Underfill materials play a critical role in reinforcing solder joints and managing heat dissipation in advanced semiconductor packages, making accurate thermal conductivity measurement essential for packaging design and reliability. This article provides a comprehensive analysis of underfill thermal conductivity testing, covering sample preparation, key test parameters, recognized standards such as ASTM E1461, ASTM D5470, and ISO 22007, and practical guidance for material qualification in electronics manufacturing.
Technical Overview
Underfill materials are epoxy-based polymer adhesives dispensed beneath flip-chip devices, ball grid arrays (BGA), wafer-level packages, and chip-scale packages to redistribute thermomechanical stress away from solder interconnects. As semiconductor packages continue to shrink while power density increases, the thermal transport behavior of underfill layers has become a first-order design parameter rather than an afterthought. The underfill occupies the thin gap between the silicon die and the substrate, so its thermal conductivity directly influences the junction-to-board thermal resistance of the assembly.
Conventional capillary underfills filled with fused silica exhibit thermal conductivity values of approximately 0.2 to 0.5 W/(m·K), while advanced formulations incorporating alumina, aluminum nitride, or boron nitride fillers can achieve values above 1 to 4 W/(m·K). Accurate measurement of these values requires carefully controlled sample preparation, selection of an appropriate transient or steady-state test method, and awareness of temperature dependence near the glass transition region. Accredited third-party laboratories routinely perform this analysis in accordance with ISO, ASTM, and related standards organizations' methods to support material qualification, supplier comparison, and thermal simulation model input.
Testing Samples
Capillary Underfill Formulations
Liquid capillary underfill materials are cured into flat plaques or disks prior to testing. The two-part or premixed resin is degassed under vacuum, cast into PTFE or silicone molds, and cured according to the manufacturer's recommended profile, typically 150 °C for 1 to 3 hours followed by an optional post-cure. Test specimens are then machined or ground to achieve parallel surfaces and the thickness required by the selected method.
No-Flow and Molded Underfills
No-flow underfills, which cure during the solder reflow process, are prepared using reflow-simulated cure cycles so the measured properties reflect the actual material state inside the package. Molded underfills used in wafer-level and panel-level packaging are molded into disks or plaques using transfer molding or compression molding tools, with filler orientation effects evaluated where anisotropy is expected.
Thermally Enhanced and Custom Filler Systems
Highly filled underfills containing boron nitride or other thermally conductive fillers may exhibit direction-dependent conductivity due to filler alignment during flow and cure. For these materials, specimens are prepared in both through-plane and in-plane orientations, and density plus specific heat capacity are measured on companion specimens to support the laser flash calculation.
Specimen Conditioning and Dimensions
- Laser flash method: disk of 10 to 12.7 mm diameter, 1 to 3 mm thickness, with thin graphite or sputtered coating on both faces
- Transient plane source method: two matching pieces, typically 20 mm or larger in diameter, 2 to 6 mm thick
- Steady-state heat flow meter method: plaques of 25 to 50 mm diameter with thickness from 0.5 to 5 mm
- Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 24 hours in accordance with ISO 291 before testing
Testing Items
A complete thermal characterization of underfill materials typically includes the following measured and derived parameters:
- Thermal conductivity, expressed in W/(m·K), at room temperature and across the operating range
- Thermal diffusivity in mm²/s, the primary direct output of the laser flash technique
- Specific heat capacity in J/(g·K), determined by differential scanning calorimetry using the sapphire method
- Density in g/cm³, measured geometrically or by the Archimedes principle at the test temperature
- Temperature-dependent property sweeps, commonly from -40 °C to 150 °C or up to 250 °C for high-temperature underfills
- Anisotropy ratio comparing through-plane and in-plane conductivity for filler-aligned systems
Testing Methods
Laser Flash Method (ASTM E1461 / ISO 22007-4)
The laser flash method is the most widely used approach for cured underfill specimens. A short laser or xenon flash pulse heats the front face of the disk specimen, and an infrared detector records the temperature rise on the rear face. Analysis of the half-rise time yields thermal diffusivity, from which thermal conductivity is calculated as the product of diffusivity, density, and specific heat capacity. The method requires only small specimens, covers a broad temperature range, and delivers measurement repeatability typically better than 3 %, though it is an indirect method whose accuracy depends on the quality of the density and heat capacity inputs.
