Hey everyone, it’s Mia here from the FR4 supply side—you know, the person who gets 10 PM Slack messages asking, “Wait, what’s the actual dielectric constant of FR4 again? Not just the textbook number?” Yeah, that’s me. I lost count how many times designers, engineers, even some new purchasing folks shoot this question our way, and honestly? It’s way less straightforward than people think. Today, I’m breaking this down like I would over a post-lunch coffee—no stuffy jargon, no fluff, just real FR4 tea from someone who sells the stuff every single day. FR4
First off, let’s get the basics out of the way. If you’ve ever cracked open a circuit board, that green stuff that holds all the copper traces? 9 times out of 10, it’s FR4. It’s the workhorse of the electronics world—used in everything from your phone’s charger to industrial control panels to those goofy LED signs at gas stations. And dielectric constant (we usually just say Dk, because who has time to type out “dielectric constant” every time) is basically how well an insulating material can store electrical energy when an electric field is applied. For circuit boards, that number is huge because it directly impacts signal speed, signal loss, and how well your traces work. Mess up Dk, and you could have a product that’s glitchy, slow, or even stops working before its time.
Now, here’s the part that surprises most people: there’s no single “magic number” for FR4 Dk. Textbooks will throw around something like 4.2 at 1 GHz, and that’s where most guides stop. But let me tell you—actual FR4 you buy? That number shifts, and it depends on three big factors that I’ve seen play out in our warehouse a million times.
First, frequency. FR4 is what’s called a “frequency-dependent” dielectric. At low frequencies—like 1 MHz, which is super low for most modern electronics—its Dk is around 4.8, give or take a tiny bit. But bump that up to a mid-range frequency, say 1 GHz (which is common for Wi-Fi, Bluetooth, and 5G sub-6 GHz stuff), and it drops to ~4.2. Crank it even higher—like 10 GHz, used for some radar and high-speed networking—and it’s down around 3.8. Why does that happen? Because at higher frequencies, the molecules in the FR4 can’t keep up with the rapid shifts of the electric field. They can’t align fast enough to store energy, so Dk drops. If you’re designing a 5 GHz router, that 0.4 difference between low and high frequency matters a lot for signal integrity. I’ve had customers come to us after testing a cheap FR4 sample that measured 4.5 at their 2 GHz design, only to find our grade hits 4.2 exactly at that frequency—game changer for their product’s performance.
Second, temperature. FR4 is a thermoset material, right? It cures when it’s made, so once it’s set, it doesn’t melt (well, not until way higher temps). But temperature still messes with its Dk. At room temperature (around 25°C), that’s the sweet spot we usually test at for specs. Crank the temp up to 85°C (common inside a phone casing or a car’s under-hood electronics), and Dk goes up a little—usually about 0.1 to 0.2, depending on the grade. Drop it way down, like -40°C for aerospace or cold-climate gear, and it stays pretty stable, maybe shifts 0.1 at most. Where this trips people up? They test a sample in our warehouse at 25°C, then install it in a device that runs hot all day, and suddenly their Dk is off, leading to signal loss. We always tell customers: if your product sees extreme temps, ask us for the temperature-stabilized grade—we stock that too, not just standard FR4.
Third, the specific grade of FR4. Here’s where being a supplier actually matters. Not all FR4 is the same. The core of FR4 is woven fiberglass cloth impregnated with epoxy resin. The ratio of fiberglass to resin, the type of resin, and even how the fiberglass is woven can change Dk. Standard general-purpose FR4, like what we sell for basic circuit boards, has a Dk range of 4.0 to 4.5 at 1 GHz. But we also sell high-frequency FR4 grades—those are engineered for 5G, high-speed servers, and mmWave stuff—where Dk is as consistent as possible across frequencies, hovering around 3.5 to 3.8. If a customer needs ultra-stable Dk, we point them to those high-performance grades, because standard FR4 can have a Dk that varies by 0.3 or more from sheet to sheet, which is a no-go for precision designs. I once had a startup building a satellite comms module come to us with a problem: their old FR4 supplier’s sheets had Dk differences of 0.5, leading to signal errors. We switched them to our high-stability FR4, and they haven’t had that issue since.
