If you’ve ever worked with polyimide (PI) materials—whether as an engineer designing high-temperature electronics, a technician assembling aerospace components, or a quality control specialist checking material consistency—you know thermal conductivity is non-negotiable. PI is our core product as a polyimide supplier, and for years, we’ve heard from clients that they struggle to test this property accurately. Too often, tests are rushed, methods are mismatched, or results are misinterpreted, leading to poor part performance or overspending on unnecessary material tweaks. Today, I want to break down how to test the thermal conductivity of polyimide correctly, straight from the perspective of a supplier that’s tested our own materials and helped hundreds of clients get reliable data. No jargon for jargon’s sake, just practical, science-backed steps that work for PI, a material that’s uniquely tricky to test compared to metals or even other polymers. Polyimide
First, let’s ground this in why PI’s thermal conductivity matters, because it’s not just a random material property. PI is used in everything from flexible printed circuits (FPCs) that wrap around smartphone cameras to insulation for jet engine wiring and thermal barriers in electric vehicle batteries. For these applications, if heat can’t move through the PI consistently, you get hot spots—frayed wiring, failed circuits, or even battery thermal runaway risks. That’s why testing isn’t a box-ticking exercise for our team here; it’s how we ensure the PI we supply meets your needs, and how we help you verify that the PI you’re using actually performs as specified.
Now, let’s get to the testing part, starting with the most critical pre-test step: sample preparation. Most people skip this, and it’s the #1 reason test results are wrong for PI. PI can be in three main forms: film, molded parts, or coated layers, and each needs a different approach. For thin PI films (our most common product, 12.5μm to 250μm thick), you can’t just grab a random scrap from a roll. You need at least three identical samples, each 10mm x 10mm in size, cut parallel to the machine direction (MD) and cross direction (CD) of the roll—PI is anisotropic, meaning thermal conductivity is different along vs. across the direction the film was stretched during manufacturing. Skip the direction step, and your MD result could be 30% higher than your CD result, leading to confusion about which value to use for design. For molded PI parts (used in structural components), you’ll need thicker samples, 3mm to 5mm thick, with flat, parallel surfaces—no rough edges from machining, because gaps or uneven surfaces will skew test readings. For coated PI layers (like on silicon wafers), the sample has to stay on the wafer, and you’ll need a reference wafer of known thermal conductivity to subtract the substrate’s contribution.
Next, choosing the right test method. This is where most people go wrong—using a method meant for metals on PI, which is a thermal insulator, not a conductor. There are three primary methods for PI, and each is suited to a specific use case. Let’s walk through them, with notes on when to use each, and what pitfalls to avoid.
The first and most common method for thin PI films (like FPC substrates) is the Laser Flash Analysis (LFA). Here’s how it works: you place your sample in a machine that heats one side with a tiny laser pulse, then measures how fast the heat travels to the opposite side with an infrared detector. The time it takes for heat to cross the sample (called the “half-time”) is used to calculate thermal diffusivity, and then thermal conductivity is calculated using the formula: k = α * ρ * Cp, where α is diffusivity, ρ is density, and Cp is specific heat capacity. For PI, you have to be careful with the temperature here—most LFA machines run at room temp, but if you need thermal conductivity at operating temperatures (like 150°C for EV applications), you’ll need a high-temperature LFA. Also, you need to coat the sample with a thin layer of graphite or gold to absorb the laser pulse—PI is translucent, so a bare sample would let the laser pass through instead of heating the surface. We’ve seen clients skip this coating step and get diffusivity values 2x lower than they should, which makes their PI seem less thermally efficient than it is.
The second method is the guarded hot plate (GHP), which is better for bulk molded PI parts or thick films. GHP works by sandwiching the sample between two heated plates, measuring the heat flow that passes through the sample when it’s at a steady temperature. This method is more accurate for absolute thermal conductivity values, because it accounts for heat loss from the edges of the sample (the “guarded” part uses a ring around the plates to prevent that). The downside is that GHP needs samples at least 5mm thick, so it’s not ideal for thin PI films. We use GHP for testing our molded PI components because it gives us data that’s perfect for structural design in aerospace and automotive parts, where thermal stability is critical. One tip for GHP: make sure the sample is completely free of voids. PI can trap small air bubbles during molding, and those voids will act as thermal barriers, lowering your k value by 10-15%. Always inspect samples with a microscope before testing, and discard any with visible defects.
The third method is the transient plane source (TPS) method, also called the hot disk method. This is great for small samples or coated layers, like PI on semiconductors. The TPS uses a thin, nickel sensor sandwiched between two halves of your sample (or between the PI layer and a reference substrate). The sensor heats up slightly, and measures its own resistance change, which lets you calculate thermal conductivity in real time. The biggest advantage here is that you don’t need to calculate diffusivity and Cp separately—though you do need to input the correct Cp value for your PI grade, which is why using our material specs here is important. If you use the wrong Cp, your k value will be off by up to 8%.
