What Is Thermal Conductivity? Understanding ASTM D5470 in Thermal Interface Materials

Whether you're designing advanced electronics, power modules, EV battery packs, or embedded AI systems, thermal interface materials (TIMs) play a critical role in keeping your components cool, efficient, and reliable.

One of the most quoted performance metrics in any TIM datasheet is thermal conductivity, usually given in W/m·K (watts per meter per kelvin). But have you ever wondered how that number is determined — and how meaningful it is to your real application?

This is where ASTM D5470 comes in — the gold-standard test method used to measure thermal conductivity and thermal resistance of solid materials.

🧪 What Is ASTM D5470?

ASTM D5470 is the Standard Test Method for Thermal Transmission Properties of Thermally Conductive Materials. It is widely used in the thermal interface materials industry to:

• Measure thermal conductivity (W/m·K)
• Determine thermal resistance (°C·cm²/W or K·cm²/W)
• Assess material performance under pressure and real operating temperatures

It is especially relevant for thermal pads, films, pastes, gels, adhesives, and gap fillers, and ensures consistent comparison between materials.

🛠 How the ASTM D5470 Test Works

The method is simple in principle, but highly precise in execution:

1. A sample of the TIM is placed between two metal plates (typically copper).
2. One plate is heated, the other is cooled, creating a temperature gradient across the sample.
3. Sensors measure the temperature drop across the sample as heat flows through it.
4. The thickness of the sample, the area, and the heat flow are all factored into the equation to calculate:

Thermal Conductivity (k) = Q × L / (A × ΔT)

Where:

• Q = heat flow (watts)
• L = thickness of the material (meters)
• A = cross-sectional area (m²)
• ΔT = temperature difference across the material (K)

The result is reported in W/m·K — the higher the number, the better the material is at conducting heat.

📏 Thermal Resistance vs. Thermal Conductivity

ASTM D5470 can also report thermal resistance (R), which is the inverse of thermal conductivity, adjusted for thickness:

Thermal Resistance (R) = Thickness / Thermal Conductivity

Low thermal resistance means heat moves quickly through the material — exactly what you want in high-performance electronics.

⚙️ Real-World Relevance: Why It Matters

In real-world applications, you don’t just want high thermal conductivity in theory — you need verified, consistent performance under pressure. ASTM D5470 accounts for factors like:

• Contact pressure: Soft TIMs can perform better under slight compression.
• Interface resistance: The test mimics real metal-to-material contact conditions.
• Temperature dependence: Materials behave differently under heat. This test reflects that.

That makes D5470 results much more reliable than theoretical or calculated values alone.

🧊 What’s a "Good" Thermal Conductivity?

Here’s how to think about thermal conductivity ranges when choosing a TIM:

🧩 Our Materials and ASTM D5470 Performance

At Sheen Thermal, all thermal conductivity values listed for our products are measured under ASTM D5470 — so you know they’re real, tested, and application-ready.

Here’s how our offerings align:

All tested under ASTM D5470 with appropriate surface pressures, simulating real-world conditions — not theoretical values.

🧠 Key Takeaways

• ASTM D5470 is the industry-standard test for measuring real thermal conductivity in TIMs
• It reflects real contact performance, under pressure and heat
• Higher thermal conductivity = faster heat transfer
• Choose materials with verified ASTM D5470 values for accurate comparisons
• Match the performance to your application — from consumer devices to industrial-grade modules

🔚 Choosing the Right TIM with Confidence

When you see thermal conductivity listed on our datasheets, you can be confident it’s backed by rigorous ASTM D5470 testing, not estimates or marketing claims.