Thermal Conductivity Testing of Heat Dissipation Materials for IC Packaging

Thermal conductivity testing of IC packaging heat dissipation materials is essential for validating the performance of thermal interface materials, heat spreaders, and advanced packaging substrates used in modern semiconductor devices. As chip power densities continue to rise with AI, 5G, and high-performance computing applications, accurate characterization of thermal properties has become a critical step in package design, material selection, and quality assurance. This article provides a comprehensive overview of the sample types, testing parameters, industry-standard test methods, and practical applications related to thermal testing of IC packaging thermal management materials.

Technical Overview

Integrated circuit (IC) packages rely on a carefully engineered thermal path to move heat away from the die and dissipate it into the surrounding environment, typically through a heat spreader, heat sink, or the printed circuit board. Every material along this path, including thermal interface materials (TIMs), die-attach adhesives, molding compounds, ceramic substrates, and metallic lids, contributes thermal resistance that directly affects junction temperature (Tj), device reliability, and long-term performance.

Thermal conductivity testing quantifies how efficiently these materials conduct heat, expressed in watts per meter-kelvin (W/m·K). Because different materials in an IC package span an enormous range of thermal conductivity, from less than 0.2 W/m·K for polymer TIMs to over 400 W/m·K for copper and graphite heat spreaders, no single test method can cover all materials with adequate accuracy. Laboratories therefore employ a portfolio of steady-state and transient techniques, selecting the appropriate method based on the expected conductivity range, sample geometry, and whether bulk conductivity or interfacial thermal resistance is the parameter of interest.

Key drivers for thermal testing in the semiconductor industry include the shrinking thermal budget of advanced packages such as flip-chip ball grid arrays (FCBGA), chiplet-based designs, 2.5D/3D stacked architectures, and fan-out wafer-level packaging. Incorrect or overstated thermal conductivity data can lead to thermal runaway, throttling, premature failure, and field returns, making independent laboratory verification an essential part of the supply chain.

Testing Samples

Thermal Interface Materials (TIM1 and TIM2)

Thermal interface materials fill the microscopic air gaps between the die and lid (TIM1) or between the lid and heat sink (TIM2). Common forms include silicone-based and non-silicone thermal greases, gap filler pads, phase change materials (PCMs), thermally conductive adhesives, and dispensed gels. These materials are typically filled with thermally conductive particles such as alumina, aluminum nitride, boron nitride, zinc oxide, silver, or graphite.

Die-Attach Materials

Die-attach layers provide both mechanical fixation and a thermal path from the die to the substrate or lead frame. Samples include solder-based attach materials such as SAC305 and high-lead alloys, sintered silver pastes, eutectic gold-tin alloys, and silver-filled epoxy adhesives. These samples often require testing in both bulk form and as-built bond line configurations.

Heat Spreaders and Lids

Metallic and composite heat spreaders distribute heat laterally away from small die. Test samples include copper and copper-alloy lids, aluminum sheets, CVD and annealed pyrolytic graphite sheets, graphite-copper composites, and vapor chamber materials. Anisotropic materials require measurement of both in-plane and through-plane thermal conductivity.

Ceramic and Organic Substrates

Package substrates based on alumina, aluminum nitride, silicon nitride, beryllium oxide, or organic build-up laminates are tested for bulk thermal conductivity to model heat flow into the board. Aluminum nitride substrates, widely used in power modules, typically require accurate verification in the 90 to 200 W/m·K range.

Molding Compounds and Encapsulants

Epoxy molding compounds (EMCs), glob-top encapsulants, and underfill materials are characterized at low conductivity ranges, typically 0.2 to 3 W/m·K, to predict heat entrapment within the package body.

Thermal Pads and Insulators

Silicone and non-silicone thermal pads, mica insulators, and ceramic-filled films used between power devices and heat sinks are tested for thermal conductivity, thermal impedance, and durometer-dependent contact performance.

Testing Items

Comprehensive thermal characterization of IC packaging materials generally includes the following measurement parameters:

• Thermal conductivity (W/m·K), the primary figure of merit for bulk heat conduction performance

• Thermal diffusivity (mm²/s), measured to derive conductivity when combined with specific heat and density

• Specific heat capacity (J/g·K), required for laser flash analysis and transient thermal modeling

• Thermal resistance (K·cm²/W or K/W), including bulk resistance and contact/interface resistance

• Thermal impedance versus pressure, evaluating gap fillers and pads under realistic clamping loads

• Bond line thickness (BLT) dependence of thermal performance for greases, adhesives, and PCMs

• Anisotropic conductivity, distinguishing in-plane from through-plane values for graphite and composite films

• Temperature-dependent conductivity, typically from -40°C to 150°C or higher for automotive and power devices

• Aging and reliability drift, including bake, thermal cycling, and power cycling effects on TIM performance

• Density and porosity, supporting data reduction and structural qualification

Testing Methods

Laser Flash Analysis (LFA)

Laser flash analysis, performed per ASTM E1461 and ISO 18755, is the preferred method for high-conductivity materials such as ceramics, metals, graphite, and substrate samples. A short laser pulse heats the front face of a small disc-shaped specimen, typically 10 to 12.7 mm in diameter and 1 to 3 mm thick, while an infrared detector records the temperature rise on the rear face. The resulting half-rise time yields thermal diffusivity, and thermal conductivity is calculated as the product of diffusivity, specific heat (measured by DSC per ASTM E1269), and bulk density. LFA covers a broad range from approximately 0.1 to over 1000 W/m·K and supports measurement temperatures from cryogenic conditions up to 1000°C.

