Thermal Conductivity Testing of Thermally Conductive Silicone Pads for Transformer Insulation Materi

Thermally conductive silicone pads are widely used in transformers to combine efficient heat dissipation with reliable electrical insulation. Accurate thermal conductivity testing, performed according to recognized standards such as ASTM D5470, ISO 22007, and the corresponding GB series, is essential for material qualification, thermal design verification, and quality control. This article explains the sample requirements, testing items, measurement methods, and typical applications of thermal performance evaluation for silicone-based transformer insulation materials.

Technical Overview

Thermally conductive silicone pads, also known as thermal gap pads or thermally conductive silicone sheets, are soft elastomeric materials filled with high-conductivity ceramic particles such as aluminum oxide, aluminum nitride, or boron nitride dispersed in a silicone polymer matrix. In transformers, these pads are installed between windings, between the core and the housing, or between coil assemblies and heat sinks, where they simultaneously serve two critical functions: conducting heat away from localized hot spots and maintaining sufficient electrical insulation to withstand operating voltages.

Because transformer miniaturization and higher power density continuously increase thermal loads, the declared thermal conductivity of a silicone pad must be verified through laboratory measurement. Manufacturers, transformer designers, and certification bodies rely on standardized thermal conductivity testing to compare materials, validate datasheet values, and confirm long-term reliability. Thermal conductivity values for commercial silicone pads typically range from approximately 0.5 W/(m·K) for general-purpose grades up to 6 W/(m·K) or higher for high-performance formulations, and even small deviations between claimed and actual performance can significantly affect transformer temperature rise calculations.

Professional thermal testing of these materials is normally carried out by accredited third-party laboratories using steady-state and transient measurement techniques, with test conditions, calibration procedures, and reporting formats defined by international and national standards organizations including ASTM International, ISO, IEC, and the GB standards system.

Testing Samples

Thermal Gap Filler Pads

Standard silicone gap filler pads are submitted in flat sheet form with typical thicknesses between 0.5 mm and 5 mm. The laboratory cuts circular or square specimens from the sheet, verifies thickness at multiple points using a precision micrometer or optical method, and records the as-received thickness because thermal impedance results depend directly on sample thickness. For ASTM D5470 testing, specimens are usually cut to match the metering bar cross-section, commonly 50 mm × 50 mm, with thickness tolerance and surface flatness controlled as specified in the test method.

Thermally Conductive Insulating Sheets for Winding Applications

Silicone sheets used as interlayer or interwinding insulation in transformers are often thinner and may incorporate fiberglass reinforcement or polymer films for enhanced dielectric strength. These samples are tested both for thermal transmission properties and for electrical characteristics. Reinforced samples must be oriented consistently, and any adhesive or protective liner layers must be removed or documented, since liners can dominate the measured contact resistance.

Sample Conditioning Requirements

Before measurement, specimens should be conditioned in a controlled environment, typically 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 24 hours, in accordance with standard practice for conditioning elastomers. Sample surfaces must be free of contamination, and for pressure-sensitive grades, testing pressure must be documented because the compliance of the silicone pad under clamping pressure significantly influences the measured thermal resistance.

Testing Items

A comprehensive thermal performance evaluation of thermally conductive silicone pads for transformer insulation covers the following test items:

• Thermal conductivity, expressed in W/(m·K), the fundamental material property used in transformer thermal modeling
• Thermal impedance or thermal resistance, expressed in K·cm²/W or °C·cm²/W, which includes both material bulk resistance and interface contact resistance
• Thermal diffusivity, expressed in mm²/s, measured primarily by transient or laser flash methods
• Specific heat capacity, expressed in J/(g·K), required to convert diffusivity data into conductivity values
• Density, expressed in g/cm³, determined in accordance with relevant ASTM or GB methods for rubber and elastomeric materials
• Dielectric breakdown strength, expressed in kV/mm, verified according to IEC 60243-2 or the corresponding ASTM D149 method to confirm the insulation function
• Volume resistivity and surface resistivity, evaluated under IEC 62631 or ASTM D257 procedures
• Hardness, typically Shore 00 or Shore A, which correlates with pad compliance and contact performance
• Flammability rating, commonly assessed under the UL 94 vertical burn classification

Testing Methods

Steady-State Heat Flow Method (ASTM D5470)

The guarded heat flow meter method described in ASTM D5470 is the most widely accepted technique for thermally conductive silicone pads used in electronics and transformer insulation. A specimen is clamped between two metering bars maintained at different temperatures, and the heat flux through the sample is calculated from the temperature gradients measured along the bars. By testing specimens of at least two different thicknesses, the laboratory can separate total thermal resistance into bulk thermal resistance, whose slope yields thermal conductivity, and the interfacial contact resistance caused by surface effects. This method closely reproduces the clamped-interface conditions found in actual transformer assemblies, which makes its thermal impedance data particularly valuable for engineering design.

