When evaluating thermal interface materials (TIMs), most people focus on one number: thermal conductivity (W/m·K). But there's another, equally important metric that reflects real-world performance at the interface: thermal impedance.
If you're looking for accuracy and predictability in your thermal design, especially with uneven surfaces, gaps, or low-pressure applications, thermal impedance is the number to trust.
Let’s break down what it means, how it’s measured under ASTM D5470, and how you can use it to select the best material for your application.
🧪 What Is Thermal Impedance?
Thermal impedance is the total thermal resistance of a thermal interface material in its applied, compressed state. It includes not only the intrinsic material resistance (based on its thickness and conductivity), but also the contact resistance between the TIM and the surfaces it touches — like a heatsink and a processor.
It’s measured in:
°C·cm²/W or K·cm²/W
This unit describes how much temperature rises per unit of power (W) across the surface area of the interface.
🔎 Thermal Impedance vs. Thermal Conductivity
Let’s clarify the difference:
| Property | Thermal Conductivity | Thermal Impedance |
|---|---|---|
| Unit | W/m·K | °C·cm²/W or K·cm²/W |
| What it measures | Material’s ability to conduct heat | Total resistance in actual interface setup |
| Depends on | Bulk material only | Thickness + contact resistance + pressure |
| More realistic for TIMs? | ❌ Theoretical | ✅ Practical and application-driven |
Bottom line:
A TIM can have excellent thermal conductivity, but if it’s too thick, not soft enough, or poorly installed, the thermal impedance will still be high. That means more heat stays trapped — not good.
🧪 How Is Thermal Impedance Measured? (ASTM D5470)
Thermal impedance is measured using the same ASTM D5470 test rig used for thermal conductivity — but with a shift in focus:
- A TIM sample is sandwiched between two metal blocks (typically copper).
- One side is heated, the other cooled — creating a thermal gradient.
- The temperature drop across the material is measured under a specific pressure and contact area.
- Instead of isolating just material properties, total resistance is calculated, including interface losses.
The result is thermal impedance — a much better predictor of real system performance than conductivity alone.
📉 What Affects Thermal Impedance?
Several factors influence the actual thermal impedance value of a TIM:
- Material conductivity (W/m·K)
- Thickness (thinner = better)
- Surface roughness (flatter is better)
- Compression force (soft TIMs conform better at low pressure)
- Contact quality (air gaps dramatically raise impedance)
🔬 Typical Thermal Impedance Ranges
| TIM Type | Thermal Impedance (°C·cm²/W) | Notes |
|---|---|---|
| High-end Carbon Pads | 0.05 – 0.15 | Thin, highly conductive, conformable |
| Silicone Thermal Pads | 0.15 – 0.45 | Balance of performance and softness |
| Silicone-Free Pads | 0.2 – 0.5 | For clean/no-silicone environments |
| Thermal Gels (1K) | 0.05 – 0.25 | Very low pressure, highly conformable |
| 2K Thermal Putty (Curing) | 0.1 – 0.3 | Holds shape after cure, stable over time |
| Thermal Adhesive Glue | 0.3 – 0.6 | Dual role: bonding and heat transfer |
📏 Our Materials – Tested Under ASTM D5470 for Thermal Impedance
Every TIM in the Sheen Thermal portfolio is tested under ASTM D5470, ensuring the thermal impedance values reflect real-use conditions, including compression and contact quality.
We publish these values so you can:
- Simulate accurate heat paths in your thermal design
- Compare TIMs fairly and confidently
- Avoid relying on overly simplified conductivity numbers
🧠 Why You Should Care About Thermal Impedance
If your application involves:
- Low mounting pressure
- Surface unevenness or warping
- Thin interfaces or low airflow
- High power density (e.g., CPUs, power modules, AI chips)
…then thermal impedance matters more than thermal conductivity. It’s the true performance number that predicts whether your TIM will actually keep components cool in the final product.
🛠 Need Help Matching TIM Performance to Your Project?
Choosing the right material isn’t just about numbers — it’s about the interaction between your surfaces, pressure, and design constraints.
Our team can help you:
- Choose materials with the right impedance for your heat load
- Balance thickness, compressibility, and conductivity
- Recommend pads, gels, or adhesives based on mechanical fit