are one of the main components of board material systems. And, within the resin system, one of the most important properties is the dielectric (insulator). The property of the dielectric that affects impedance and wave velocity is the relative dielectric constant (er). As shown in Table 1, this property varies with both resin content and the frequency at which it is measured.

Table 1. Properties of a Typical Hi Tg FR-4 Laminate System

When people use “FR-4” they assume that there is one common dielectric constant for all FR-4 materials. Specifically, they assume FR-4 means the material has an er of 4.7. This er has not existed for 30 years, but the assumption was the source of lots of impedance errors. Table 2 shows the er of commonly used laminates none of which are much above 4.0. Additional critical information includes Tan (f) loss tangent, DBV (dielectric breakdown voltage) and WA (water absorption).

Table 2. Properties of Several Common PCB Materials Systems

• Tg = glass transition temperature
• WA = water absorption
• DBV = dielectric breakdown voltage
• Tan (f) = loss tangent
• er = relative dielectric constant

All materials include woven glass reinforcement except Teflon.

Specifying the Best Laminate for Your Design

One of the things that plagued the industry over the years is that the data sheets on materials historically did not include enough information to allow product developers to choose laminates based on the performance parameters of their designs. Within the broad category of “FR-4”, many materials are “typical” without enough specifics to ensure the right laminate was selected for a given product design.

As designs became more complex, the need for further refinement of laminate information rose exponentially. For instance, in recent years, one common challenge was missing information on glass weave styles. Not having this information led to serious issues with jitter and skew. Fortunately, laminate manufacturers now provide information on the glass weave styles such as that shown Table 3.

Table 3. List of Glass Weave Styles Available in PCB Laminates

The differences in these weave styles for some example materials are shown below. These FR4-grade materials span a large range of dielectric constant values, which is a function of the glass type and resin content used in the laminate material. The openings in the glass weave are clearly visible, and before mechanical spread glass became widespread, these glass weave openings were responsible for severe fiber weave effects. These material examples shown below are for FR4-grade prepregs in commercially available materials, but the same images could be found for core materials.

Newer materials and more advanced versions of legacy materials have increasingly moved to spread glass. Standard glass weaves will maintain the openings shown in the above image, which would lead to creative zig-zag routing patterns being used for high-speed parallel buses and differential pairs.

Thanks to spread glass, this is much less necessary today as long as spread glass is specified in the fabrication notes for a design. A designer could also specify a commercially available material that is built with spread glass, and this material should be called out in the fabrication notes and stackup table in the master drawings.

Not all PCB materials suppliers offer the same level of information on the specific glass weaves or whether the material contains spread glass. For example, a given material brand name could have laminate options that include spread glass or standard glass, or the material could be available with either type of glass. Table 4 shows an example of the laminate information containing glass style and resin content needed to design a stackup.

The table below shows resin content, glass type, and dielectric constant data for the FR408HR laminate system from Isola Group. Note that this table is only for cores, but there is also a corresponding table for prepregs. Note that Isola has historically done an excellent job of making this information publicly available or providing the information on request.

Table 4. Typical Laminate Information Needed to Design a PCB Stackup

From Prototyping to Production

As with other critical aspects of design, it’s imperative that the engineering team be very specific when identifying a particular laminate for a product, and then be equally diligent about ensuring that the same laminate is used throughout the entire product manufacturing cycle from prototyping to full production. Sometimes, a cost-conscious production manufacturer substitutes one material for another to save the customer money. This type of situation occurred about five years ago when a production manufacturer substituted a material that had the cheaper one-ply glass material rather than the design-correct, but more expensive, two-ply. This resulted in boards failing and the manufacturer having to buy all of the assembled boards, each of which had $5-6K worth of components.

In truth, our industry got into the bad habit of letting fabricators change the artwork, the stackup,or the material either as part of their routine or as a way of “saving” their customers money. This may not have had much impact when the speeds of design were slow, but that kind of latitude is impossible with today’s high-speed designs. This is why it’s important to have test structures built into your boards and to make sure a “traveler” goes through the fabrication facility with each board. Product developers should insist on this document so they can see which materials went into their boards throughout the product development cycle.

Summary

While the use of FR-4 as a laminate designator has a basis in history, it does not mean it offers performance metrics. In reality, using  “FR-4” materials for a particular design can lead to severe consequences including product failure. To ensure your product will work as designed the first time, you must factor in the critical performance-related characteristics into the final material choice. It’s also imperative to specify that the same laminate be used throughout the entire production cycle from prototype through to full production.

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Reference

1. Ritchey, Lee W. and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design, Volumes 1 and 2.”

Frequently Asked Questions

What does "FR-4" actually mean, and why is it misleading to use it as a laminate spec?

FR-4 is not a specific laminate material; it's a UL flammability rating, meaning "flame retardant class 4." Historically, when choices were mainly polyimide or epoxy systems, "FR-4" became shorthand for "not polyimide." Over time, many different epoxy-based and blended resin systems—some with high-Tg and varied electrical/thermal properties—have all been labeled "FR-4." As a result, specifying "FR-4" alone says almost nothing about performance, cost, or construction, and two boards both called "FR-4" can behave very differently.

Why can't I assume a single dielectric constant for FR-4, and what problems does that cause?

The relative dielectric constant (er) for FR-4-class materials varies with resin content, glass content, and measurement frequency; it has not been a single "4.7" value for decades. In practice, many commonly used laminates are at or below ~4.0, and the exact value depends on the specific material and stackup details. Assuming a fixed er leads to impedance and timing errors, especially in high-speed designs. Designers should use material-specific data (er vs. frequency, loss tangent, resin/glass percentages) when building stackups and simulations, and consult the dielectric constant of fr4 provided in each vendor’s data.

What role do glass weave styles play, and how does spread glass help high-speed designs?

Woven fiberglass reinforcement forms part of the dielectric and its weave pattern creates local variations in glass-to-resin ratio. Standard weaves have visible openings that can induce fiber-weave effects—like differential skew and jitter—when high-speed traces randomly cross resin-rich or glass-rich regions. Spread glass reduces or eliminates these openings, making the dielectric more uniform and mitigating those effects. To benefit, explicitly specify spread glass in fabrication notes and call out a material option that includes spread glass in the stackup and master drawings.

What information should I insist on when selecting a laminate within the "FR-4" family?

Go beyond the FR-4 label and gather laminate-specific data, including: dielectric constant vs. frequency, loss tangent, resin content, glass style(s), Tg (glass transition temperature), water absorption, and dielectric breakdown voltage. Ensure you have this for both cores and prepregs. Many suppliers now publish detailed tables (e.g., glass type and resin content for each thickness option); use these to build a predictable stackup and to control signal integrity. When in doubt, request the fr4 material datasheet to verify these parameters.

How do I keep my prototype and production boards consistent, and why is this critical?

Precisely specify the material (including glass style, resin content, and whether spread glass is required) and enforce it from prototype through volume production. Do not allow substitutions "to save cost" without engineering approval—seemingly small changes (e.g., one-ply vs. two-ply glass) can cause failures and expensive rework. Include test structures on the board, and require a traveler document to track the exact materials and process used for each build. Consistency is essential as today's high-speed designs leave no margin for uncontrolled material changes.