Thermal Conductivity Testing of Power Device Packaging Materials: Standards, Methods, and Applicatio

Thermal conductivity testing of power device packaging materials is essential for verifying heat dissipation performance in IGBT modules, SiC power devices, and wide-bandgap semiconductor assemblies. This article explains the key test items, applicable standards such as ASTM E1461, ASTM E1530, ISO 22007-2, and GB/T 22588, sample requirements, and measurement methods used by accredited third-party laboratories to characterize ceramic substrates, thermal interface materials, molding compounds, and die-attach materials.

Technical Overview

Modern power semiconductor devices, including silicon IGBTs, silicon carbide (SiC) MOSFETs, and GaN HEMTs, operate at continuously increasing power densities and switching frequencies. A large proportion of the electrical losses in these devices is converted into heat at the die level, and the junction temperature directly governs long-term reliability, on-state performance, and thermal runaway behavior. Effective heat removal from the chip junction to the cooling system therefore depends on the thermal conductivity of every material in the packaging stack, from die-attach solder and ceramic substrate to thermal interface material (TIM) and baseplate.

Packaging materials used in power modules span an extremely wide conductivity range, from approximately 0.2 W/(m·K) for unfilled silicone gels to more than 200 W/(m·K) for sintered silver and aluminum nitride ceramics. Because no single measurement technique covers this entire range with acceptable accuracy, thermal conductivity testing must be matched to the material class, sample geometry, anisotropy, and target service temperature. Accredited third-party laboratories typically apply laser flash, guarded heat flow meter, and transient methods in combination to deliver complete thermal characterization, including temperature-dependent conductivity profiles from cryogenic conditions up to 1000 °C or higher.

Testing Samples

Ceramic Substrates and Insulated Metal Substrates

Direct bonded copper (DBC) and active metal brazed (AMB) substrates based on alumina (Al2O3), aluminum nitride (AlN), and silicon nitride (Si3N4) are the dominant insulation and heat-spreading layers in power modules. Testing also covers insulated metal substrates (IMS), copper-clad laminates, FR-4 prepregs, and build-up dielectric films used in embedded power packaging.

Thermal Interface Materials

Thermal greases, thermally conductive pads, gap fillers, phase change materials, and graphite sheets are tested to quantify their effective conductivity under controlled contact pressure. These materials fill the air gaps between baseplate and heat sink and are frequently the limiting resistance in the overall thermal stack.

Encapsulants and Molding Compounds

Epoxy molding compounds (EMC), silicone gels, glob top coatings, underfills, and potting compounds are characterized as molded coupons or cured plaques. Filler loading, cure state, and post-mold conditioning all influence the measured conductivity and are controlled during specimen preparation.

Die-Attach and Interconnect Materials

Solder alloys such as SnAgCu, high-lead solders, sintered nano-silver pastes, and silver-filled conductive adhesives are evaluated in bulk specimen form or as bonded joints. Thermal conductivity, together with thermal resistance across the bond line, determines the die-to-substrate heat transfer efficiency.

Baseplates, Heat Spreaders, and Semiconductor Materials

Copper and copper alloy baseplates, aluminum silicon carbide (AlSiC) composites, molybdenum disks, diamond-reinforced composites, and bare semiconductor wafers or dies (Si, SiC) are tested to complete the thermal description of the module.

Testing Items

Standard test items for power device packaging materials include the following parameters:

• Thermal conductivity, expressed in W/(m·K), across the temperature range of interest
• Thermal diffusivity, expressed in mm²/s, typically measured by flash methods
• Specific heat capacity, expressed in J/(g·K), measured by differential scanning calorimetry or flash calorimetry
• Thermal resistance and thermal impedance of interface materials, expressed in K·mm²/W or °C/W, as a function of bond line thickness and clamping pressure
• Through-plane versus in-plane (anisotropic) conductivity for laminates, graphite films, and oriented composites
• Temperature-dependent conductivity curves for thermal simulation and derating analysis

Testing Methods

Laser Flash Analysis (LFA)

Laser flash analysis, performed in accordance with ASTM E1461, ISO 18755, JIS R 1611, and GB/T 22588, is the preferred method for high-conductivity ceramics, metals, and semiconductor materials. A short laser pulse heats the front face of a disc-shaped specimen, and an infrared detector records the temperature rise on the rear face. Thermal diffusivity is derived from the half-rise time, and thermal conductivity is calculated from diffusivity, specific heat capacity, and bulk density. Typical specimens are discs of 6 to 25 mm diameter and 0.5 to 3 mm thickness, and measurements can extend from room temperature up to approximately 1000 °C or beyond.

