Thermally conductive potting compounds: λ values explained
When power electronics overheat, even the best heat sink is useless—unless the heat can escape from the encapsulated component. Thermally conductive potting compounds with a high λ value do just that: they protect electronics from environmental influences while simultaneously dissipating heat loss in a targeted manner. But what exactly does the λ value mean, which fillers increase thermal conductivity, and when is it worthwhile to use thermally conductive potting compounds?
Why thermal conductivity is crucial for casting compounds
Modern electronic assemblies operate in increasingly smaller spaces with rising power densities. LED drivers, DC/DC converters, battery management systems, and motor controllers generate heat loss that must be dissipated. Standard epoxy- or silicone-based potting compounds offer excellent protection against moisture, chemicals, and mechanical stress, but they also act as thermal insulators.
The consequences of insufficient heat dissipation are measurable: for every 10 Kelvin increase in junction temperature, the service life of semiconductors is statistically halved (Arrhenius law). Hot spots occur when heat is not distributed evenly. Power components must be throttled (derating), which means that systems cannot reach their full performance. In critical applications such as e-mobility battery packs or high-performance LED modules, overheating leads to failures or safety risks.
Thermally conductive potting compounds solve this problem by containing thermally conductive fillers. These form heat conduction paths through the polymer matrix and enable heat transfer from the component to the housing or circuit board. Modern formulations thus combine the protective function of classic potting compounds with active thermal management.
What is the λ value (lambda)?
The λ value—also known as thermal conductivity—describes how well a material conducts heat. The physical unit is watts per meter and Kelvin (W/m·K). A higher λ value means better heat conduction.
Typical λ values for comparison:
- Copper: 390 W/m·K (excellent heat conductor)
- Aluminum: 235 W/m·K
- Standard epoxy resin: 0.2–0.3 W/m·K
- Standard silicone: 0.15–0.25 W/m·K
- Thermal conductive potting compound: 0.5–3.0 W/m·K
- High-performance thermal paste: up to 15 W/m·K
The measurement is performed according to standardized methods such as ASTM D5470 (laser flash method) or ISO 22007 (hot disk method). A defined heat flow is passed through a material sample and the resulting temperature difference is measured. Important: The λ value is determined in the cured state – manufacturer specifications always refer to fully cross-linked casting compounds.
Practical tip: λ value versus thermal resistance
The λ value is a material property, but it does not say anything about the actual cooling effect. The decisive factor is the thermal resistance Rth of the entire potting layer: Rth = d / (λ × A), where d is the layer thickness and A is the area. A 5 mm thick layer with λ = 1 W/m·K dissipates heat less effectively than a 2 mm layer with λ = 0.8 W/m·K. You should therefore optimize your choice of material and geometry.
Comparison: Standard potting vs. thermally conductive
The differences between conventional and thermally conductive potting compounds go beyond the λ value. The table shows typical property profiles:
| property | Standard casting compound | Thermally conductive potting compound |
|---|---|---|
| Thermal conductivity λ | 0.2–0.3 W/m·K | 0.6–3.0 W/m·K |
| filler content | 0–20% by weight | 40–75% by weight |
| Viscosity (uncured) | 1,000–10,000 mPa·s | 10,000–80,000 mPa·s |
| Shore hardness (cured) | Shore A 30–80 | Shore A 50–90 / Shore D 30–60 |
| density | 1.0–1.2 g/cm³ | 1.8–2.8 g/cm³ |
| processing | Pouring, dosing, vacuum optional | Vacuum degassing recommended, agitator required |
| Price (relative) | € | €€–€€€ |
The high filler content of thermally conductive potting compounds poses challenges: viscosity increases significantly, which makes venting more difficult. The higher density requires adapted dosing systems. In addition, fillers tend to sediment if the material is left to stand for a long time before processing. In return, you get significantly improved heat dissipation with consistent electrical insulation properties.
