Thermal Resistance Determination of Thermally Conductive Gels for Photovoltaic Modules

Thermal resistance determination of thermally conductive gels for photovoltaic (PV) modules is a critical verification step for ensuring efficient heat dissipation from solar cells, bypass diodes, and module-level power electronics. By measuring thermal resistance and apparent thermal conductivity according to recognized standards such as ASTM D5470 and ISO 22007-2, accredited third-party laboratories help module manufacturers and material suppliers confirm thermal performance, support IEC 61215 and IEC 61730 reliability assessments, and reduce junction-temperature-related field failures. This article explains the test samples, measured parameters, standard test methods, and typical applications of thermal resistance testing for thermal interface gels used in PV modules.

Technical Overview

Thermally conductive gels, also known as thermal gap filler gels, are soft, curable elastomeric materials, most commonly silicone-based matrices loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, zinc oxide, or boron nitride. In photovoltaic modules they are dispensed between heat-generating components, including solar cells, bypass diodes inside junction boxes, and module-level power electronics, and heat-dissipating structures such as backsheets, front glass, aluminum frames, and housings. Because trapped air has a thermal conductivity of only about 0.026 W/(m·K), replacing interfacial air gaps with a gel exhibiting 1–6 W/(m·K) substantially reduces the temperature drop across the interface and lowers the operating junction temperature of sensitive components.

Thermal resistance quantifies the resistance to heat flow across the material joint and is defined as the temperature difference divided by the heat flow: R = ΔT / Q, expressed in K/W. When normalized by the heat-transfer area, the result is the area-normalized thermal impedance, expressed in K·cm²/W or m²·K/W. The total joint resistance is the sum of the bulk resistance of the gel, which equals the bond-line thickness divided by the thermal conductivity, and the two contact resistances at the gel-substrate interfaces. Accurate determination of thermal resistance therefore requires strict control of bond-line thickness, contact pressure, temperature, and surface conditions.

The principal test methods are the steady-state heat flow meter method described in ASTM D5470, the transient plane source method described in ISO 22007-2, and the laser flash method described in ASTM E1461 for derived thermal conductivity. Specimen conditioning generally follows ISO 291, and the results directly support reliability and safety assessments of PV modules under the IEC 61215 and IEC 61730 series of standards, as well as related national material test standards such as GB/T methods for the thermal conductivity of rubbery and polymeric materials.

Testing Samples

Bulk Gel Materials (One-Part and Two-Part Systems)

Uncured gel is supplied as a dispensable paste. Two-part systems are mixed at the manufacturer-specified ratio using planetary or centrifugal mixing to ensure homogeneity and minimize entrapped air, and one-part systems are used as received. The mixed material is degassed under vacuum before dispensing and cured according to the supplier-recommended schedule, for example 30 minutes at 100 °C or 24 hours at 23 °C, with cure verification by Shore 00 hardness or mass stability.

Cured Gel Specimens (Disks and Plaques)

Cured specimens are prepared as circular disks or square plaques matching the metering area of the test apparatus, typically 25.4 mm or 50 mm in diameter. The recommended specimen thickness ranges from 0.5 mm to 3 mm, and at least three different thicknesses are recommended so that contact resistance can be separated from bulk resistance. The two faces must be flat, smooth, and parallel, and the thickness is measured at the actual test pressure because compliant gels deform under load.

As-Installed Gel Joints

To reproduce real module interfaces, the gel can be sandwiched between representative substrates such as low-iron tempered glass, anodized aluminum, copper or tinned busbars, and multilayer backsheet films. A defined bond line is maintained using precision shims or spacers, and the compression ratio is typically set between 10% and 40% of the free-standing gel thickness to reflect in-service mounting conditions.

Sampling and Conditioning

A representative sample is taken from each production lot, and a minimum of three to five specimens are tested for statistical validity. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours in accordance with ISO 291 before measurement.

Testing Items

The core deliverables of a thermal resistance test program for PV thermal gels include:

• Total thermal resistance (K/W) and area-normalized thermal impedance (K·cm²/W or m²·K/W)
• Apparent thermal conductivity (W/(m·K))
• Interfacial (contact) thermal resistance at the gel-substrate interfaces (K·cm²/W)
• Bond-line thickness and compression ratio at the test pressure
• Thermal resistance as a function of applied pressure, typically 20 kPa to 500 kPa
• Temperature dependence of thermal resistance, typically from 25 °C to 125 °C
• Retention of thermal resistance after environmental aging, including damp heat at 85 °C and 85% relative humidity and thermal cycling from −40 °C to +85 °C in line with IEC 61215 test sequences
• Supporting characterizations such as density, specific heat, and hardness when required for thermal modeling

Testing Methods

Steady-State Heat Flow Meter Method (ASTM D5470)

The specimen is sandwiched between two metal metering bars, usually polished copper or aluminum, whose surfaces contain embedded temperature sensors at precisely known positions. The upper bar is heated and the lower bar is cooled, establishing one-dimensional heat flow through the specimen under a controlled compressive load applied by a pneumatic or screw-driven mechanism. After reaching steady state, the temperature drop across the specimen and the heat flux through the joint are determined, and the total thermal resistance is calculated as R = ΔT / Q. By testing a series of at least three thicknesses and plotting resistance against thickness, the slope of the regression line yields the reciprocal of apparent thermal conductivity, while the intercept at zero thickness gives the combined contact resistance. The method is applicable to materials with thermal conductivity from approximately 0.1 W/(m·K) to 40 W/(m·K) and is the industry-preferred approach for thermal interface materials because it captures both bulk and interfacial effects.