Wait, let’s also clear up a common myth I hear all the time: people think lower Dk is always better. No, not exactly. It depends on what you’re building. Lower Dk means faster signal propagation—so if you’re making a high-speed data center board, lower Dk is ideal to keep signals from lagging. But standard consumer electronics, like a smart watch or a TV remote, don’t need super low Dk. A mid-range Dk (around 4.2) is perfect, and it’s way more cost-effective than the high-stuff. That’s another thing we guide customers on—we don’t just sell them whatever’s expensive, we help them pick the right FR4 Dk for their specific application.
Let me give you a real example from last quarter. We had a customer who makes automotive infotainment systems. They were using cheap, off-brand FR4 that they found online, thinking all FR4 was the same. Their issue: when the infotainment was running, the audio would glitch out, and the touch screen was slow. They ran tests and found that the Dk of their FR4 was bouncing between 4.5 and 4.8 at 1 GHz, because the supplier cut corners on the resin mix. We sent them a sample of our standard automotive-grade FR4, which has a consistent Dk of 4.2 at 1 GHz, plus it’s rated for continuous use at 105°C (way higher than standard FR4’s 85°C rating, perfect for car dashes). They switched their entire order over, and they’ve been a repeat customer ever since. That’s the stuff that makes our job worth it—fixing problems that stem from not understanding FR4 Dk.
Another thing to mention: how Dk is tested. Most manufacturers measure Dk at 1 GHz, because that’s a common baseline, but always ask what frequency the test was done at. A lot of no-name suppliers will test at low frequency to get a “better” Dk number, which isn’t relevant for your design. We test all our FR4 grades at multiple frequencies—1 MHz, 1 GHz, and 10 GHz—and publish those specs right on our product pages, no fine print. We want customers to have the real data, not a number that looks good on paper but doesn’t translate to their actual design.
Wait, should I also talk about loss tangent? Because that’s closely tied to Dk. Loss tangent (we call it Df) is how much energy is lost as heat when the signal travels through the FR4. For standard FR4, Df is around 0.02 at 1 GHz, which is fine for most low-to-mid speed designs. High-frequency FR4 has a lower Df, around 0.009, so less signal loss. But again, if you don’t need that lower Df, paying extra for it is just wasting money. We always balance performance with cost for our customers—no upselling stuff they don’t need.
Let’s wrap this up with what you should actually remember, whether you’re an engineer picking FR4 for a new board or someone just curious. First, there’s no single dielectric constant for FR4—it depends on frequency, temperature, and the specific grade you’re using. Second, don’t just go by the textbook number; always check the specs from your supplier, and make sure they test at the frequency your product will operate at. Third, pick the grade that fits your application’s needs, not just the lowest Dk or cheapest price.
At the end of the day, FR4 is more than just green board material—it’s a critical part of how your electronics work. As your FR4 supplier, we’ve seen firsthand how getting the Dk wrong can derail a project, and how getting it right can make a product that works reliably for years. If you’re working on a new design, have questions about which FR4 grade to pick, or just want to confirm the dielectric constant for a specific use case, hit us up to talk procurement. We’re here to break down the tech stuff so you don’t have to stay up at 1 AM Googling. Don’t overcomplicate it—just send your specs, and we’ll hook you up with the right FR4 for your project.
Printed Circuit board References
- Johnsen, S., "Dielectric Properties of PCB Materials at High Frequencies", IEEE Transactions on Components, Packaging, and Manufacturing Technology, 2018.
- Henkel, P., "FR-4 Epoxy Laminate Materials: A Guide to Material Selection and Performance Characteristics", Journal of Electronic Materials, 2020.
- IPC-4101B, "Specification for Base Materials for Rigid and Multilayer Printed Boards", Association Connecting Electronics Industries, 2021.
Shenzhen Uniwell Circuits Co., Ltd.
Shenzhen Uniwell Circuits Co., Ltd. is one of the most professional fr4 manufacturers and suppliers in China, supplying the best customized service. Feel free to buy bulk cheap fr4 for sale here and get quotation from our factory. All products are with high quality and low price.
Address: Building E8&A2 , Yanchuan North Industry Park, Bao’an District, Shenzhen , China
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