Potting compounds & electrical casting resins for electronics: epoxy, silicone, or PU?

At a Glance: Which Potting Compound for Electronics?

Silicone is suitable for high temperatures, flexibility, and thermally or mechanically sensitive assemblies. Polyurethane (PU) is the tough-elastic all-rounder for vibration and media contact. Epoxy offers the highest mechanical strength and chemical resistance, but is rigid. For heat dissipation, all three classes offer thermally conductive variants (higher λ-value). The decisive factors for selection are temperature range, flexibility, thermal conductivity, and media contact; the specific values are provided in the respective technical data sheet.

What are potting compounds?

Potting compounds are liquid or pasty materials that completely encase electronic assemblies and, after curing, provide permanent protection. Unlike conformal coatings, which form only a thin protective layer of 25 to 75 micrometers, potting compounds fill the entire cavity around the electronics. The result is a solid, durable protection against moisture, vibrations, chemicals, temperature fluctuations, and mechanical stress.

Potting compounds prevent corrosion and electrochemical migration caused by moisture, increase the creepage resistance between adjacent conductors, fix components against vibrations and impacts, dissipate heat loss in thermally conductive variants, and protect against chemical influences such as oils, fuels, and cleaning agents. In safety-critical applications, they also serve as tamper protection, as potted assemblies cannot be opened without destruction.

Full encapsulation or selective coating?

Before the material question is clarified, a fundamental decision must be made: Is the assembly to be fully potted (Potting) or only selectively coated (Encapsulation)?

Rule of thumb: IP68/IP69K required → Potting. Repairability needed → Encapsulation. Power dissipation above 5 W → Potting with thermally conductive compound. Weight critical → Encapsulation.

The three material classes compared

Silicone potting compounds

Silicones are the most versatile material class for electronic potting compounds. They remain elastic over an extremely wide temperature range (−60 °C to +200 °C, special types up to +300 °C). The low mechanical stress protects sensitive components and solder joints. For LED applications, silicones are often the only sensible choice: special optical formulations are transparent, do not yellow, and have a suitable refractive index.

Typical applications: LED modules, automotive control units, outdoor electronics, solar inverters, sensors, medical electronics, aerospace.

Epoxy Potting Compounds

Epoxy resins offer the highest mechanical strength (Shore D 70 to 90), excellent adhesion to metals and ceramics, and the highest dielectric strength (up to 25 kV/mm). The main disadvantage: practically not repairable after curing, brittleness under temperature fluctuations, narrower temperature range (−40 to +130 °C).

Typical applications: High-voltage power supplies, transformers, ignition electronics, underwater electronics, tamper protection.

Polyurethane potting compounds (PU)

PU positions itself between epoxy and silicone: a balanced property profile at the lowest cost. Shore hardness adjustable (Shore A 60 to Shore D 50), good abrasion resistance. Main disadvantage: hygroscopic, UV-sensitive, narrowest temperature range (−40 to +120 °C).

Typical applications: Industrial controls, switch-mode power supplies (indoor), E-mobility chargers, BMS modules, building automation.

Comparison table

Thermally Conductive Potting Compounds: The λ Value Decides

Modern electronics operate in ever smaller spaces with increasing power densities. Standard potting compounds tend to be thermally insulating (0.16 to 0.20 W/(m·K)); they protect the electronics but also trap heat within the component.

Rule of thumb: An increase in operating temperature of 10 K can roughly halve the service life of electronic components in many cases.

The λ-value (thermal conductivity, W/(m·K)) describes how well a material conducts heat. Still air: 0.025; unfilled silicones: 0.16 to 0.20; filled silicones: 0.30 to 0.42; hybrid systems: up to 1.05; aluminum: 237.

Thermal conductivity is enhanced by mineral or ceramic fillers: aluminum oxide (Al₂O₃), boron nitride (BN), or silicon carbide (SiC). The higher the filler content, the better the thermal conductivity, but also the higher the viscosity.

When does thermally conductive potting pay off? From approx. 1 W power loss per cm² of component area. For standard sensors: 0.16 to 0.20 W/(m·K). For power electronics: 0.30 to 0.50 W/(m·K). For critical thermal management with fire protection: Permabond MT3836 with 1.05 W/(m·K) and UL 94 V-0.

SILITECH Potting Compound Range

SILITECH AG carries potting compounds of all material classes from stock in Switzerland, from simple protective encapsulation to thermally conductive high-performance potting.

Silicone potting compounds from Elkem (Bluesil) and Dow

One-component systems (CAF series)

The CAF series from Elkem comprises 1K silicone elastomers that cure at room temperature through contact with atmospheric moisture. Ready to use, no mixing required.

The CAF product numbers do not indicate Shore hardness. CAF stands for «Compound à Froid» (cold-curing compound). The technical data sheet is always decisive for the correct selection.

Two-component systems (addition curing)

Addition-curing 2K silicones cure via platinum catalysis without by-products. Precisely controllable pot life and curing times, almost no shrinkage.

Which silicone system for which application? For transparent potting: RTV 141, ESA 7250, or DOWSIL EI-2888. If heat dissipation is critical: RTV 147/148 (λ = 0.31) or ESA 7252 (λ = 0.42). For simple sealing without mixing: CAF series. For flame retardancy UL 94 V0: ESA 7252. For outdoor LEDs without primer: DOWSIL EI-2888.

PU casting resins (SILIRESIN Biothan)

Bio-based PU casting resins based on renewable raw materials. Label-free (neither resin nor hardener), VOC 0.0%, shrinkage < 0.1%.

Notable: Biothan 2 MD 207 E achieves a performance profile with λ = 0.455 W/(m·K) and UL 94 V-0 that exceeds many silicone potting compounds, at a significantly lower price.

Epoxy and hybrid potting compounds (Permabond, Loctite)

Classic Epoxies

Flexible modified epoxies (MT series): for electronics potting

Permabond's MT series combines epoxy chemistry with flexibility. Soft to medium-hard, high elongation at break, good substrate adhesion.

Thermally Conductive Hybrid Potting Compound

MT3836 is particularly interesting where heat dissipation and flame retardancy are required simultaneously, such as in battery management systems, power electronics, and e-mobility charging modules. With λ = 1.05 W/(m·K), it significantly outperforms all silicone potting compounds in the range.

Permabond PU structural adhesives (also for potting)

Thermal Greases

Material selection by application

Processing instructions

Mixing Ratio and Dosing

All 2K potting compounds require precise adherence to the mixing ratio. Deviations of more than ±5% result in incomplete curing, a tacky surface, or reduced mechanical strength.

Vacuum Degassing

Air bubbles significantly reduce dielectric strength and create thermal weak points. Vacuum degassing at 30 to 50 mbar is essential for high-quality potting compounds. Low-viscosity systems (RTV 141: 4'000 mPa·s) deaerate more easily than high-viscosity ones (RTV 147: 150'000 mPa·s).

Curing

Most silicone potting compounds cure at RT and can be accelerated by heat: 4 h at 60 °C, 2 h at 100 °C, or 1 h at 150 °C. Heating too quickly (> 3 °C/min) can cause stress cracks.

Caution, inhibition with addition silicones: Contact with sulfur-containing rubbers, tin-catalyzed silicones, amine-cured epoxies, or tin-stabilized PVC can block platinum catalysis. When in doubt, perform a preliminary test on a small area.

Frequently Asked Questions