Transient Plane Source Method (ISO 22007-2)

A double-spiral nickel sensor is clamped between two pieces of the cured gel, and a constant electrical power step is applied. The sensor simultaneously acts as a heat source and a resistance thermometer, and the recorded temperature rise over time is fitted to a heat-conduction model to yield thermal conductivity directly. The method requires only small specimens, is fast, and is well suited for screening formulations and comparing intrinsic material conductivity, although it does not reproduce the contact resistance of a compressed joint.

Laser Flash Method (ASTM E1461)

A short laser or flash-lamp pulse heats the front face of a thin cured disk, typically 12.7 mm in diameter and 1–2 mm thick, and an infrared detector records the temperature response of the rear face to determine thermal diffusivity. Thermal conductivity is then derived as the product of diffusivity, specific heat capacity measured by differential scanning calorimetry, and density. The laser flash method offers high accuracy for homogeneous cured materials and is often used to cross-verify steady-state results.

Test Procedure and Conditions

• Cure and condition the specimens, and verify complete crosslinking before testing.
• Measure specimen dimensions and record the thickness at the applied test pressure.
• Install the specimen between the metering surfaces with a thin layer of thermal grease on the reference surfaces where required by the standard.
• Apply the specified contact pressure and allow the assembly to reach thermal equilibrium, typically within 30 to 90 minutes.
• Record steady-state temperatures, heat flux, and thickness; repeat for each pressure and temperature setpoint.
• Calculate total resistance, thermal impedance, apparent thermal conductivity, and contact resistance, and report all test conditions in full.

Applications

Junction Box and Bypass Diode Thermal Management

Thermal gels are widely used to pot or line junction boxes, conducting diode heat into the box housing and the module frame. Verified low thermal resistance keeps bypass diode junction temperatures below their rated limits during shading and reverse-current events, protecting modules during IEC 61215 bypass diode thermal tests and extending service life.

Module-Level Power Electronics Integration

Microinverters, power optimizers, and rapid-shutdown devices mounted behind modules rely on gels to transfer losses to the frame and glass. Thermal resistance data allow designers to predict junction temperatures with thermal simulation and validate derating margins under peak irradiance.

Cell-to-Backsheet and Frame Heat Dissipation

As module power classes increase and glass-glass bifacial designs become common, gels improve lateral heat spreading and reduce hot-spot risk. Measured impedance values feed directly into finite-element thermal models used for yield and degradation estimation.

Material Qualification and Quality Control

Module manufacturers use third-party thermal resistance reports to qualify new gel suppliers, release incoming production lots, and benchmark formulations, while material producers rely on the same data for research and development of filler systems and cure chemistries.

FAQ

What thermal resistance value is considered acceptable for a PV thermal gel?

Acceptance limits are design-specific, but commercially available PV thermal gels typically exhibit apparent thermal conductivity between 1 W/(m·K) and 6 W/(m·K), corresponding to an area-normalized thermal impedance of roughly 0.1 to 1.5 K·cm²/W at a 1 mm bond line. The acceptance criterion is ultimately derived from the allowable junction temperature of the diode or electronic device being protected.

Why do ASTM D5470 and ISO 22007-2 sometimes give different numbers?

ASTM D5470 measures a compressed joint under steady-state conditions and therefore includes contact resistance, which depends on pressure and surface finish, whereas ISO 22007-2 measures the intrinsic thermal conductivity of the cured material. Both values are valid, but they answer different engineering questions and should not be compared directly.

How is contact resistance separated from bulk resistance?

By testing a thickness series of at least three specimens at identical pressure and plotting total resistance against thickness, the linear regression intercept represents the total contact resistance and the slope yields the bulk resistance per unit thickness, from which apparent thermal conductivity is calculated.

Does the thermal resistance of a gel change over time?

It can. Filler settling, oil bleeding, cure reversion, and interfacial delamination during damp heat or thermal cycling may increase thermal resistance. Measuring thermal resistance before and after IEC 61215-type aging sequences verifies long-term stability and is recommended for qualification programs.

How many specimens and how much time are required?

A standard characterization typically requires five cured specimens or approximately 50 g of uncured gel, plus an additional set for any aging study. Routine testing is usually completed within three to five working days, while programs including environmental aging may require several weeks.