Fillers and their effect
The thermal conductivity of the casting compound depends directly on the type, quantity, and shape of the fillers used. Polymer matrices (epoxy, silicone, polyurethane) themselves are poor heat conductors—only the fillers create continuous heat conduction paths. The following materials are used:
Aluminum oxide (Al₂O₃)
The most commonly used filler for thermally conductive potting compounds. Aluminum oxide offers excellent value for money and achieves λ values of 0.8–1.5 W/m·K at high fill levels (60–70 wt%). The white particles are electrically insulating, chemically inert, and available in various grain sizes. By combining different particle sizes (bimodal distribution), the packing density can be optimized—fine particles fill the spaces between coarse ones.
Boron nitride (BN)
Hexagonal boron nitride is also known as "white graphite" and exhibits pronounced thermal anisotropy: heat is conducted excellently along the crystal planes. Casting compounds with BN filler achieve λ values of 1.5–3.0 W/m·K with a low dielectric constant – ideal for high-frequency applications. The disadvantage: boron nitride is significantly more expensive than aluminum oxide and more difficult to process, as the platelet-shaped particles tend to orient themselves.
Aluminum nitride (AlN)
With an intrinsic thermal conductivity of over 200 W/m·K, aluminum nitride is one of the most effective ceramic fillers. Casting compounds achieve λ values of up to 2.5 W/m·K. AlN is electrically insulating and, unlike boron nitride, does not exhibit anisotropy. Its high cost and sensitivity to moisture are limiting factors – AlN reacts with water to form aluminum oxide and ammonia, which is why careful drying and storage are required.
Metallic fillers (silver, aluminum)
Silver flakes or aluminum powder enable thermal conductivities above 3 W/m·K—but at the expense of electrical insulation. These electrically conductive potting compounds are used where ground connections or EMC shielding are specifically required. They are unsuitable for classic insulation applications.
Applications
Thermally conductive potting compounds are used wherever electronics need to be both protected and cooled:
LED lighting and high-performance LEDs
LED chips achieve power densities of several watts per square millimeter. The junction temperature determines brightness, color location, and service life. Thermally conductive silicone potting compounds (λ = 0.8–1.2 W/m·K) encapsulate LED modules and conduct heat to the aluminum heat sink. Mechanical flexibility compensates for thermal expansion, while UV stability ensures long-term transparency. Bluesil formulations for LED applications achieve Shore A 40–60 at λ values around 1.0 W/m·K.
Power electronics and frequency converters
IGBT modules, MOSFET bridge circuits, and DC/DC converters generate considerable losses during switching operation. Epoxy-based potting compounds with λ = 1.5–2.0 W/m·K offer the necessary mechanical strength (Shore D 50–70) and temperature resistance up to 150°C. The high dielectric strength protects against leakage currents, while heat dissipation prevents overheated barrier layers. Typical layer thicknesses: 3–8 mm.
E-mobility: Battery management systems and charging electronics
Automotive applications require temperature resistance from –40°C to +125°C, mechanical robustness against vibration, and long-term durability. Thermally conductive polyurethane or silicone potting compounds protect BMS boards in high-voltage battery packs and dissipate heat. At the same time, they must comply with UL94-V0 flame retardancy. Requirements for λ values: at least 1.0 W/m·K for active cooling, up to 2.0 W/m·K for passive systems.
Power supplies and power sources
Switching power supplies combine high component density with permanent thermal load. Transformers, rectifiers, and electrolytic capacitors benefit from thermally conductive potting, which directs heat to metal housings or base plates. With 2-component silicones, the long pot life (20–60 minutes) allows even complex geometries to be completely filled.
Selection criteria: Determining the correct λ value
Higher thermal conductivity always sounds better at first – but it comes with higher costs, more difficult processing, and often higher mechanical hardness. The choice of material should be based on thermal calculations:
1. Determine power loss
What thermal power P (in watts) must be dissipated? Values from data sheets for power semiconductors or measurements during operation.
2. Set the temperature difference
What temperature difference ΔT between the component and heat sink is permissible? Typically: 20–40 Kelvin, depending on the maximum junction temperature and ambient temperature.
3. Calculate thermal resistance
Rth = ΔT / P (unit: K/W). This is the maximum permissible thermal resistance of the encapsulation layer.
