Thermally conductive pads are critical thermal interface materials in electric vehicle power battery packs, where their compression behavior directly determines surface wetting, interfacial thermal resistance, and long-term mechanical reliability. This article presents a professional technical guide to compression performance testing of power battery thermal pads, covering sample types, testing items, test methods, applicable standards such as ASTM D575, ISO 7743, and ASTM D395, typical applications, and frequently asked questions.
Technical Overview
Thermally conductive pads (also called thermal gap pads or thermal interface pads) are highly filled polymer composites, typically silicone-based or silicone-free elastomers loaded with ceramic or graphitic fillers such as alumina, aluminum nitride, or boron nitride. In lithium-ion power battery packs, these pads are installed between cells, between cells and liquid cooling plates, and between modules and pack covers to replace thermally insulating air gaps with a compliant, heat-conducting medium. Because battery pack components introduce dimensional tolerances of several millimeters, the pad must deform under relatively low clamping pressure to conform to uneven surfaces while avoiding excessive mechanical stress on the cells.
Compression performance is therefore one of the most important qualification attributes of a battery thermal pad. Key engineering metrics include the compression-deflection (force-displacement) curve, deflection at rated pressure, compressive stress at a given strain, compression set, stress relaxation, and hardness. A pad that is too stiff cannot fill gaps at achievable stack pressures, while a pad with poor compression-set resistance gradually loses contact pressure over the vehicle life, causing interfacial thermal resistance to rise and localized hot spots to develop. Testing is generally performed in accordance with recognized standards such as ASTM D575, ISO 7743, ASTM F36, ASTM D395, ISO 815-1, ISO 3384, and ASTM D2240, either in-house or at accredited third-party laboratories.
Testing Samples
Silicone-Based Thermal Gap Pads
Silicone gap pads are the most widely used category in power battery applications, offering stable elasticity across a broad temperature range (-40 °C to 150 °C or higher), typical hardness of 20 to 80 Shore 00, and thermal conductivity from approximately 1 to 8 W/m·K. They may be supplied with or without a fiberglass reinforcement carrier, which significantly influences compression behavior.
Non-Silicone Thermal Gap Pads
Acrylic, polyurethane, or other silicone-free gap pads are selected where silicone volatiles (siloxane migration) must be avoided, such as near electrical contacts or optical components within or adjacent to the battery system. Their compression characteristics often differ from silicone grades, with lower elongation and different recovery behavior.
Thermal Gap Fillers and Putty Materials
Ultra-soft, putty-like gap fillers achieve very high deflection (often exceeding 50%) at low pressure and are increasingly used in cell-to-pack designs. These materials require careful specimen handling and dedicated fixturing because they exhibit viscoelastic, near-fluid behavior under load.
Specimen Configuration
Typical specimens are die-cut discs (commonly 29 mm diameter) or square plaques (for example, 50 mm × 50 mm) cut from production sheets at nominal product thickness. Where permitted by the test standard, thin pads may be stacked to reach a minimum testable thickness, although stacking must be clearly reported because it biases the compression-deflection curve. Finished die-cut parts can also be tested to evaluate the as-installed construction, including any reinforcement carrier.
Testing Items
Compression performance evaluation of power battery thermal pads typically includes the following test items:
- Compression-deflection curve (force versus displacement, or stress versus strain) at room temperature
- Deflection percentage at specified pressures, for example 50 kPa, 100 kPa, 200 kPa, or 350 kPa
- Compressive stress at specified deflection levels, commonly 10%, 25%, 40%, and 50% compression
- Compression set under constant deflection (for example 25%) at elevated temperatures such as 70 °C, 100 °C, or 125 °C for 22 h, 70 h, or 168 h
- Compression set under constant load, where applicable
- Stress relaxation and force retention under sustained compression at room and elevated temperatures
- Hardness (Shore 00 or Shore A) as a complementary control metric
- Cyclic compression durability and loading-unloading hysteresis behavior
- Thickness recovery after load removal
Testing Methods
Specimen Conditioning
Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum period (typically 40 hours) in accordance with ISO 291 or ASTM D618. Thickness is measured at multiple points using a low-pressure thickness gauge, and the average original thickness is recorded as the reference for all deflection calculations. Test report data should include nominal thickness, filler construction, carrier orientation, and the number of layers if stacked.
Compression-Deflection Testing (ASTM D575 / ISO 7743)
The compression-deflection test is performed on a universal testing machine fitted with rigid, parallel compression platens and a calibrated load cell (accuracy typically Class 0.5 or better). The specimen is centered between the platens and compressed at a controlled crosshead speed, typically 10 to 12 mm/min per ASTM D575 or ISO 7743; because thermal pads are strain-rate sensitive, slower rates (for example, 1 mm/min) may be specified to simulate slow stack-up loading, and the selected rate must always be reported. Force and displacement are recorded continuously to generate the compression curve, from which deflection percentage (original thickness minus compressed thickness, divided by original thickness) and compressive stress are calculated. Commonly reported data points include deflection at 100 kPa or 350 kPa and stress at 25% compression. A minimum of three specimens is recommended, with median values reported.