Transient Plane Source Method (ISO 22007-2)
The transient plane source, or hot disc, technique places a double-spiral nickel sensor between two pieces of the cured underfill. A controlled electrical heating step generates a transient temperature response from which conductivity and diffusivity are extracted simultaneously. This direct method is well suited to isotropic materials, requires no separate heat capacity measurement, and can resolve both through-plane and in-plane conductivity in a single experiment when extended anisotropic analysis is applied.
Steady-State Heat Flow Method (ASTM D5470 / ASTM C518)
Steady-state methods sandwich the specimen between metered plates maintained at different temperatures and measure the heat flux through the sample. Testing a series of thicknesses allows separation of contact resistance from bulk resistance, giving a direct bulk thermal conductivity value. This approach is particularly useful for thin underfill films and thermally enhanced formulations, and it mirrors the compression conditions encountered in real package assemblies.
Supporting Measurements
Differential scanning calorimetry per ASTM E1269 or ISO 11357 provides specific heat capacity and confirms the glass transition temperature, which is essential for interpreting conductivity changes at elevated temperatures. Thermogravimetric analysis verifies cure extent and filler loading, while density measurements complete the input data set for laser flash calculations.
Applications
- Qualification of capillary underfills for flip-chip and micro-BGA assemblies in consumer and mobile electronics
- Thermal design input for 2.5D interposers, 3D stacked die, and high-bandwidth memory packaging where the underfill path carries a significant heat flux
- Automotive electronics and power module packaging requiring validated properties across the -40 °C to 150 °C range
- Formulation development and filler optimization by material suppliers comparing resin chemistries and loading levels
- Generation of accurate material libraries for finite element thermal and thermomechanical simulation of packages
- Incoming material inspection and lot-to-lot consistency verification in high-reliability manufacturing
FAQ
What is the typical thermal conductivity range of underfill materials?
Standard silica-filled capillary underfills generally measure between 0.2 and 0.5 W/(m·K), while thermally enhanced grades with boron nitride or alumina fillers can reach 1 to 4 W/(m·K). Reported values should always be referenced to the specific test standard and temperature used.
Which test method should be selected for underfill thermal conductivity?
The laser flash method is preferred for broad temperature sweeps and small specimens, the transient plane source method suits direct measurement of isotropic cured samples, and the steady-state heat flow method best reproduces compressed interface conditions. Selection depends on sample geometry, filler loading, and whether diffusivity or conductivity is the primary deliverable.
Can liquid underfill be tested directly without curing?
No. Thermal conductivity testing requires a dimensionally stable cured specimen. Liquid underfills are cast and cured following the intended process cure profile so the cured network and filler distribution reflect the material as it exists in the final package.
What temperature range can be covered?
Laser flash systems routinely operate from cryogenic temperatures up to 250 °C or higher, covering the entire operating envelope of most underfills. Testing near or above the glass transition temperature should be noted, as conductivity and diffusivity shift measurably through the transition.
How should results from different laboratories or methods be compared?
Comparison is valid only when the same standard, specimen thickness, cure condition, and test temperature are used. Inter-method differences of 5 to 10 % are common between transient and steady-state techniques, so consistency of methodology matters more than the absolute identity of the laboratory.
What measurement uncertainty can be expected?
For well-prepared cured specimens, laser flash conductivity results typically carry combined uncertainties of 5 to 8 %, dominated by specific heat capacity and density inputs. Steady-state measurements on flat, parallel specimens can achieve comparable or better precision when contact resistances are properly characterized.