4. Determine the required λ value
λ = d / (Rth × A), where d is the casting layer thickness in meters and A is the heat transfer area in square meters. Include a safety factor of 1.3–1.5 to compensate for manufacturing tolerances and aging.
Sample calculation: An LED module generates 10 W of heat loss, which must be dissipated via a 5 mm thick potting layer with an area of 50 cm². Permissible temperature difference: 30 K.
- Rth = 30 K / 10 W = 3 K/W
- λ = 0.005 m / (3 K/W × 0.005 m²) = 0.33 W/m·K
- With safety factor 1.4: λ ≥ 0.46 W/m·K
A casting compound with λ = 0.8 W/m·K would be sufficiently dimensioned here.
Additional selection criteria: chemical resistance (coolants, oils), temperature range, Shore hardness (vibration absorption), electrical insulation strength (creepage current resistance CTI), processability (pot life, venting), and approvals (UL, REACH, RoHS).
Processing tips
The high viscosity and filler content of thermally conductive potting compounds require adapted processing techniques:
Mixing and homogenizing
Fillers settle during storage. Thorough stirring is essential before processing. For 2-component systems, both components should be homogenized separately before mixing. Planetary mixers or twin-screw mixers ensure even distribution of fillers. Mixing time: at least 2–3 minutes for container sizes over 1 kg.
vacuum degassing
Air pockets drastically reduce effective thermal conductivity – an air bubble with λ = 0.026 W/m·K acts as a thermal barrier. Degassing at 10–50 mbar for 5–10 minutes after mixing removes any air that has been stirred in. For large casting volumes, casting can also be carried out directly in the vacuum chamber. Caution: Vacuuming for too long shortens the pot life of reactive systems.
Dosage and flow characteristics
The viscosity is often between 20,000 and 60,000 mPa·s – significantly higher than with standard potting compounds. Gear pumps or eccentric screw pumps are more suitable than piston dispensers. For complex assemblies, pour the casting material slowly from the lowest point and allow it to rise so that air can escape. Heating to 30–40°C reduces viscosity and improves flow behavior, but shortens the pot life.
curing
The high filler content slows down heat dissipation during exothermic reactions. Uncontrolled heat generation can occur with thick casting layers (> 20 mm) and fast-reacting epoxy systems. Remedy: Gradual curing (e.g., 2 hours at 60°C, then 4 hours at 80°C) or use of slow-curing formulations. Silicone casting compounds cure without exothermic reactions.
Aftercare and quality control
After curing, the quality of the casting should be checked: visual inspection for bubbles, Shore hardness measurement to check cross-linking, thermographic imaging under load to verify heat dissipation. For safety-critical applications, the dielectric strength can be checked on a random basis.
Frequently asked questions (FAQ)
Conclusion: Measurably improve thermal performance
Thermally conductive potting compounds are more than just an upgrade—they enable electronic designs that would not work thermally with standard potting. The λ value indicates the material's capability, but the actual cooling effect depends on layer thickness, surface area, and the overall system. Aluminum oxide-filled systems offer the best price-performance ratio for most applications, while boron nitride and aluminum nitride remain reserved for high-performance thermal management.
The processing requires more care than standard encapsulation—homogenization, degassing, and customized dosing technology are crucial for reproducible results. In return, you receive measurable benefits: lower barrier layer temperatures, longer service life, higher system performance, and improved reliability.
When selecting a product, the rule is: as much thermal conductivity as necessary, not as much as possible. A well-thought-out thermal calculation prevents over-engineering and keeps costs within limits. Bluesil casting compounds offer tailored solutions for different λ requirements – from flexible silicone formulations with λ = 0.8 W/m·K to highly filled systems with λ = 2.5 W/m·K.
Thermally conductive potting compounds for your application
Our materials specialists will help you select the ideal thermally conductive potting compound—with thermal calculations, samples, and processing advice. SILITECH offers Bluesil potting systems with λ values ranging from 0.6 to 3.0 W/m·K for electronics, LEDs, e-mobility, and industrial applications.
