What Is Thermal Resistance? Thermal Resistance vs Thermal Conductivity Explained

In thermal design, two terms are constantly confused: thermal resistance and thermal conductivity. Understanding the difference is essential for selecting thermal grease, heat sinks and interface materials correctly. This article explains what thermal resistance really means, how it relates to thermal conductivity, and why engineers must consider both.

What Is Thermal Resistance?

Thermal resistance (Rth, °C/W or °C·cm²/W) describes how strongly an object or interface opposes the flow of heat. It is the temperature rise produced per watt of heat passing through: a CPU dissipating 100 W across a thermal resistance of 0.3 °C/W will run 30°C above the heat sink base. Lower thermal resistance means cooler components.

Thermal Resistance vs Thermal Conductivity

Thermal conductivity (W/m·K) is an intrinsic material property - it tells you how well a bulk material conducts heat. Thermal resistance depends not only on the material but also on geometry: thickness and area. A very conductive material can still have high resistance if the layer is thick.

Why Interface Thermal Resistance Matters Most

In an assembled product, the largest temperature drop often occurs not inside the heat sink but at the interface between the component and the heat sink. This "contact resistance" is created by microscopic air gaps. Applying thermal grease replaces the air with a conductive compound and collapses the interface resistance. The improvement is why a few milligrams of thermal paste can lower CPU temperature by 10-20°C.

Which Is More Important: Thermal Resistance or Thermal Conductivity?

For TIM selection, the honest answer is: thermal impedance (resistance per area) is more important than bulk conductivity. A pad with 6.0 W/m·K but 3 mm thick can perform worse than a grease layer of 1.0 W/m·K and 0.05 mm thickness, because the grease layer is a hundred times thinner. Always compare the effective thermal impedance at the real application thickness.

How to Measure and Compare

1. Use the ASTM D5470 test method for TIMs;
2. Compare impedance values at the same pressure (typically 50-100 psi);
3. For grease, use the expected bond line thickness in your assembly;
4. For pads, use the compressed thickness at your clamping force;
5. Verify with an in-product temperature test.

Practical Selection Rules

• High heat flux and tight stack-up: thin grease layer wins;
• Large, uneven gaps: pad with adequate thickness and softness;
• Vibration and long life: non-curing grease or phase-change material;
• Automated high-volume lines: screen-printed grease is cost-effective;
• Field serviceability: pads or reworkable non-curing grease.

Thermal Resistance in a Complete System

The total thermal path from a chip junction to the surrounding air is a series of resistances: junction-to-case, case-to-heat-sink (the TIM interface) and heat-sink-to-air. Designers sum these values to predict the junction temperature. If the TIM resistance is a large share of the total, improving the TIM gives the biggest temperature drop; if the heat sink dominates, changing the grease will barely help. This is why system-level thinking matters more than picking the "best" paste on paper.

FAQ

What is a good thermal resistance for thermal grease?

High-performance greases achieve interface impedance below 0.1 °C·cm²/W at a 25-50 µm bond line. Compare values measured by the same standard and pressure.

Why do data sheets list both conductivity and impedance?

Conductivity describes the material; impedance describes the installed layer. Suppliers list both so engineers can compare products at realistic thickness.

Conclusion

Thermal resistance and thermal conductivity answer different questions. Conductivity tells you the quality of the material; resistance tells you the real temperature impact in your assembly. When comparing thermal grease, pads or heat sinks, always convert everything to thermal resistance at your actual thickness - that number decides the temperature. Contact Hanast for thermal grease samples with complete thermal impedance data sheets.