Guarded Heat Flow Meter (GHFM)

The guarded heat flow meter method, covered by ASTM E1530 and related to the heat flow meter technique of ASTM C518 and GB/T 10295, is a steady-state technique suited to polymers, molding compounds, and materials with low to moderate conductivity. The specimen is sandwiched between temperature-controlled plates under a defined compressive load, and thermal conductivity is calculated from the measured heat flux and the temperature gradient across a calibrated reference. Disc or square specimens of about 50 mm across and 1 to 20 mm thick are commonly used.

Transient Plane Source (TPS)

The transient plane source, or hot disc, method standardized in ISO 22007-2 and GB/T 32064 places a double-spiral sensor between two pieces of the specimen. A controlled heating pulse is applied, and the temperature response is recorded over a few seconds. The technique covers a broad conductivity range of roughly 0.01 to 500 W/(m·K) and can separately determine in-plane and through-plane conductivity, which is valuable for anisotropic substrates and graphite-based interface materials.

Transient Line Source (TLS)

The transient line source, or hot wire, method described in ISO 22007-3 and ASTM D7896 is applied to thermal greases, pastes, liquids, and soft gap fillers that cannot sustain steady-state contact geometry. A thin probe containing a heater and thermocouple is immersed in the sample, and conductivity is extracted from the temperature rise versus logarithmic heating time.

Steady-State Method for Interface Materials

ASTM D5470 defines the standard measurement of thermal impedance for thermally conductive electrical insulation materials. The specimen is compressed between meter bars at controlled pressure, and apparent conductivity is determined at several thicknesses to separate bulk conductivity from contact resistance. This method is essential for TIM qualification in power module assembly.

Applications

Thermal conductivity test data for power packaging materials support design, qualification, and manufacturing control across many industries:

• Electric vehicle traction inverters, on-board chargers, and DC fast-charging power modules
• Renewable energy systems, including wind turbine converters, string and central solar inverters, and energy storage power conversion systems
• Industrial motor drives, servo amplifiers, welding equipment, and induction heating systems
• Rail transit traction converters and auxiliary power units
• Data center power supplies, UPS systems, and high-density server power stages
• Material development programs for filled polymers, advanced ceramics, and sintered bonding pastes
• Incoming material inspection, supplier change verification, and reliability qualification of power module assemblies

FAQ

Which test method should be selected for a specific packaging material?

Laser flash analysis is recommended for ceramics, metals, and wafers above roughly 5 W/(m·K), while guarded heat flow meter and transient plane source methods suit polymers, molding compounds, and interface materials. Greases and liquids are best measured by the transient line source method, and finished interface products should additionally be evaluated by ASTM D5470 for bond line dependent impedance.

What sample size and preparation are required?

Laser flash specimens are typically 12.7 mm discs of 1 to 3 mm thickness with parallel, opaque faces. Heat flow meter and TPS specimens require flat surfaces and thicknesses within the instrument working range, usually 10 to 50 mm. Laboratories can often machine, mold, or cure specimens from customer raw materials when finished coupons are unavailable.

Can testing be performed at elevated or sub-ambient temperatures?

Yes. Laser flash systems commonly operate from below -50 °C up to 1000 °C or higher, and heat flow meter and TPS instruments cover ranges from approximately -20 °C to 300 °C, allowing complete temperature-dependent characterization for electrothermal simulation.

How is measurement quality assured?

Accredited third-party laboratories verify instruments with certified reference materials such as Pyroceram 9606, Pyrex glass, and pure metals, calibrate thickness and heating power traceably, and report measurement uncertainty together with full test conditions, including atmosphere, orientation, and contact pressure where applicable.

How long does a typical thermal conductivity test take?

Single-temperature measurements are usually completed within a few working days, while multi-temperature profiles across five or more set points, including specimen preparation and data review, are typically completed within one to two weeks depending on sample quantity.