Flame Retardancy Testing of Thermally Conductive Pads for Power Batteries

Flame retardancy testing of thermally conductive pads is a critical qualification step for thermal interface materials installed inside electric vehicle power battery modules and packs. This technical article reviews the applicable flammability standards — including UL 94, IEC 60695, GB/T 2408, and ISO 4589-2 — along with specimen requirements, test procedures, and acceptance criteria commonly specified by EV manufacturers and verified by accredited third-party laboratories.

Technical Overview

Thermally conductive pads are highly filled elastomeric thermal interface materials (TIMs) placed between lithium-ion cells, module cold plates, and pack structures in electric vehicle (EV) power batteries. They typically consist of a silicone or non-silicone polymer matrix loaded with ceramic fillers such as aluminum oxide, zinc oxide, aluminum nitride, or boron nitride, and are engineered to transfer heat away from cells while providing electrical insulation, gap filling, and vibration damping.

Because a power battery pack stores a large amount of electrochemical energy, every organic material placed inside the pack must demonstrate resistance to ignition and must not accelerate flame propagation during a thermal runaway event. Flame retardancy testing therefore verifies that thermal pads comply with material-level flammability standards such as UL 94, IEC 60695-11-10 and IEC 60695-11-20, GB/T 2408, GB/T 2406.2 / ISO 4589-2 for limiting oxygen index, and the IEC 60695-2 glow wire series. Most EV manufacturers and module integrators specify a minimum rating of UL 94 V-0, frequently combined with a limiting oxygen index (LOI) of 28% or higher, for thermal pads used in cell-to-plate and cell-to-cell positions.

Testing Samples

Silicone-Based Thermally Conductive Pads

Platinum-cured silicone matrices with high ceramic loading are the most common category, available in thicknesses from approximately 0.5 mm to 10 mm. Because flammability classification is thickness-dependent, testing must cover both the minimum and maximum commercial thicknesses offered for the material grade.

Non-Silicone Thermally Conductive Pads

Acrylic, polyurethane, or epoxy-based pads are selected where silicone outgassing is prohibited, for example near optical sensors or in silicone-sensitive electronics within the battery management system. These formulations are tested with the same flammability methods and acceptance thresholds as silicone products.

Thermally Conductive Gap Fillers

Soft, highly conformable gap filler pads and cure-in-place compounds used to bridge large stack-up tolerances between modules and liquid cooling plates are tested in their cured pad form at the representative production thickness.

Thermally Conductive Foam Gaskets

Foamed thermal gaskets used for compression and sealing functions cannot be evaluated with the rigid-bar classifications alone and are assessed using the foam-specific UL 94 classifications (HBF, HF-2, HF-1).

Typical specimen geometry for flammability testing includes bars of 125 mm × 13 mm cut at the relevant thickness for UL 94 horizontal and vertical burning, and bars of approximately 100 mm × 10 mm × 4 mm (or the supplied thickness) for limiting oxygen index. Specimens are conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for at least 48 hours before testing.

Testing Items

• UL 94 vertical burning classification (V-0, V-1, V-2) in accordance with UL 94, IEC 60695-11-10, and GB/T 2408 (vertical method)
• UL 94 horizontal burning classification (HB) in accordance with UL 94 and GB/T 2408 (horizontal method)
• UL 94 foam flammability classifications (HBF, HF-2, HF-1) for foamed gasket materials
• UL 94 advanced flammability classifications (5VA, 5VB) using the 500 W flame source per IEC 60695-11-20
• Limiting oxygen index (LOI) per GB/T 2406.2, ISO 4589-2, and ASTM D2863
• Glow wire ignition temperature (GWIT) and glow wire flammability index (GWFI) per the IEC 60695-2 series
• Horizontal burn rate per ISO 3795 and GB 8410 for automotive interior components

Testing Methods

UL 94 Vertical Burning Test (V-0 / V-1 / V-2)

A bar specimen is clamped vertically above a layer of surgical cotton. A 20 mm blue flame from a small burner is applied to the lower edge of the specimen for 10 seconds, removed, and the after-flame time (t1) is recorded. After the specimen self-extinguishes, the flame is applied a second time for 10 seconds, and the second after-flame time (t2) and after-glow time (t3) are recorded, together with observation of any flaming drips that ignite the cotton indicator.

