Measuring Thermal Conductivity of Thermal Interface Materials Using MTPS
Fatigue crack growth compact tension specimen prepared per ASTM E647 with crack tip monitoringWhat Are Thermal Interface Materials?
Thermal interface materials (TIMs) are substances applied between heat-generating electronic components and heat sinks, spreaders, or cooling assemblies to minimize thermal contact resistance at the interface. Common TIM types include thermal greases, gap pads, phase-change materials (PCMs), thermally conductive adhesives, and indium foils. TIMs are critical to the thermal management of power electronics, CPUs/GPUs, LED lighting, EV battery packs, and RF power amplifiers across the electronics, EV, and telecommunications industries.
Accurate measurement of TIM thermal conductivity is essential for materials selection, design validation, and quality control.
What Is the MTPS Method?
The Modified Transient Plane Source (MTPS) method is a transient thermal conductivity measurement technique particularly suited to TIM materials. It uses a single-sided, non-contacting sensor (a flat spiral heating element embedded in a sensor card) that acts simultaneously as a heat source and temperature sensor.
The MTPS method is standardized in ASTM D7984 (Standard Test Method for Measurement of Thermal Effusivity of Fabrics Using a Modified Transient Plane Source Instrument) and in ISO 22007-2 (Transient plane heat source method for solid and liquid materials).
How MTPS Works
- The sensor is placed on the surface of the TIM specimen (or sandwiched between two specimens)
- A brief, defined electrical pulse heats the sensor spiral
- The temperature response of the sensor during and after heating is recorded with millisecond resolution
- Thermal conductivity (λ) and volumetric heat capacity (ρCp) are extracted simultaneously from the time-temperature response curve using analytical solutions to the heat diffusion equation
Key advantages of MTPS:
- Simultaneous measurement of thermal conductivity and effusivity (= √(λ·ρCp))
- No special specimen geometry required—flat surfaces are sufficient
- Short measurement time (seconds to minutes)
- Applicable to a wide range of materials (0.01–100 W/m·K)
- Non-destructive; specimen is not altered by the measurement
Thermal Conductivity of Common TIM Materials
| TIM Type | Thermal Conductivity (W/m·K) |
| Thermal grease (standard) | 1–5 |
| Phase-change material | 3–8 |
| Thermally conductive gap pad | 1–15 |
| Graphite sheet | 5–800 (in-plane) |
| Indium foil | 80 |
| Solder (Sn63/Pb37) | 50 |
| Diamond-filled compound | 10–30 |
Factors Affecting TIM Thermal Conductivity Measurement
- Bond line thickness (BLT): TIM performance in a real assembly depends not just on bulk thermal conductivity but on the thermal resistance of the compressed TIM layer (R_TIM = BLT / λ). ASTM D5470 measures total thermal resistance including contact resistance.
- Filler particle distribution: TIM thermal conductivity is sensitive to how uniformly conductive filler particles (Al₂O₃, BN, AlN, diamond) are dispersed.
- Specimen surface flatness: Poor contact between sensor and TIM due to surface roughness introduces contact resistance that lowers apparent thermal conductivity.
- Pressure: Greases and gap pads compress under load; measurement under application-representative pressure is important.
MTPS vs. ASTM D5470 for TIM Characterization
| Feature | MTPS (ASTM D7984) | ASTM D5470 |
| Measures | Bulk thermal conductivity | Thermal resistance (system) |
| Includes contact resistance | No | Yes |
| Specimen preparation | Minimal | Requires flat plates |
| Speed | Very fast (seconds) | Slower (steady-state) |
| Best for | Material screening | System-level validation |
Both methods are used together in comprehensive TIM characterization programs.
Why Choose Infinita Lab for TIM Thermal Conductivity Testing?
Infinita Lab offers comprehensive thermal interface material testing including MTPS (ASTM D7984), steady-state thermal conductivity (ASTM D5470), and thermal impedance characterization. Our nationwide accredited laboratory network supports TIM material selection, qualification, and production quality control for electronics thermal management applications.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the difference between thermal conductivity and thermal resistance for TIMs? Thermal conductivity (λ, W/m·K) is an intrinsic material property. Thermal resistance (R_TIM, K/W or K·cm²/W) is an assembly-level property that combines material conductivity, bond line thickness, and interface contact resistance. In practice, thermal resistance determines device temperature—a TIM with high conductivity but thick bond line or poor contact may perform worse than a moderate-conductivity TIM with thin bond line and good contact.
Why is graphite sheet sometimes listed with very high thermal conductivity but not used for all TIM applications? Graphite sheets (pyrolytic or compressed expanded graphite) have extremely high in-plane thermal conductivity (200–800 W/m·K) but very low through-plane conductivity (3–10 W/m·K). For most TIM applications where heat flows perpendicular to the sheet (through-plane direction), the effective thermal conductivity is the low through-plane value, not the impressive in-plane value.
How does filler particle size and loading affect TIM thermal conductivity? Higher filler loading (volume fraction of conductive particles) generally increases thermal conductivity, but viscosity also increases with loading, limiting pumpability and dispensability. Larger particles reduce viscosity at equivalent loading but may increase surface roughness and BLT. Bimodal particle size distributions (mix of large and small particles) maximize packing efficiency and thermal conductivity at acceptable viscosity.
Can MTPS measure thermal conductivity of liquid TIMs (greases) as well as solid pads? Yes. MTPS is applicable to both solid and liquid TIM materials. For liquid greases, the specimen is contained between two flat plates and the sensor is pressed against the top surface. The non-invasive, single-sided MTPS approach is particularly advantageous for liquid and paste TIMs that cannot be formed into self-supporting solid specimens.
What thermal conductivity is needed for a TIM to be effective in high-power electronics? For moderate power density applications (standard PC CPUs, LED lighting), TIMs with λ ≥ 1 W/m·K are adequate. For high-power applications (power amplifiers, EV power modules, data center processors), λ ≥ 5–10 W/m·K is typically required. For the most demanding applications (SiC modules, GaN power devices with power densities >200 W/cm²), λ > 20–50 W/m·K is necessary to maintain acceptable junction temperatures.
