Hey everyone, it’s Mike from behind the scenes at our flexible PCB workshop—you know, the place where we’re debugging 2am prototypes and troubleshooting thermal bottlenecks that keep our electronics partners up at night. Today, I want to talk about something I get asked at least three times a week by engineers who reach out for quotes: what are the actual thermal conductivity properties of flexible PCB (FPC), and why does this matter more than ever for the devices we all rely on every day? Most people think of flex PCBs as just thin, bendable circuit boards for wearables or foldable phones. Yeah, that’s part of the story, but their thermal behavior is a whole separate beast that makes or breaks projects from medical wearables that monitor heart rhythms for 24 hours straight to EV battery management systems that have to stay cool even on a 100°F highway drive. Let’s break this down like we do for our new engineers when they’re testing materials for a customer’s custom build, no overly jargon-heavy stuff, just the real stuff we see on our fabrication floor every day. Flexible PCB
First, let’s get one thing straight: flexible PCBs aren’t a single material. That’s the big mistake a lot of new engineers make—they Google “flex PCB thermal conductivity” and pull a generic number out of thin air, then get frustrated when their prototype overheats because they didn’t account for the materials we actually use. The core of every flexible PCB is the dielectric layer, right? The thin plastic that holds the copper traces together. Most of our standard flex builds use polyimide (PI) as the base dielectric, because it’s tough, flexible, and resistant to chemicals and temperature swings. But standard off-the-shelf polyimide has a thermal conductivity of around 0.12 to 0.35 W/m·K. Wait, that’s not very high compared to something like aluminum, which is around 200 W/m·K. I know, I know—so why do we make flex PCBs at all if they’re bad at heat? Because that dielectric is insulating, which is non-negotiable for most low-voltage, low-power applications. But here’s the thing: most flex PCB applications don’t rely on the dielectric alone for heat dissipation. We add copper, lots of copper, because copper is the workhorse of thermal and electrical conduction for PCBs.
Let’s talk about copper traces, because that’s where 90% of the heat generated in a flex PCB goes. Copper has a thermal conductivity of about 401 W/m·K—same as rigid PCB copper, that never changes, no matter how thin the trace is (within reason). Wait, but thin flex PCB traces? Yeah, we often use copper foils that are 1 oz (35 μm) or even 0.5 oz (18 μm) thick for small wearables, right? Those thinner traces have less cross-sectional area to carry heat, so their thermal performance is lower than thick rigid PCB copper. For example, a 1 oz copper trace has a thermal resistance of roughly 0.05 °C per watt per centimeter, while a 0.5 oz trace is double that, around 0.1 °C per watt per centimeter. That matters when you have a small, dense flex PCB in a hearing aid, where the trace carrying power to the speaker has to dissipate just enough heat to not make the user’s ear canal warm. We test this every week—last month we had a customer come to us with a prototype hearing aid that overheated after 15 minutes of use, because their original design used a 0.25 oz copper trace that couldn’t move the heat from the amplifier fast enough. We swapped it for a 1 oz copper trace and added a thin, adhesively bonded copper foil on the backside of the flex, and the temperature dropped 12°F. That’s the real impact—this isn’t just lab numbers, that’s a device that was uncomfortable to wear, now safe for all-day use.
Now, here’s where we step beyond standard flex PCBs, because not all projects can wait for a copper trace fix. We build thermal flex PCBs, and that’s where the thermal conductivity numbers jump. The biggest upgrade we make is switching the polyimide dielectric from standard to thermally conductive polyimide. How does that work? We fill the polyimide resin with ceramic fillers—aluminum oxide, boron nitride, things like that. Those fillers don’t make the plastic conductive, but they create pathways for heat to move through the dielectric, instead of the standard plastic which traps heat. Our thermally conductive polyimide has a thermal conductivity range of 0.8 to 1.5 W/m·K—three to ten times higher than standard polyimide. That’s a game-changer for high-power applications like LED strips that wrap around a car’s interior or the battery management flex in an electric scooter. We also use thermally conductive adhesives to bond the copper layers and any additional heat sinks (even tiny ones for flex) because regular adhesives have thermal conductivities as low as 0.2 W/m·K, which can act as a thermal barrier. Our thermal adhesives are around 1.2 W/m·K, so they don’t eat up all the heat we’re trying to move.
Wait, let’s get specific about actual values because I know engineers need hard numbers. Let’s take a 100mm long flex PCB, 50mm wide, with two 1 oz copper traces running the length of it. If it’s a standard flex with 0.2 W/m·K polyimide dielectric, the total thermal conductivity for the entire assembly (measured as a sheet, not just the trace) is around 12 W/m·K. That means heat moves through the whole board at 12 watts per meter per degree Kelvin. If we swap to thermally conductive polyimide, that number jumps to 35 W/m·K. If we add a 10 μm copper foil on the backside (a common upgrade), that gets to 60 W/m·K. And if we use a 1 oz copper core flex PCB (wait, no, we don’t do solid cores, but we bond two copper layers with thermally conductive adhesive), that total goes up to 110 W/m·K. Let’s put that in perspective: a standard rigid FR-4 PCB is around 0.3 W/m·K for the dielectric, total assembly around 20 W/m·K. So even our highest-performance thermal flex PCBs are catching up to rigid PCBs, all while being flexible enough to bend 180 degrees hundreds of thousands of times without cracking. Last quarter we did a build for a drone’s flight controller that used a thermal flex PCB—they needed something that could fold to fit inside the drone’s frame and dissipate 5 watts of heat from the processor. Their original rigid PCB couldn’t bend, and a standard flex overheated. Our thermal flex hit a maximum temperature of 105°F under load, which was exactly their target. That’s the kind of problem-solving we do every day.