SILITECH AG
Worbstrasse 173
3073 Gümligen
Switzerland
Tel: +41 31 398 50 70
Email: info@silitech.ch
Thermally conductive potting compounds: λ values explained
When power electronics overheat, even the best heat sink is useless—unless the heat can escape from the encapsulated component. Thermally conductive potting compounds with a high λ value do just that: they protect electronics from environmental influences while simultaneously dissipating heat loss in a targeted manner. But what exactly does the λ value mean, which fillers increase thermal conductivity, and when is it worthwhile to use thermally conductive potting compounds?
Why thermal conductivity is crucial for casting compounds
Modern electronic assemblies operate in increasingly smaller spaces with rising power densities. LED drivers, DC/DC converters, battery management systems, and motor controllers generate heat loss that must be dissipated. Standard epoxy- or silicone-based potting compounds offer excellent protection against moisture, chemicals, and mechanical stress, but they also act as thermal insulators.
The consequences of insufficient heat dissipation are measurable: for every 10 Kelvin increase in junction temperature, the service life of semiconductors is statistically halved (Arrhenius law). Hot spots occur when heat is not distributed evenly. Power components must be throttled (derating), which means that systems cannot reach their full performance. In critical applications such as e-mobility battery packs or high-performance LED modules, overheating leads to failures or safety risks.
Thermally conductive potting compounds solve this problem by containing thermally conductive fillers. These form heat conduction paths through the polymer matrix and enable heat transfer from the component to the housing or circuit board. Modern formulations thus combine the protective function of classic potting compounds with active thermal management.
What is the λ value (lambda)?
The λ value—also known as thermal conductivity—describes how well a material conducts heat. The physical unit is watts per meter and Kelvin (W/m·K). A higher λ value means better heat conduction.
Typical λ values for comparison:
- Copper: 390 W/m·K (excellent heat conductor)
- Aluminum: 235 W/m·K
- Standard epoxy resin: 0.2–0.3 W/m·K
- Standard silicone: 0.15–0.25 W/m·K
- Thermal conductive potting compound: 0.5–3.0 W/m·K
- High-performance thermal paste: up to 15 W/m·K
The measurement is performed according to standardized methods such as ASTM D5470 (laser flash method) or ISO 22007 (hot disk method). A defined heat flow is passed through a material sample and the resulting temperature difference is measured. Important: The λ value is determined in the cured state – manufacturer specifications always refer to fully cross-linked casting compounds.
Practical tip: λ value versus thermal resistance
The λ value is a material property, but it does not say anything about the actual cooling effect. The decisive factor is the thermal resistance Rth of the entire potting layer: Rth = d / (λ × A), where d is the layer thickness and A is the area. A 5 mm thick layer with λ = 1 W/m·K dissipates heat less effectively than a 2 mm layer with λ = 0.8 W/m·K. You should therefore optimize your choice of material and geometry.
Comparison: Standard potting vs. thermally conductive
The differences between conventional and thermally conductive potting compounds go beyond the λ value. The table shows typical property profiles:
| property | Standard casting compound | Thermally conductive potting compound |
|---|---|---|
| Thermal conductivity λ | 0.2–0.3 W/m·K | 0.6–3.0 W/m·K |
| filler content | 0–20% by weight | 40–75% by weight |
| Viscosity (uncured) | 1,000–10,000 mPa·s | 10,000–80,000 mPa·s |
| Shore hardness (cured) | Shore A 30–80 | Shore A 50–90 / Shore D 30–60 |
| density | 1.0–1.2 g/cm³ | 1.8–2.8 g/cm³ |
| processing | Pouring, dosing, vacuum optional | Vacuum degassing recommended, agitator required |
| Price (relative) | € | €€–€€€ |
The high filler content of thermally conductive potting compounds poses challenges: viscosity increases significantly, which makes venting more difficult. The higher density requires adapted dosing systems. In addition, fillers tend to sediment if the material is left to stand for a long time before processing. In return, you get significantly improved heat dissipation with consistent electrical insulation properties.