Now, a common question we get: what results should I expect for polyimide? As a supplier, we make several grades of PI, and their thermal conductivity ranges from 0.12 W/mK for standard unfilled PI to 0.8 W/mK for thermally filled PI (filled with boron nitride or aluminum oxide particles). Standard PI is an electrical insulator, so its thermal conductivity is low—think of it as a thermal barrier, which is what makes it great for high-temperature insulation. But if you need PI that moves heat, like in LED heat sinks, you’ll want our thermally filled grades, which use our proprietary particle dispersion process to avoid clumping, so the heat flows evenly without sacrificing PI’s chemical resistance. If your test comes back lower than our spec, don’t panic—first check your sample preparation: did you test MD vs CD? Did you coat LFA samples? Did you account for voids? We’ve had clients send us test results that were 20% lower than our spec, turn out to have used a sample with a scratch that created a tiny air gap, skewing the GHP reading.
Another key point: testing at operating temperature, not just room temperature. Most material data sheets list thermal conductivity at 25°C, but PI’s k value changes with temperature. For example, standard unfilled PI’s thermal conductivity stays roughly constant up to 200°C, then decreases slightly as the polymer chains soften. Filled PI might increase slightly as the filler particles conduct more heat at higher temps, because the polymer matrix becomes more flexible and the particles are able to connect more. If your application is at 125°C inside an EV battery, testing at room temp won’t give you usable data—you’ll need a method that can run at your operating temp, which is why our in-house testing lab has LFA machines that go up to 500°C, so we can provide temperature-corrected data for every grade we supply.
Let’s talk about common mistakes we see all the time, to save you the headache. First, using the wrong sample size for thin films: if your PI film is 25μm thick, you can’t use GHP, because the heat will conduct through the plates’ interfaces instead of the sample. Stick to LFA or TPS for thin films. Second, mixing up anisotropy: as I mentioned earlier, PI’s thermal conductivity is directional. If you’re designing a flexible FPC that will bend along the MD, you need the MD k value, not the average of MD and CD. If you test both, you’ll get a range, so you need to know what direction is relevant for your part. Third, not calibrating your equipment. Every thermal test machine needs calibration with a reference material of known k value—like a quartz standard or a copper sample. We calibrate our in-house machines monthly, and we always advise clients to do the same, especially if they’re getting data used for design or compliance.
If you’re testing PI for a new project, here’s a quick step-by-step guide to follow, tailored to our experience:
- Define your application: is it thin film, bulk molded, or coated on a substrate? What’s your operating temperature range?
- Source three identical samples, cut to the correct size and orientation (MD/CD for films) for your chosen test method.
- Inspect samples for voids, scratches, or defects—discard any that are not perfect.
- Choose a calibrated test method: LFA for thin films at temps up to 500°C, GHP for bulk parts, TPS for small/coated samples.
- Input the correct Cp value for your specific PI grade (we provide these free to all our clients, just reach out).
- Run three tests per sample, average the results, and compare to our grade specs—don’t rely on a single test result, because small variations can happen at the micro level.
As a polyimide supplier, our goal isn’t just to send you PI rolls or molded parts—it’s to make sure you have the data to use them correctly. If you’ve tested PI and gotten results that don’t align with what you expected, or if you need help choosing the right test method, or if you just want to confirm that our PI grades will work for your application, our technical team is here to walk through it with you. We’ve spent 15 years refining our testing processes, and we’ve helped thousands of engineers avoid costly mistakes from incorrect thermal conductivity data. Whether you’re prototyping a new flexible circuit, designing aerospace wiring insulation, or building an EV battery thermal management system, thermal conductivity is make-or-break, and getting the test right is the first step to making your project a success.
Don’t settle for generic test data that doesn’t account for PI’s unique properties. Reach out to us to discuss your testing needs, or to get a custom sample of our PI grade with temperature-specific thermal conductivity data tailored to your operating conditions. We’re here to partner with you, not just supply materials.
OLED Material References
- Turi, E. A. (Ed.). (1996). Polyimides: Fundamentals and Applications. CRC Press.
- Zhang, Y., et al. (2018). Thermal conductivity of polyimide films: A review. Polymer Testing, 69, 371-382.
- ASTM E1461-22, Standard Test Method for Thermal Diffusivity by the Flash Method. ASTM International.
- ASTM D5470-17, Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials. ASTM International.
Hubei Jiutian Bio-medical Technology Co., Ltd.
Hubei Jiutian Bio-medical Technology Co., Ltd. is one of the most professional polyimide manufacturers and suppliers in China, also supports customized service with low price. Please feel free to buy bulk cheap polyimide made in China here from our factory. For free sample, contact us now.
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