Thermal conductive pads are the dominant thermal interface materials installed between battery cells, modules, and liquid cooling plates in new energy vehicle battery packs, and their effective heat transfer performance depends heavily on the compression applied during assembly. Compression-dependent thermal conductivity testing evaluates bulk thermal conductivity, thermal impedance, deflection, and contact resistance under realistic clamping pressures in accordance with standards such as ASTM D5470 and ISO 22007-2. This article explains the test principles, sample requirements, testing items, methods, and typical applications of compression thermal conductivity testing for EV battery thermal pads.
Technical Overview
Thermal conductive pads, also known as thermal gap pads or thermal interface materials (TIMs), are soft, elastomer-based sheets filled with thermally conductive ceramic or graphite particles. In new energy vehicle (NEV) battery packs, they are placed between cylindrical, prismatic, or pouch cells and the cooling plate, and between cells within a module, to fill air gaps, reduce contact thermal resistance, and equalize temperature across the pack. Because the pad only performs well when it conforms to surface irregularities under clamping load, its thermal behavior must be characterized as a function of applied pressure, not merely at free thickness.
Compression thermal conductivity testing simulates real assembly conditions by measuring the thermal transmission properties of the pad while a controlled compressive load is applied. The steady-state method described in ASTM D5470 is the industry benchmark because it simultaneously captures thermal impedance, deflection, and contact resistance at defined pressures. Supplementary methods, including the transient plane source technique (ISO 22007-2), the laser flash method (ASTM E1461 / ISO 22007-4), and rubber compression tests (ASTM D575, ISO 7743, ASTM D395, ISO 815), provide a complete dataset covering conductivity, deflection behavior, and long-term mechanical stability.
Manufacturers of battery packs, module integrators, and material suppliers rely on accredited third-party laboratories to generate comparable, traceable data. Results are used for material screening, design-in calculations of junction-to-coolant thermal resistance, incoming inspection, and validation of supplier datasheets before mass production release.
Testing Samples
Silicone-Based Thermal Gap Pads
The most common sample type, consisting of silicone rubber filled with aluminum oxide, zinc oxide, aluminum nitride, or boron nitride particles. Typical thicknesses range from 0.5 mm to 5.0 mm, with nominal thermal conductivity ratings from 1.0 W/m·K to 8.0 W/m·K and hardness from Shore 00 30 to Shore A 50. These pads are tested for thermal impedance versus pressure as well as bulk conductivity.
Non-Silicone Thermal Pads
Acrylic, polyurethane, or polyolefin-based pads used where silicone outgassing or silicone migration must be avoided, for example near optical sensors, relays, or connectors inside the battery pack. These materials are often highly compressible and benefit from deflection-versus-pressure characterization alongside thermal testing.
High-Deflection Gap Filler Pads
Very soft, putty-like or gel-like pads designed to absorb tolerance stack-up of several millimeters between cells and cold plates. Because their effective thickness changes dramatically under load, testing at multiple clamping pressures is essential to establish the thermal resistance curve used in pack-level thermal modeling.
Graphite and Ceramic-Filled Composite Pads
Pads incorporating oriented graphite films or high-loading ceramic fillers for anisotropic or high-conductivity applications. These samples may require directional testing, since in-plane and through-thickness conductivity can differ by an order of magnitude.
Samples are typically supplied as flat sheets or die-cut discs. For steady-state testing, specimens at least 25 mm × 25 mm or 50 mm diameter are preferred, with surfaces kept clean and free of release liners. For laser flash testing, disc specimens of approximately 12.7 mm diameter and 1–3 mm thickness are prepared. Specimens are conditioned at standard laboratory atmosphere (23 °C ± 2 °C, 50% ± 10% relative humidity) for at least 24 hours before testing.
Testing Items
The scope of a compression thermal conductivity test program for battery thermal pads typically includes the following parameters:
- Through-thickness thermal conductivity (W/m·K) at ambient and elevated temperatures
- Thermal impedance and total thermal resistance (K·cm²/W or m²·K/W) as a function of clamping pressure
- Apparent thermal conductivity under defined compression, calculated at measured compressed thickness
- Contact thermal resistance at the pad-to-substrate interfaces
- Deflection (compression) versus pressure curve, typically from 0.03 MPa to 1.0 MPa
- Compressed thickness at each load step, used for thermal resistance calculation
- Thermal diffusivity (mm²/s) and specific heat capacity (J/g·K) for laser flash derivation
- Hardness (Shore 00 or Shore A) per ASTM D2240 / ISO 868 / ISO 7619-1
- Density per ASTM D792 / ISO 1183
- Compression set per ASTM D395 Method B / ISO 815 / GB/T 7759, evaluating permanent deformation after 22 hours at 70 °C or 125 °C
Testing Methods
Steady-State Heat Flow Method (ASTM D5470)
The reference method for thermal interface materials. The specimen is placed between two polished metal metering bars containing calibrated heat flux sensors and embedded thermocouples. A known heat flow is established, and a compression load is applied through the metering stack, typically at pressure steps such as 0.07, 0.14, 0.35, 0.69, and 1.38 MPa (10, 20, 50, 100, and 200 psi). At each step, the apparatus records thermal impedance and the compressed thickness. By testing multiple thicknesses of the same material and plotting thermal resistance versus thickness, the slope yields the inverse of intrinsic thermal conductivity while the intercept yields total contact resistance. This method directly reproduces the installed condition inside a battery pack and is the preferred basis for supplier datasheets.