Compression Set Testing (ASTM D395 / ISO 815-1 / GB/T 7759.1)
Compression set is evaluated under constant deflection (Method B of ASTM D395, or ISO 815-1 / GB/T 7759.1). The specimen is compressed between parallel plates to a defined deflection, typically 25% of the original thickness, using spacers or a fixed-gap fixture to prevent over-compression. The assembly is placed in a circulating-air oven at the selected aging temperature (for example 100 °C or 125 °C, reflecting battery pack operating conditions) for the specified duration (22 h, 70 h, or 168 h). After removal and a recovery period of 30 minutes at room temperature, the final thickness is measured. Compression set is calculated as the original thickness minus the recovered thickness, divided by the original thickness minus the spacer thickness, multiplied by 100%. Battery-grade thermal pads are commonly required to exhibit compression set below approximately 5% to 15% depending on the material class and design requirement.
Stress Relaxation Testing (ISO 3384)
In stress relaxation testing, the specimen is held at constant deflection (or constant strain) while the decay of compressive force is monitored over time, either at room temperature or in an environmental chamber at elevated temperature. This test quantifies force retention, which predicts whether the pad will maintain adequate interfacial pressure across the service life of the battery pack. Test durations range from hours in screening studies to thousands of hours for durability validation.
Hardness Testing (ASTM D2240 / ISO 868 / GB/T 531.1)
Hardness is measured using a durometer calibrated to the appropriate scale: Shore 00 for ultra-soft gap fillers and soft pads, and Shore A for firmer pad grades. Hardness provides a rapid incoming-inspection indicator and is correlated, though not identical, to compression stiffness. Specimens must meet minimum thickness requirements for the chosen durometer scale; stacked layers are used where necessary.
Cyclic Compression and Durability Evaluation
For design validation, pads may be subjected to repeated compression cycles between defined strain limits (for example, between 10% and 40% deflection for thousands of cycles) to assess hysteresis, energy loss, and permanent growth or thinning. Compression testing is also frequently combined with thermal cycling in accordance with IEC 60068 environmental test methods to replicate real battery pack duty cycles, after which compression set and deflection characteristics are re-measured.
Applications
Compression performance testing of power battery thermal pads supports the full product lifecycle, from material development to pack-level validation:
- Material qualification and datasheet verification for thermal pad manufacturers, confirming deflection-at-pressure and compression-set values claimed in technical documentation
- Incoming quality control at battery pack and module assembly plants, ensuring batch-to-batch consistency of supplied gap pads and gap fillers
- Design and tolerance stack-up analysis, where measured compression curves feed finite element models to confirm that pads fill worst-case gaps under achievable assembly forces without over-stressing prismatic, pouch, or cylindrical cells
- Thermal management validation between cells and liquid cold plates, cell-to-cell sheets, module-to-lid interfaces, busbar joints, and battery management system electronics
- Reliability and aging programs combining compression, thermal cycling, vibration, and high-temperature storage to verify long-term contact pressure retention
- Independent verification at accredited third-party laboratories for customer audits, regulatory submissions, and comparative benchmarking of competing materials
FAQ
Which standards are most commonly applied to thermal pad compression testing?
ASTM D575 and ISO 7743 govern compression-deflection (compressive stress-strain) testing; ASTM D395, ISO 815-1, and GB/T 7759.1 cover compression set; ISO 3384 addresses stress relaxation in compression; and ASTM D2240, ISO 868, and GB/T 531.1 cover durometer hardness. ASTM F36 is sometimes referenced for gasket-style compressibility evaluation.
What pressure range is representative of power battery applications?
Most battery pack interfaces impose contact pressures of roughly 50 kPa to 350 kPa (approximately 7 to 50 psi), with some module designs reaching higher values. Datasheets commonly report deflection at 50 psi (345 kPa); ultra-soft gap fillers are often characterized at lower pressures such as 10 to 20 psi because they are designed for large gaps and delicate cell surfaces.
Why must thin pads sometimes be stacked for testing, and how does this affect results?
Very thin pads fall below the minimum thickness required for reliable measurement and stable buckling behavior. Stacking several layers to reach the minimum test thickness is permitted by some standards, but interlayer slip and surface effects make stacked results appear slightly softer than a monolithic specimen, so the stacking condition must be explicitly documented in the report.
How do test temperature and duration influence compression set results?
Higher temperatures accelerate viscoelastic flow and crosslink or filler network changes, increasing measured compression set. A pad showing acceptable set at room temperature may degrade significantly at 125 °C; therefore, aging temperature and duration are always selected to reflect the actual thermal environment of the battery pack, and comparing results across different conditions is not valid.
Does a fiberglass reinforcement carrier change compression test results?
Yes. A carrier limits lateral expansion of the elastomer, raising compressive stiffness at a given deflection and typically reducing compression set. Specimens must be tested in the final construction and orientation, and carrier-bearing results should not be directly compared with unreinforced material data.
What are typical acceptance criteria for battery-grade thermal pads?
While requirements are set by each pack design, common targets include deflection of at least 25% to 40% at the rated assembly pressure, compression set below approximately 10% to 15% after high-temperature aging, hardness within the datasheet tolerance band, and force retention of at least 70% to 80% after prolonged stress relaxation exposure. Verification against these targets should be carried out using calibrated equipment and, where independent confirmation is required, at accredited third-party laboratories.