Transient Plane Source Method (ISO 22007-2)

The transient plane source technique, standardized in ISO 22007-2, uses a double-spiral nickel sensor sandwiched between two pieces of the silicone material. A controlled electrical pulse heats the sensor while its resistance change is recorded, allowing simultaneous calculation of thermal conductivity, thermal diffusivity, and specific heat capacity from a single measurement. The method is fast, requires relatively small samples, and is well suited to isotropic elastomeric materials with moderate conductivity, making it a popular screening tool for material development and incoming inspection.

Laser Flash Method (ASTM E1461 / ISO 22007-4)

The laser flash method determines thermal diffusivity by applying a short laser pulse to the front face of a small disc-shaped specimen and recording the temperature rise on the rear face with an infrared detector. When combined with independently measured density and specific heat capacity, thermal conductivity is calculated as the product of the three values. The technique offers excellent accuracy at elevated temperatures, so it is frequently used when silicone pad performance must be characterized across the operating temperature range of a transformer, for example from −40 °C to +150 °C.

Supporting Electrical and Mechanical Verification

Because the material must provide both thermal and insulating functions, thermal testing is often accompanied by dielectric strength testing under IEC 60243-2, insulation resistance measurement, and hardness verification. An accredited third-party laboratory typically issues a combined report presenting thermal conductivity alongside dielectric withstand results, which allows transformer manufacturers to qualify a silicone pad grade with a single test program.

Applications

Verified thermal performance data for silicone insulation materials directly supports the following transformer-related applications:

• Dry-type power and distribution transformers, where pads conduct winding heat toward cooling ducts and enclosures
• Switch-mode power supply transformers and high-frequency magnetic components in server, telecom, and charger platforms
• Traction and auxiliary transformers in rail and electric vehicle powertrains, where vibration damping and insulation coordination are equally important
• EV charging modules and onboard chargers that demand compact, high-power magnetic assemblies
• Solar inverter transformers and reactors operating under continuous thermal cycling
• Gap filling between transformer cores, bobbins, potting housings, and heat-dissipating chassis surfaces
• Material qualification, incoming inspection, and supplier comparison programs for transformer manufacturers
• Certification support where declared thermal parameters must be substantiated by standardized laboratory measurement

FAQ

What thermal conductivity range is typical for silicone pads used in transformers?

General-purpose silicone pads generally offer thermal conductivity between 0.8 W/(m·K) and 2 W/(m·K), while high-ceramic-loading formulations reach 3 W/(m·K) to 6 W/(m·K) or more. Selecting a grade involves balancing conductivity against hardness, dielectric strength, and cost, and declared values should always be confirmed by laboratory testing.

Which test standard should be chosen for thermal conductivity measurement?

ASTM D5470 is preferred when interface resistance and in-situ clamped performance matter, as in transformer assemblies. ISO 22007-2 is convenient for rapid multi-parameter screening, and the laser flash method under ASTM E1461 or ISO 22007-4 is best for wide-temperature-range characterization. Regional GB standards may apply for domestic compliance in certain markets.

Why do thermal conductivity values from different methods sometimes differ?

Steady-state methods measure conductivity through the thickness of the pad under pressure and include real contact interfaces, whereas transient and laser flash methods measure volumetric properties under different boundary conditions. Differences in applied pressure, thickness measurement, and contact resistance explain typical variations, which is why test reports must state the method and conditions used.

What sample dimensions are required for testing?

For ASTM D5470, specimens are commonly 50 mm × 50 mm with thicknesses of 1 mm to 3 mm, and at least two thickness levels are recommended for separating bulk conductivity from contact resistance. Transient plane source testing usually requires two matching pieces of approximately 40 mm to 50 mm in diameter, while laser flash testing uses small discs typically 10 mm to 12.7 mm in diameter.

How long does thermal conductivity testing take?

Once samples are conditioned, steady-state testing usually requires one to two weeks including calibration and reporting, transient plane source measurements can be completed within a few working days, and laser flash programs with extended temperature sweeps may take two to three weeks. Accredited third-party laboratories can confirm turnaround times based on the exact test matrix.

Is thermal conductivity testing sufficient to qualify a silicone pad for transformer insulation?

No. Thermal data must be combined with dielectric breakdown strength, insulation resistance, flame retardancy, and aging assessments to fully qualify the material. A complete qualification program evaluates both the heat transfer function and the long-term electrical insulation reliability of the silicone pad in its intended transformer environment.