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

To the point: Which potting compound is suitable 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. Thermally conductive variants (higher λ-value) are available for all three classes to facilitate heat dissipation . The key selection criteria are temperature range, flexibility, thermal conductivity, and media contact—specific parameters can be found in the respective technical data sheet.

What are potting compounds?

Potting compounds are liquid or paste-like materials that completely encapsulate electronic assemblies and provide permanent protection after curing. 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 robust, long-lasting protection against moisture, vibration, chemicals, temperature fluctuations, and mechanical stress.

Potting compounds prevent corrosion and electrochemical migration caused by moisture, increase tracking resistance between adjacent conductors, secure components against vibrations and shocks, dissipate heat loss (in thermally conductive versions), 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 damage.

Full encapsulation or selective coating?

Before the material question is resolved, a fundamental decision must be made: Will the assembly be completely potted or only selectively coated?

Rule of thumb: IP68/IP69K required → potting. Repairability necessary → encapsulation. Power dissipation above 5 W → potting with thermally conductive compound. Weight critical → encapsulation.

A comparison of the three material classes

Silicone potting compounds

Silicones are the most versatile material class for electronic potting. 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 70–90), excellent adhesion to metals and ceramics, and the highest dielectric strength (up to 25 kV/mm). Their biggest disadvantages: practically irreparable after curing, brittleness under temperature changes, and a 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 is adjustable (Shore A 60 to Shore D 50), and it offers good abrasion resistance. Its main disadvantages are its hygroscopic nature, UV sensitivity, and narrow temperature range (−40 to +120 °C).

Typical applications: industrial controls, switched-mode power supplies (indoor), e-mobility chargers, BMS modules, building automation.

Comparison table

Thermally conductive potting compounds: The λ-value is decisive

Modern electronics operate in increasingly smaller spaces with rising power densities. Standard potting compounds tend to have a thermally insulating effect (0.16–0.20 W/(m·K)) — they protect the electronics, but at the same time trap the heat within the component.

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

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

Thermal conductivity is increased 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 become worthwhile? From approximately 1 W power dissipation per cm² of component area. For standard sensors: 0.16–0.20 W/(m·K). For power electronics: 0.30–0.50 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 stocks potting compounds of all material classes from Switzerland — from simple protective coatings to high-performance thermally conductive potting compounds.

Silicone potting compounds from Elkem (Bluesil) and Dow

Single-component systems (CAF series)

Elkem's CAF range comprises 1K silicone elastomers that cure at room temperature upon contact with atmospheric moisture. Ready to use, no mixing required.

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

Two-component systems (additional networking)

Addition-curing two-component silicones cure via platinum catalysis without byproducts. Precisely controllable pot life and curing times, virtually no shrinkage.

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

PU electrocasting resins (SILIRESIN Biothane)

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

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

Epoxy and hybrid potting compounds (Permabond, Loctite)

Classic epoxides

Flexible modified epoxides (MT series) — for electronics potting

Permabonds MT series combines epoxy chemistry with flexibility. Soft to medium strength, 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—for example, in battery management systems, power electronics, and e-mobility charging modules. With a thermal conductivity of λ = 1.05 W/(m·K), it significantly outperforms all other silicone potting compounds in the product range.

Permabond PU structural adhesives (also for potting)

thermal paste

Material selection according to application

Processing instructions

Mixing ratio and dosage

All two-component potting compounds require precise adherence to the mixing ratio. Deviations of more than ±5% lead to incomplete curing, a sticky surface, or reduced mechanical strength.

Vacuum degassing

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

Curing

Most silicone potting compounds cure at room temperature and can be accelerated by heat: 4 hours at 60°C, 2 hours at 100°C, or 1 hour at 150°C. Excessive heating (> 3°C/min) can cause stress cracking.

Caution — Inhibition in addition silicones: Contact with sulfur-containing rubbers, tin-catalyzed silicones, amine-cured epoxides, or tin-stabilized PVC can block platinum catalysis. In case of doubt, conduct a preliminary test on a small area.

Frequently Asked Questions