• V-0: each after-flame time ≤ 10 s; total after-flame time ≤ 50 s for the five specimens; no flaming drips igniting the cotton; after-glow ≤ 30 s
• V-1: each after-flame time ≤ 30 s; total after-flame time ≤ 250 s; no flaming drips igniting the cotton
• V-2: timing criteria identical to V-1, but flaming drips that ignite the cotton are permitted

UL 94 Horizontal Burning Test (HB)

The specimen is mounted horizontally with reference marks at 25 mm and 100 mm from the free end, and a burner flame is applied at a 45° angle. The material passes if burning extinguishes before the 100 mm mark, or if the measured burn rate does not exceed 40 mm/min for thicknesses of 3 mm to 13 mm and 75 mm/min for thinner specimens.

Limiting Oxygen Index (LOI)

The specimen is ignited at its top edge and burns downward in a glass chimney with a controlled oxygen/nitrogen mixture. The LOI is the minimum oxygen concentration at which candle-like burning is sustained over 50 mm of specimen length or 180 seconds of burning time. Higher LOI values indicate greater intrinsic flame resistance, and a value of 28% or above is commonly specified for battery pack materials.

Glow Wire Tests (GWFI / GWIT)

A nickel-chromium glow wire is electrically heated to a temperature between 550 °C and 960 °C and pressed against the specimen with a force of approximately 1 N for 30 seconds, with tissue paper positioned beneath the specimen. The glow wire flammability index identifies the highest temperature at which the specimen does not ignite or does not ignite the tissue paper by flaming drops, while the glow wire ignition temperature identifies the temperature 25 K above the highest temperature at which ignition does not occur.

Applications

Flame retardancy test data for thermal pads directly supports safety design and regulatory compliance across the electric vehicle and energy storage value chain. Typical applications include:

• Cell-to-cold plate and cell-to-cell thermal interfaces in prismatic, pouch, and cylindrical battery modules
• Pack-level interface and compression pads between modules, busbars, and enclosure structures
• Stationary energy storage system (ESS) racks and containers, where fire propagation control is a primary safety requirement
• Power electronics within the battery system, including onboard chargers, DC-DC converters, and inverter modules
• Battery management system (BMS) circuit boards and 48 V mild-hybrid electrical architectures

Test reports issued by accredited third-party laboratories are routinely used for supplier qualification, incoming material inspection, change management after formulation or thickness revisions, and technical documentation submitted to vehicle OEMs and regulatory bodies.

FAQ

What flame retardancy rating is typically required for thermal pads in EV power batteries?

The dominant industry requirement is UL 94 V-0, often combined with an LOI of at least 28%. For materials located near high-temperature power electronics, glow wire ratings such as GWFI 960 °C may additionally be specified.

Should the pad be tested at minimum or maximum thickness?

Both. Flammability classification is thickness-dependent, and the test conclusion is only valid for the thickness interval actually tested. Suppliers should test the thinnest and thickest commercial versions of each material grade.

Are GB/T 2408 and UL 94 equivalent?

GB/T 2408 is technically aligned with IEC 60695-11-10 and specifies the same horizontal and vertical 50 W flame procedures as UL 94, so results are generally comparable. However, for customer-specific certification purposes, testing should be performed under the exact standard version and within the accredited scope of the laboratory.

How many specimens are required for a test program?

A UL 94 vertical test typically requires two sets of five bar specimens per thickness, while the horizontal test uses three specimens. LOI determination generally requires a series of ten to fifteen specimens to bracket the critical oxygen concentration, and all specimens must complete conditioning beforehand.

Do silicone-based pads automatically pass V-0?

No. Although silicone chemistry offers intrinsic thermal stability, the final classification depends on the filler system, flame-retardant additives, thickness, and cell structure of the specific formulation, so verification by testing at the production thickness is always required.

What is a typical turnaround time for flame retardancy testing?

For a standard scope covering UL 94 and LOI at one or two thicknesses, accredited third-party laboratories generally complete testing within one to two weeks, including the mandatory 48-hour specimen conditioning period.