Thermal gels are widely used as thermal interface materials (TIMs) in electronics, power modules, and battery systems, where the measured thermal conductivity directly determines heat dissipation performance and long-term reliability. This article provides a professional technical overview of thermal conductivity determination for thermal gels in accordance with recognized methods such as ASTM D5470 and ISO 22007-2, and explains how test results should be evaluated, interpreted, and compared against specifications. Whether you are a materials engineer, quality manager, or design engineer, understanding the factors that influence reported thermal conductivity values is essential for making sound material selection and acceptance decisions.
Technical Overview
Thermal gel is a soft, highly filled polymer material—typically a silicone or non-silicone elastomer matrix loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, or boron nitride. It is dispensed or pre-formed between heat-generating components and heat sinks, where it displaces air and substantially reduces interfacial thermal resistance. The single most important performance indicator of a thermal gel is its thermal conductivity (λ, expressed in W/(m·K)), because this value governs how efficiently heat is transported through the interface under a given thickness and contact pressure.
Unlike rigid solids, thermal gels present unique measurement challenges: they are compressible, may be tested in cured or uncured states, exhibit pressure- and temperature-dependent contact behavior, and can show filler orientation effects that produce anisotropic conductivity. As a result, reported thermal conductivity values are method- and condition-dependent, and a rigorous evaluation of test results is required before they are used for design calculations, supplier qualification, or incoming inspection. Testing is typically performed by accredited third-party laboratories or in-house laboratories operating under ISO/IEC 17025 quality systems, using internationally recognized standards.
Testing Samples
Sample Forms
Thermal gels can be submitted for testing in several forms, and the sample form must be documented in the final evaluation because it influences the measured value:
- Cured plaques or sheets with controlled thickness, prepared under the manufacturer's specified cure schedule.
- As-dispensed gel beads or lines that are compressed and cured between fixture plates to simulate real application conditions.
- Pre-formed gel pads or "putty" blanks cut to the required specimen dimensions.
- Multi-thickness specimen sets (e.g., 0.5 mm, 1.0 mm, 2.0 mm) used for separating contact resistance from bulk resistance.
Sample Preparation and Conditioning
Specimens should be free of voids, wrinkles, and contamination, with parallel surfaces and thickness uniformity within the tolerance specified by the test method. Thickness must be measured at multiple points using a calibrated gauge under defined pressure, since gel thickness is compressible and directly enters the conductivity calculation. Cure completion should be verified, because residual crosslinking or volatiles can alter thermal properties. Standard conditioning (e.g., 23 ± 2 °C and 50 ± 5 % relative humidity) prior to testing is recommended to ensure reproducibility.
Testing Items
A comprehensive thermal characterization and result evaluation for thermal gel typically includes the following items:
- Thermal conductivity, λ (W/(m·K)) — the core reported parameter.
- Thermal impedance or total thermal resistance (K·cm²/W or K·mm²/W), which captures both bulk and interfacial effects.
- Thermal contact resistance (K·cm²/W), obtained by extrapolation to zero thickness.
- Thermal diffusivity (mm²/s), typically required for transient or laser flash methods.
- Specific heat capacity, cp (J/(g·K)), required when conductivity is derived from diffusivity.
- Density (g/cm³), measured per ISO 1183 or ASTM D792 as an input to derived calculations.
- Through-plane versus in-plane conductivity, where anisotropy is suspected.
Testing Methods
Steady-State Heat Flow Method (ASTM D5470)
ASTM D5470 is the most widely referenced method for thermal interface materials. The specimen is clamped between two calibrated meter bars; a steady heat flux is established, and the temperature drop across the specimen is measured. By testing at two or more thicknesses, the total thermal resistance can be extrapolated to zero thickness to obtain contact resistance, and the slope of the resistance–thickness curve yields the true bulk thermal conductivity. The applied clamping pressure and mean test temperature must be reported, as gels show significant pressure sensitivity.
Transient Plane Source Method (ISO 22007-2)
ISO 22007-2 uses a spiral nickel sensor sandwiched between two specimen pieces. A controlled heating pulse is applied, and the temperature response is analyzed to yield thermal conductivity, diffusivity, and volumetric heat capacity simultaneously. The method is fast and requires no external contact agent, but for highly filled and anisotropic gels the isotropic analysis model can bias the result, and through-plane versus in-plane values must be evaluated carefully.
Laser Flash Method (ASTM E1461 / ISO 22007-4)
The laser flash method measures thermal diffusivity of a small disc specimen. Thermal conductivity is then calculated as λ = α · ρ · cp, combining diffusivity (α), density (ρ), and specific heat capacity (cp). Because three independently measured quantities contribute to the result, the combined uncertainty is higher than for direct steady-state methods, and the derived conductivity should be evaluated with a full uncertainty budget.
