Performance Testing of Thermal Conductive Pads for Power Batteries: Standards, Methods, and Key Para

Performance testing of thermal conductive pads for power batteries evaluates the thermal conductivity, mechanical compliance, electrical insulation, and long-term reliability of gap filler materials installed between battery cells, modules, and cooling plates. As electric vehicle power density and fast-charging requirements continue to rise, qualified thermal interface materials have become critical for controlling cell temperature gradients and mitigating thermal runaway risk. This article outlines the main sample types, test items, international standards such as ASTM D5470, ISO 22007, IEC 60243, UL 94 and GB series standards, and testing methods commonly performed by accredited third-party laboratories.

Technical Overview

Thermal conductive pads, also known as thermal gap pads or thermal interface materials (TIMs), are soft, highly compliant polymer sheets filled with thermally conductive ceramic particles such as alumina, aluminum nitride, boron nitride, or graphite. In power battery systems, these pads are installed between cylindrical, prismatic, or pouch cells and liquid cooling plates, or between cells and module housings, where they fill manufacturing tolerances, conform to surface irregularities, transfer heat away from the cells, dampen vibration, and in most designs provide electrical insulation between live cell surfaces and grounded cooling structures.

Because a power battery pack operates across a wide temperature range and is subject to continuous mechanical stress, vibration, and aging, the performance of the thermal pad directly affects cell temperature uniformity, charging speed, cycle life, and overall system safety. A pad that hardens, bleeds silicone oil, loses contact pressure, or releases volatile siloxanes over time can dramatically increase thermal resistance and create reliability risks in adjacent electronics. Performance testing therefore focuses not only on the as-received thermal conductivity, but on the full picture of mechanical, electrical, fire-safety, and durability behavior under realistic operating conditions.

Testing is typically conducted in accredited third-party laboratories operating under ISO/IEC 17025 quality systems, using internationally recognized methods from ASTM, ISO, IEC, UL, EN, JIS, and GB standard organizations. The resulting data supports material qualification, incoming inspection, supplier comparison, and design verification within the broader battery safety framework defined by standards such as GB 38031 and IEC 62660.

Testing Samples

Silicone-Based Thermal Gap Pads

Silicone elastomer pads are the most widely used category in power battery applications. They are supplied as cured sheets with typical thicknesses of 0.5 mm to 10 mm and nominal thermal conductivity ratings from 1 W/m·K up to 10 W/m·K. Silicone pads offer excellent compression compliance, wide service temperature ranges of approximately -40 °C to +200 °C, and stable performance over long service periods, but require evaluation of oil bleeding and volatile siloxane release.

Silicone-Free Thermal Pads

Silicone-free pads based on polyurethane, acrylic, or other polymer matrices are selected where siloxane contamination of electrical contacts, optical components, or relays must be strictly avoided. These materials are tested with the same thermal and mechanical regime as silicone pads, with additional attention to long-term heat aging stability, since their polymer backbones often have narrower temperature margins.

Reinforced and Die-Cut Pad Products

Many battery designs use pads laminated with a fiberglass carrier, polyimide film, or single-sided adhesive layer to improve handling, dielectric strength, or dimensional stability. These are tested in their final laminated construction, because the carrier film changes the effective thermal impedance and mechanical compression behavior of the finished part.

Thermal Gels and Putty-Type Gap Fillers

Dispensable thermal gels and putty-like gap fillers that cure in place are increasingly used in cell-to-pack architectures. Although chemically similar to pads, they require dedicated test fixtures that reproduce the compressed in-situ thickness, and their cured properties are evaluated using the same parameter set as sheet materials.

Unless otherwise specified, all samples are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours before testing, in accordance with ISO 291 or ASTM D618, to ensure reproducible and comparable results.

Testing Items

Comprehensive performance evaluation of power battery thermal pads covers the following parameter groups:

• Thermal performance: thermal conductivity, thermal resistance and thermal impedance, thermal diffusivity, specific heat capacity, and pressure-dependent contact resistance.
• Physical and mechanical properties: hardness (Shore 00 / Shore A), density, tensile strength, elongation at break, tear strength, compression force-deflection curve, and compression set.
• Electrical properties: dielectric breakdown strength, volume resistivity, and surface resistivity.
• Reliability and durability: heat aging, damp heat aging, thermal cycling, low-temperature flexibility, compression set under heat, oil bleed resistance, and outgassing behavior.
• Fire safety: flammability rating and glow-wire ignition performance.
• Chemical compliance: RoHS and REACH restricted substances, halogen content, and volatile organic compound emissions.

Testing Methods

Thermal Conductivity Testing

The reference method for thermal interface materials is ASTM D5470, the guarded heat flow meter method. A pad specimen is compressed between two metering bars at controlled pressure, typically 0.1 MPa to 0.7 MPa, and the total thermal impedance is measured as a function of thickness. Measuring multiple thicknesses allows separation of bulk thermal resistance from contact resistance and yields the intrinsic through-plane thermal conductivity. This pressure-dependent measurement reproduces real installation conditions and is the preferred qualification method for battery gap pads.

For rapid material screening, ISO 22007-2 transient plane source (hot disk) testing provides isotropic thermal conductivity values. Alternatively, ASTM E1461 laser flash analysis measures thermal diffusivity, which is combined with specific heat capacity from ISO 11357 DSC and density from ISO 1183 or ASTM D792 to calculate thermal conductivity. These methods are useful for incoming inspection and formulation comparison, while ASTM D5470 remains the definitive interface performance test.