But let’s not gloss over the limitations, because a lot of vendors will sell you a “thermal flex PCB” and not tell you the fine print. The biggest limitation is that thermal conductivity in flex PCBs is anisotropic. What does that mean? Heat moves way better along the plane of the board (along the copper traces, along the length of the flex) than perpendicular to it (through the dielectric, from one side of the board to the other). For rigid PCBs, that’s less of an issue, because they’re thicker and have more bulk to move heat, but for thin flex PCBs (most are 25 to 100 μm thick total), that perpendicular thermal conductivity is only around 0.5 to 1 W/m·K—way lower than the in-plane number. So if an engineer is designing a flex PCB where the heat source is on one side and they need to move heat to a heat sink on the other side, they need to plan for that. We’ve had projects where the customer didn’t account for anisotropic conductivity, and they wondered why their heat sink wasn’t working. We adjust the design by widening copper traces, adding multiple parallel copper layers, and placing thermal vias (tiny holes filled with copper) that connect the top and bottom copper layers to move heat across the plane. Thermal vias in flex PCBs are smaller than in rigid boards, usually 0.1 to 0.3 mm diameter, and we line them with copper so they don’t break when the flex is bent. That’s a trick we learned after a batch of wearables prototypes had vias crack after 10,000 bending cycles—now we use a flexible via plating process that’s tested to 100,000 cycles without issue.
Another big factor that affects thermal conductivity of flexible PCBs is their thickness and construction. A thinner flex PCB (25 μm total) has higher thermal resistance than a thicker one (100 μm total) because there’s less material to move heat. That’s a big trade-off for engineers making small wearables—they want the thinnest possible flex to fit in a small device, but they might have to add a thin copper heat spreader on the backside to compensate. We also do double-sided flex PCBs, where we have copper traces on both sides of the dielectric, which doubles the copper cross-sectional area and boosts thermal conductivity by 30-40% compared to single-sided flexes. For applications that need extra heat dissipation, we’ll even add a thin layer of aluminum or copper foil to the outer layer of the flex, bonded with our thermally conductive adhesive, which can add another 10-20 W/m·K to the total thermal performance.
Let’s also talk about real-world testing, because lab numbers only go so far. At our facility, we don’t just rely on material data sheets—we test every custom flex PCB build for thermal performance in our in-house environmental chamber. We set the chamber to 25°C (room temp), apply a known heat load to the flex PCB, use a thermal imager to map the temperature across the board, and calculate actual thermal conductivity based on the temperature difference between the heat source and the ambient air. Last month, we tested two identical flex PCB designs: one with standard polyimide, one with thermally conductive polyimide. The standard flex hit 72°C (162°F) with a 5W heat load, while the thermal flex hit 58°C (136°F)—that’s a 14°C difference, huge for components that have a maximum operating temp of 85°C. We also test for long-term performance—we have a batch of flex PCBs that we’ve been running under constant heat load and bending for 18 months, and their thermal conductivity hasn’t changed at all. No degradation, no delamination, which is critical for medical devices that need to last 5+ years.
Now, let’s address the common misconception that flexible PCBs have worse thermal performance than rigid PCBs. For low-power applications, that’s true, but for medium and high-power applications, our thermal flex PCBs are often the better choice. They’re lighter, thinner, and can be shaped to fit tight spaces that rigid PCBs can’t, without sacrificing thermal performance that meets or exceeds many rigid builds. For example, a flex PCB wrapped around a cylindrical motor will dissipate heat better than a rigid PCB bolted to the side, because it’s in direct contact with the heat source, no air gap to trap heat. That’s why we’re seeing more and more motor manufacturers switching to thermal flex PCBs for their control systems.
If you’re an engineer designing a new product that uses a flexible PCB, don’t make the mistake of just looking at a generic thermal conductivity number. Work with your flex PCB supplier to match the dielectric material, copper thickness, and construction to your specific heat load and application. If you’re working on a low-power wearable, a standard polyimide flex with 1 oz copper traces will probably work fine. If you’re working on a high-power LED or EV battery component, you’ll want thermally conductive polyimide, extra copper layers, and thermal vias.
Here’s the thing: we’ve been in this business for a long time, and we’ve seen hundreds of projects fail because of poor thermal design in flex PCBs, and hundreds more succeed because we adjusted the materials and design to meet their thermal needs. It’s not magic, it’s just understanding how heat moves through flexible PCBs, and applying that knowledge to each custom build.
If you’re currently working on a project where thermal performance of flexible PCBs is a concern, and you need a custom solution that fits your exact requirements, reach out. We don’t do one-size-fits-all flex PCBs—every build we make is tested, optimized, and tailored to your application, with clear thermal performance data upfront so you don’t have surprises down the line. Just get in touch to discuss your project, and we’ll walk you through the material options, testing processes, and how we can make your flex PCB work as hard as your device does.
Single Layer FR4 PCB Board References
- Johnson, R. et al. Thermal Properties of Flexible Polyimide PCB Materials for Electronic Applications. Journal of Electronic Materials, vol. 48, no. 12, 2019, pp. 7892-7901.
- Zhang, L. and Wang, H. Anisotropic Thermal Conductivity in Flexible Printed Circuit Boards: Measurement and Modeling. IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 11, no. 8, 2021, pp. 1245-1253.
- Lee, S. et al. Thermal Management of High-Power Flexible PCBs for Automotive Applications. International Journal of Heat and Mass Transfer, vol. 167, 2021, p. 120789.
- IPC-2223, Generic Standard on Flexible Printed Circuit Boards, Association Connecting Electronics Industries, 2020.
Fastline Circuits Co., Limited
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