Guarded Heat Flow Meter Method (ASTM E1530)
A steady-state comparative method suitable for moderately conductive polymer sheets. The specimen is sandwiched between heated and cooled plates with a transducer referencing a known standard. It provides bulk conductivity at modest loads and is often used for screening incoming material lots.
Transient Plane Source Method (ISO 22007-2)
A spiral nickel sensor is clamped between two pieces of the pad material and excited with a controlled electrical pulse. The sensor simultaneously acts as heat source and temperature recorder, yielding thermal conductivity, diffusivity, and specific heat capacity within seconds. Because the sample is clamped during measurement, moderate pressure can be applied, and the method is well suited to soft, highly compressible gap pads and to rapid quality control testing.
Laser Flash Method (ASTM E1461 / ISO 22007-4 / GB/T 22588)
A short laser pulse heats the front face of a disc specimen, and an infrared detector records the temperature rise on the rear face to determine thermal diffusivity. Combined with density (ASTM D792) and specific heat capacity measured by differential scanning calorimetry (ISO 11357-4), thermal conductivity is calculated. This method offers high precision and a wide temperature range from -40 °C to 200 °C, and is commonly used to characterize anisotropic graphite composite pads in different orientations.
Compression Deflection Testing (ASTM D575 / ISO 7743)
The pad is compressed between parallel plates at a standard rate, and force versus deflection is recorded. The resulting compression curve defines the load required to achieve a target deflection percentage, which battery designers use to balance thermal contact against mechanical stress on cells and busbars.
Compression Set and Aging Evaluation (ASTM D395 / ISO 815)
Specimens are compressed to a fixed deflection and exposed to elevated temperature for a defined period. The permanent thickness loss after recovery is reported as compression set. This test predicts whether the pad will maintain contact pressure over the vehicle lifetime, a critical reliability concern because loss of interfacial contact directly increases thermal resistance.
Applications
Electric Vehicle Battery Packs
Pack and module designers use pressure-dependent thermal resistance data to select pad thickness and hardness for cell-to-cold-plate and cell-to-cell interfaces. Accurate compression testing supports thermal runaway propagation mitigation, fast-charging capability, and temperature uniformity targets of 5 °C or less across the pack.
Energy Storage Systems
Container-scale battery storage systems apply the same materials between rack cells and liquid or air-cooled plates. Compression test data at operating temperature informs derating calculations and long-term warranty models for grid storage projects.
Power Electronics and Charging Hardware
Onboard chargers, DC fast-charging modules, inverters, and battery management system power boards use thermal pads between power semiconductors and heat sinks. The same compression-dependent test protocols apply, particularly for high-conductivity pads rated above 4 W/m·K.
Material Development and Supplier Qualification
Material manufacturers use the full test matrix to characterize new formulations, verify lot consistency, and support customer design-in, while battery OEMs use independent laboratory data to validate datasheet claims during supplier audits and incoming inspection.
FAQ
Why must thermal pads be tested under compression instead of at free thickness?
At free thickness, a pad retains air gaps at both interfaces, giving inflated thermal resistance. Under the clamping pressures typical of battery assembly, typically 0.1 MPa to 0.7 MPa, the pad deflects, wets out the surfaces, and reduces contact resistance substantially. Testing under compression reproduces the installed state, so measured impedance values can be used directly in pack thermal models.
What is the difference between thermal conductivity and thermal impedance?
Thermal conductivity is an intrinsic material property in W/m·K describing heat flow through the bulk material. Thermal impedance is a system-level property that includes bulk resistance plus the two contact resistances at the interfaces, expressed in K·cm²/W. Battery design depends on impedance under real load, while conductivity enables material-to-material comparison.
Which standard is most appropriate for battery thermal pads?
ASTM D5470 is the industry reference for thermal interface materials because it measures impedance, deflection, and contact resistance simultaneously under controlled pressure. ISO 22007-2 and laser flash methods (ASTM E1461, ISO 22007-4) are excellent for bulk conductivity verification, and rubber standards such as ISO 7743 and ISO 815 complete the mechanical characterization.
What pressures and temperatures are used in a typical test?
Standard pressure steps for ASTM D5470 testing are 0.07 to 1.38 MPa (10 to 200 psi), with battery-relevant evaluations concentrated between 0.1 and 0.7 MPa. Tests are commonly performed at 25 °C, with extended programs at 50 °C, 85 °C, or 100 °C to reflect operating conditions inside the pack.
How are bulk conductivity and contact resistance separated?
Thermal resistance is measured at two or more specimen thicknesses under identical pressure. The slope of resistance versus thickness gives the intrinsic resistance of the material, from which conductivity is calculated, and the intercept gives the total contact resistance of the two interfaces.
What sample size and quantity are required?
For steady-state testing, specimens of 25 mm × 25 mm or 50 mm diameter at three thicknesses (for example 1.0, 2.0, and 3.0 mm) are recommended. For laser flash testing, 12.7 mm diameter discs are standard. A minimum of three replicate specimens per condition is advised for statistical validity.
How long does a compression thermal conductivity test program take?
A single-pressure impedance curve can be completed within one working day once thermal equilibrium is reached at each step. A full program including multiple thicknesses, multiple pressures, elevated temperature points, and compression set aging typically requires five to ten working days at an accredited third-party laboratory.