Fillers and their effect
The thermal conductivity of the casting compound depends directly on the type, quantity, and shape of the fillers used. Polymer matrices (epoxy, silicone, polyurethane) themselves are poor heat conductors—only the fillers create continuous heat conduction paths. The following materials are used:
Aluminum oxide (Al₂O₃)
The most commonly used filler for thermally conductive potting compounds. Aluminum oxide offers excellent value for money and achieves λ values of 0.8–1.5 W/m·K at high fill levels (60–70 wt%). The white particles are electrically insulating, chemically inert, and available in various grain sizes. By combining different particle sizes (bimodal distribution), the packing density can be optimized—fine particles fill the spaces between coarse ones.
Boron nitride (BN)
Hexagonal boron nitride is also known as "white graphite" and exhibits pronounced thermal anisotropy: heat is conducted excellently along the crystal planes. Casting compounds with BN filler achieve λ values of 1.5–3.0 W/m·K with a low dielectric constant – ideal for high-frequency applications. The disadvantage: boron nitride is significantly more expensive than aluminum oxide and more difficult to process, as the platelet-shaped particles tend to orient themselves.
Aluminum nitride (AlN)
With an intrinsic thermal conductivity of over 200 W/m·K, aluminum nitride is one of the most effective ceramic fillers. Casting compounds achieve λ values of up to 2.5 W/m·K. AlN is electrically insulating and, unlike boron nitride, does not exhibit anisotropy. Its high cost and sensitivity to moisture are limiting factors – AlN reacts with water to form aluminum oxide and ammonia, which is why careful drying and storage are required.
Metallic fillers (silver, aluminum)
Silver flakes or aluminum powder enable thermal conductivities above 3 W/m·K—but at the expense of electrical insulation. These electrically conductive potting compounds are used where ground connections or EMC shielding are specifically required. They are unsuitable for classic insulation applications.
Applications
Thermally conductive potting compounds are used wherever electronics need to be both protected and cooled:
LED lighting and high-performance LEDs
LED chips achieve power densities of several watts per square millimeter. The junction temperature determines brightness, color location, and service life. Thermally conductive silicone potting compounds (λ = 0.8–1.2 W/m·K) encapsulate LED modules and conduct heat to the aluminum heat sink. Mechanical flexibility compensates for thermal expansion, while UV stability ensures long-term transparency. Bluesil formulations for LED applications achieve Shore A 40–60 at λ values around 1.0 W/m·K.
Power electronics and frequency converters
IGBT modules, MOSFET bridge circuits, and DC/DC converters generate considerable losses during switching operation. Epoxy-based potting compounds with λ = 1.5–2.0 W/m·K offer the necessary mechanical strength (Shore D 50–70) and temperature resistance up to 150°C. The high dielectric strength protects against leakage currents, while heat dissipation prevents overheated barrier layers. Typical layer thicknesses: 3–8 mm.
E-mobility: Battery management systems and charging electronics
Automotive applications require temperature resistance from –40°C to +125°C, mechanical robustness against vibration, and long-term durability. Thermally conductive polyurethane or silicone potting compounds protect BMS boards in high-voltage battery packs and dissipate heat. At the same time, they must comply with UL94-V0 flame retardancy. Requirements for λ values: at least 1.0 W/m·K for active cooling, up to 2.0 W/m·K for passive systems.
Power supplies and power sources
Switching power supplies combine high component density with permanent thermal load. Transformers, rectifiers, and electrolytic capacitors benefit from thermally conductive potting, which directs heat to metal housings or base plates. With 2-component silicones, the long pot life (20–60 minutes) allows even complex geometries to be completely filled.
Selection criteria: Determining the correct λ value
Higher thermal conductivity always sounds better at first – but it comes with higher costs, more difficult processing, and often higher mechanical hardness. The choice of material should be based on thermal calculations:
1. Determine power loss
What thermal power P (in watts) must be dissipated? Values from data sheets for power semiconductors or measurements during operation.
2. Set the temperature difference
What temperature difference ΔT between the component and heat sink is permissible? Typically: 20–40 Kelvin, depending on the maximum junction temperature and ambient temperature.
3. Calculate thermal resistance
Rth = ΔT / P (unit: K/W). This is the maximum permissible thermal resistance of the encapsulation layer.