Mechanical and Physical Testing

Hardness is measured with a durometer according to ASTM D2240 or ISO 868, typically on the Shore 00 scale for very soft gap fillers and Shore A scale for firmer pads. Tensile strength and elongation at break are determined on dumbbell specimens using ASTM D412 or ISO 37. Compression set is measured per ASTM D395 Method B or ISO 815-1 after extended deflection at elevated temperature, and quantifies the permanent loss of pad thickness that would reduce contact pressure in service. Compression force-deflection characterization, performed on a universal testing machine, records the stress-strain response at 10 % to 50 % deflection, which is essential input for battery pack structural simulation. Density is verified according to ASTM D792 or ISO 1183 as a quick indicator of filler loading consistency between production batches.

Electrical Insulation Testing

Dielectric breakdown strength is evaluated according to IEC 60243-1 or ASTM D149 using a rising-voltage ramp between parallel electrodes, and is reported in kV/mm or as breakdown voltage at the specified pad thickness. Volume and surface resistivity are measured per IEC 62631-3-1 or ASTM D257 using a guarded electrode configuration. These tests confirm that the pad safely isolates cells at battery pack working voltages, which now commonly reach 400 V to 800 V and above in modern electric vehicles.

Reliability and Environmental Testing

Long-term heat aging, for example 1000 hours or longer at 150 °C, tracks the retention of hardness, thermal conductivity, and dielectric strength after simulated service exposure. Damp heat testing at 85 °C and 85 % relative humidity per IEC 60068-2-78 evaluates hydrolytic stability. Thermal cycling between -40 °C and +125 °C or +150 °C per IEC 60068-2-14 or GB/T 2423.22, often for 500 to 1000 cycles, reveals delamination, cracking, or hardening caused by repeated thermal expansion mismatch. Outgassing is assessed using ASTM E595 total mass loss and collected volatile condensable materials criteria, and volatile cyclic siloxanes are quantified by gas chromatography, which is particularly important for silicone pads installed near electrical contacts, sensors, or connectors.

Flame Retardancy Testing

Flammability is verified according to UL 94, with most power battery specifications requiring a V-0 rating at the installed pad thickness, supported by equivalent methods such as IEC 60695-11-10 or GB/T 2408. Glow-wire flammability index and glow-wire ignition temperature are determined per IEC 60695-2 series methods. These fire-safety characteristics align the pad material with the overall thermal propagation prevention strategy required by battery-level safety standards such as GB 38031.

Applications

Qualified thermal conductive pads are used throughout electrified transport and stationary energy systems:

• Electric, hybrid, and plug-in hybrid vehicle battery modules, where pads bridge the gap between cells and liquid cooling plates and between adjacent cells to equalize temperature.
• Cell-to-pack and cell-to-chassis battery architectures, where large-area pads and gels accommodate stack-up tolerances of several millimeters.
• Stationary energy storage systems and battery containers, where long-term aging resistance and flame retardancy are decisive.
• Power electronics subsystems, including IGBT and SiC power modules, inverters, on-board chargers, DC-DC converters, and motor controllers.
• EV charging infrastructure, 5G base station power units, rail transit equipment, and industrial power supplies, where materials may additionally be assessed under EN 45545-2 fire safety requirements for railway applications.

FAQ

What thermal conductivity value is sufficient for a power battery thermal pad?

Most commercial power battery gap pads fall between 1 W/m·K and 6 W/m·K, with high-end formulations reaching 8 to 10 W/m·K. However, effective heat transfer depends more on thermal impedance, meaning the combination of conductivity, pad thickness, and contact resistance under real clamping pressure, than on the nominal bulk conductivity figure alone. ASTM D5470 testing at the design pressure and thickness is the most reliable basis for material selection.

Why is ASTM D5470 preferred over laser flash testing for gap pads?

Laser flash and hot disk methods measure the intrinsic bulk conductivity of the material but exclude the two contact interfaces that dominate total thermal resistance in soft, thick pad installations. ASTM D5470 measures the complete joint under controlled compression, reproducing actual pack assembly conditions, which is why battery OEM specifications typically reference it for qualification.

What hardness and compression range are typical for battery applications?

Soft gap fillers usually measure between Shore 00 40 and Shore 00 80, while reinforced pads commonly fall between Shore A 10 and Shore A 40. Design compression is generally 20 % to 50 % of the free thickness, providing sufficient contact pressure for low interfacial resistance without imposing excessive stress on prismatic cells or pouch cell stacks.

Why is outgassing testing important for silicone thermal pads?

Silicone polymers can release low-molecular-weight cyclic siloxanes and silicone oil during operation at elevated temperature. These volatiles may deposit on electrical contacts, connector surfaces, or sensor elements, causing contact failure or contamination. Outgassing and oil bleed testing verifies that the pad formulation is sufficiently post-cured and low-volatility for its intended installation environment.

How is long-term reliability demonstrated?

Reliability is demonstrated through a combined aging program: extended heat aging at 150 °C, damp heat exposure at 85 °C and 85 % relative humidity, and thermal cycling from -40 °C to +125 °C or higher, each followed by re-measurement of thermal conductivity, hardness, dielectric strength, and compression set. Property retention of 80 % or higher after 1000 hours or 1000 cycles is a commonly accepted pass criterion in automotive material specifications.

What samples and lead time are typically required?

Standard screening normally requires flat sheets of at least 100 mm × 100 mm in the production thickness, with a minimum of two to three sheets plus cut specimens for tensile and dielectric testing. A basic thermal, mechanical, and electrical characterization package is usually completed within five to ten working days, while full reliability programs involving 1000-hour aging or 1000-cycle testing run in parallel over several weeks. Accredited third-party laboratories can tailor the test matrix to the applicable OEM specification and regional regulatory framework.