Other Reference Standards
- ASTM C518 — heat flow meter apparatus, applicable to lower-conductivity elastomeric specimens.
- ISO 22007 series — determination of thermal conductivity and diffusivity of plastics.
- GB/T 11205, GB/T 10294, and GB/T 22588 — Chinese national standards for thermal conductivity and diffusivity of rubber, insulation, and solid materials.
- JIS and EN standards for thermal transmission properties, where regional compliance is required.
Evaluation of Test Results
Information Required for Valid Evaluation
A test report cannot be meaningfully evaluated without the test conditions. The evaluation should confirm that the report clearly states the test standard, instrument type, specimen thickness, clamping pressure, mean test temperature, number of specimens, and whether the material was tested cured or uncured. Missing condition data is the most common reason for misleading comparisons between reports.
Method-Dependent Values
Values obtained by different methods are not directly interchangeable. Steady-state methods (ASTM D5470) return apparent through-plane conductivity under defined pressure and may include residual contact effects if thickness extrapolation is not performed. Transient methods (ISO 22007-2) tend to reflect bulk properties within the sensing volume and can read differently for anisotropic gels. A typical evaluation should expect deviations of 10–20 % between methods for the same material, and comparisons against datasheet limits must always reference the same method and conditions.
Contact Resistance and Apparent Conductivity
The measured total resistance follows the relationship R_total = R_contact + L/λ. For soft gels, R_contact is small relative to gap pads but not negligible, and it decreases with increasing pressure. If a laboratory reports only a single-thickness result, the value is an apparent conductivity that may understate the bulk value. Proper evaluation verifies that multi-thickness testing and zero-thickness extrapolation were used whenever the bulk conductivity is the parameter of interest.
Uncertainty, Repeatability, and Reproducibility
Typical expanded measurement uncertainty for thermal conductivity of polymeric TIMs is in the range of ±5 % to ±10 % for steady-state methods, and higher for diffusivity-derived results. Evaluation should check that: duplicate specimens agree within the laboratory's stated repeatability limit (commonly 5 %); at least three specimens were tested; and the report includes the measurement uncertainty. Round-robin experience shows inter-laboratory reproducibility can deviate further, so borderline results within the uncertainty band should trigger retesting rather than immediate rejection.
Acceptance Criteria and Data Interpretation
- Compare the reported conductivity to the specification limit using the same method, temperature, and pressure conditions stated in the specification.
- Confirm the measured value exceeds the minimum requirement even after subtracting the measurement uncertainty, or define an agreed tolerance band with the supplier.
- Investigate results that fall more than 15–20 % below datasheet values for specimen preparation errors, incomplete cure, or filler settling.
- For design use, rely on thermal impedance data at the actual application thickness and pressure rather than bulk conductivity alone.
Applications
Reliable thermal conductivity data and its correct evaluation are critical across all major thermal gel applications:
- Electric vehicle battery packs — gap filling between cylindrical, prismatic, or pouch cells and cooling plates, where conductivity values drive pack thermal design margins.
- Power electronics and IGBT/SiC modules — interface between devices and baseplates or heat sinks in inverters and converters.
- Telecom and 5G infrastructure — heat spreading from high-power RF amplifiers and base-station processors.
- Servers, CPUs, and GPUs — automated dispensing of gel as an alternative to pads and greases in high-volume assemblies.
- LED lighting and displays — thermal coupling of emitters to metal-core boards and housings.
- Automotive ECUs and charging systems — durable interfaces validated under thermal cycling and vibration.
FAQ
Why do thermal conductivity results differ between ASTM D5470 and ISO 22007-2?
The two methods measure different physical situations. ASTM D5470 is a steady-state through-plane measurement under clamping pressure that can include contact effects, while ISO 22007-2 is a transient bulk measurement. For highly filled gels with anisotropic filler orientation, the difference can be significant. Results should only be compared when the method, thickness, pressure, and temperature are matched.
Which value should be used for thermal design?
For interface design, thermal impedance measured at the actual bond-line thickness and application pressure is more relevant than bulk conductivity. Bulk conductivity is best used for material-to-material comparison and supplier qualification.
Does clamping pressure affect the measured conductivity?
Yes. Gels are compliant, so higher pressure improves surface wetting and reduces contact resistance, raising the apparent conductivity. This is why the pressure must always be reported and controlled during evaluation.
How many specimens should be tested?
A minimum of three specimens per condition is standard practice, with duplicate agreement typically required within 5 %. Multi-thickness testing is recommended whenever contact resistance must be separated from bulk conductivity.
What should be done if a result is marginally below specification?
If the reported value falls within the measurement uncertainty of the limit, the result should be treated as indeterminate. Retesting with a larger specimen set, verifying cure state and thickness measurement, and confirming identical test conditions are the appropriate next steps before making an accept or reject decision.