4. Determine the required λ value
λ = d / (Rth × A), where d is the casting layer thickness in meters and A is the heat transfer area in square meters. Include a safety factor of 1.3–1.5 to compensate for manufacturing tolerances and aging.
Sample calculation: An LED module generates 10 W of heat loss, which must be dissipated via a 5 mm thick potting layer with an area of 50 cm². Permissible temperature difference: 30 K.
- Rth = 30 K / 10 W = 3 K/W
- λ = 0.005 m / (3 K/W × 0.005 m²) = 0.33 W/m·K
- With safety factor 1.4: λ ≥ 0.46 W/m·K
A casting compound with λ = 0.8 W/m·K would be sufficiently dimensioned here.
Additional selection criteria: chemical resistance (coolants, oils), temperature range, Shore hardness (vibration absorption), electrical insulation strength (creepage current resistance CTI), processability (pot life, venting), and approvals (UL, REACH, RoHS).
Processing tips
The high viscosity and filler content of thermally conductive potting compounds require adapted processing techniques:
Mixing and homogenizing
Fillers settle during storage. Thorough stirring is essential before processing. For 2-component systems, both components should be homogenized separately before mixing. Planetary mixers or twin-screw mixers ensure even distribution of fillers. Mixing time: at least 2–3 minutes for container sizes over 1 kg.
vacuum degassing
Air pockets drastically reduce effective thermal conductivity – an air bubble with λ = 0.026 W/m·K acts as a thermal barrier. Degassing at 10–50 mbar for 5–10 minutes after mixing removes any air that has been stirred in. For large casting volumes, casting can also be carried out directly in the vacuum chamber. Caution: Vacuuming for too long shortens the pot life of reactive systems.
Dosage and flow characteristics
The viscosity is often between 20,000 and 60,000 mPa·s – significantly higher than with standard potting compounds. Gear pumps or eccentric screw pumps are more suitable than piston dispensers. For complex assemblies, pour the casting material slowly from the lowest point and allow it to rise so that air can escape. Heating to 30–40°C reduces viscosity and improves flow behavior, but shortens the pot life.
curing
The high filler content slows down heat dissipation during exothermic reactions. Uncontrolled heat generation can occur with thick casting layers (> 20 mm) and fast-reacting epoxy systems. Remedy: Gradual curing (e.g., 2 hours at 60°C, then 4 hours at 80°C) or use of slow-curing formulations. Silicone casting compounds cure without exothermic reactions.
Aftercare and quality control
After curing, the quality of the casting should be checked: visual inspection for bubbles, Shore hardness measurement to check cross-linking, thermographic imaging under load to verify heat dissipation. For safety-critical applications, the dielectric strength can be checked on a random basis.
Frequently asked questions (FAQ)
Conclusion: Measurably improve thermal performance
Thermally conductive potting compounds are more than just an upgrade—they enable electronic designs that would not work thermally with standard potting. The λ value indicates the material's capability, but the actual cooling effect depends on layer thickness, surface area, and the overall system. Aluminum oxide-filled systems offer the best price-performance ratio for most applications, while boron nitride and aluminum nitride remain reserved for high-performance thermal management.
The processing requires more care than standard encapsulation—homogenization, degassing, and customized dosing technology are crucial for reproducible results. In return, you receive measurable benefits: lower barrier layer temperatures, longer service life, higher system performance, and improved reliability.
When selecting a product, the rule is: as much thermal conductivity as necessary, not as much as possible. A well-thought-out thermal calculation prevents over-engineering and keeps costs within limits. Bluesil casting compounds offer tailored solutions for different λ requirements – from flexible silicone formulations with λ = 0.8 W/m·K to highly filled systems with λ = 2.5 W/m·K.
Thermally conductive potting compounds for your application
Our materials specialists will help you select the ideal thermally conductive potting compound—with thermal calculations, samples, and processing advice. SILITECH offers Bluesil potting systems with λ values ranging from 0.6 to 3.0 W/m·K for electronics, LEDs, e-mobility, and industrial applications.
SILITECH AG
Worbstrasse 173
3073 Gümligen
Switzerland
Tel: +41 31 398 50 70
Email: info@silitech.ch
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