ASTM D5470 Steady-State Heat Flow Method

ASTM D5470 is the industry-standard technique for thermal interface materials. The specimen is placed between two polished metal meter bars maintained at a controlled temperature gradient, and heat flux is calculated from the temperature profiles measured along the bars. By testing at multiple thicknesses or pressures, the method separates bulk thermal conductivity from thermal contact resistance at the material surfaces. This is the only technique that directly measures the total thermal impedance that a TIM will present in an actual package, making it essential for greases, gap pads, phase change materials, and adhesives. Instrument reference compliance with ASTM D5470 should be verified, and pressure and temperature conditions should match the intended application.

Transient Plane Source (Hot Disk, ISO 22007-2)

The transient plane source method uses a nickel foil sensor that acts as both heater and resistance thermometer, sandwiched between two halves of the sample. It rapidly measures thermal conductivity, diffusivity, and effusivity over a range of roughly 0.005 to 500 W/m·K, making it suitable for polymers, molding compounds, elastomeric pads, and anisotropic materials when appropriate sensor geometries are used.

Guarded Heat Flow Meter (ASTM E1530)

For moderate to low conductivity materials such as encapsulants, polymers, and organic laminates in the range of 0.1 to 20 W/m·K, the guarded heat flow meter provides steady-state results with good repeatability. The sample is placed between heated and cooled plates, and a calibrated heat flux transducer determines thermal conductivity.

Guarded Hot Plate (ASTM C177 / ISO 8302)

The guarded hot plate is an absolute steady-state method regarded as the primary reference for insulation-class and low-conductivity materials. It is used primarily for calibration verification and for thick polymeric specimens where high accuracy at low conductivity is required.

Modified Hot Wire and Thermocouple Probe Methods

Transient hot-wire techniques (ASTM D7896) and needle probes are used for pastes, uncured compounds, liquids, and soft gap fillers where rigid sample preparation is impractical. These methods are useful for incoming inspection and comparative screening.

Junction Temperature and Structure Function Validation

For as-built packages, transient thermal testing per JESD51-14 (JEDA) and the JESD51 series uses electrical test methods, such as the temperature-sensitive parameter of a diode, to extract the thermal resistance junction-to-case (Rth J-C) and to resolve the contribution of each layer in the thermal stack through structure function analysis. This validates how material-level data translates into package-level performance.

Applications

• Material selection and qualification for CPU, GPU, ASIC, and SoC packages in data center and AI accelerator products

• Incoming quality control and supplier data verification for thermal greases, pads, and gap fillers in contract assembly lines

• Thermal modeling and simulation input, providing accurate conductivity values for finite element and CFD package models

• Power semiconductor packaging, including IGBT modules, SiC and GaN devices, sintered silver die attach, and DCB/AMB substrate qualification

• Automotive electronics validation against AEC-Q100 and related stress test qualifications, including temperature-dependent and post-aging thermal performance

• Advanced packaging development for chiplets, 2.5D interposers, 3D stacked die, and high-bandwidth memory (HBM) thermal stack engineering

• Reliability studies tracking pump-out, dry-out, and delamination effects in TIMs after thermal cycling and accelerated aging

• Failure analysis support, quantifying degradation of thermal paths in returned or destructively analyzed units

FAQ

Why do supplier datasheet values for thermal greases often not match measured results?

Datasheet values are frequently derived from bulk measurements of cured or uncompressed samples under ideal conditions, while in-package performance depends on bond line thickness, applied pressure, surface roughness, and contact resistance. ASTM D5470 testing at application-relevant pressures and thicknesses provides a realistic apparent thermal conductivity and total impedance that better predicts field behavior.

Which test method should be used for a specific material?

As a general guideline, use ASTM D5470 for TIMs and interface materials, laser flash (ASTM E1461) for ceramics, metals, graphite, and substrates above roughly 5 W/m·K, transient plane source or guarded heat flow meter for polymers and molding compounds, and junction-based transient testing (JESD51 series) for complete packaged devices.

What sample dimensions and preparation are required?

Laser flash samples are typically 10 to 12.7 mm discs, 1 to 3 mm thick, with flat parallel faces. ASTM D5470 specimens are commonly 25 to 50 mm squares or discs at the intended bond line thickness. Accurate thickness measurement is critical because thickness error propagates directly into conductivity error.

Can anisotropic materials like graphite sheets be tested?

Yes. In-plane and through-plane conductivity differ significantly in graphite and composites. Laser flash with lateral masking or specialized in-plane holders, along with anisotropic-capable transient plane source sensors, can resolve directional properties, and the reported values should clearly state the measurement direction.

What temperature range can be tested?

Most IC packaging materials are characterized between -40°C and 150°C to cover consumer, industrial, and automotive use conditions. Laser flash systems can extend to 500°C or higher for substrate ceramics, while ASTM D5470 systems commonly operate from approximately -20°C to 150°C.

How is thermal interface resistance separated from bulk conductivity?

By measuring total thermal resistance at several bond line thicknesses and plotting resistance versus thickness, the slope yields bulk thermal conductivity and the intercept yields the total contact resistance at the two interfaces. This approach, defined in ASTM D5470, is fundamental to characterizing TIM performance.