As electronic devices become increasingly miniaturized and high-powered, heat dissipation has emerged as a critical bottleneck affecting performance and lifespan. Core components like chips, batteries, and LEDs generate significant heat during operation. If not effectively managed, this can lead to excessive temperature rises, performance degradation, or even thermal runaway. Traditional metal heat sinks, while excellent conductors of heat, lack electrical insulation and struggle to conform to irregular surfaces. Conversely, conventional silicone rubber is flexible and electrically insulating but is a poor conductor of heat (with a thermal conductivity of only about 0.2 W/m·K). Enter thermal conductive silicone rubber—a material designed to maintain the intrinsic advantages of silicone rubber while adding the seemingly contradictory ability to conduct heat efficiently.

Key Concepts in Thermal Conductive Silicone Rubber

The core concept involves introducing high thermal conductivity fillers into the insulating silicone rubber matrix to create continuous or quasi-continuous heat conduction pathways. Since amorphous polymers like silicone rubber severely scatter phonons (the primary carriers of heat), their intrinsic thermal conductivity is very low. Inorganic fillers such as aluminum oxide (Al₂O₃), boron nitride (BN), aluminum nitride (AlN), and magnesium oxide (MgO) have crystalline structures with long phonon mean free paths, achieving thermal conductivities ranging from 30 to 300 W/m·K. When these fillers are uniformly dispersed at high volume fractions (typically >50%) within the silicone matrix and form interconnected networks, they facilitate efficient heat transfer along filler pathways.

Challenges and Solutions in Filler Addition

Adding more fillers isn't always better; excessive amounts can result in increased hardness, loss of elasticity, processing difficulties, and potential compromise of electrical insulation if conductive fillers like metals or graphite are used. Therefore, the selection and combination of fillers are crucial:

Spherical Al₂O₃: Offers good flowability and cost-effectiveness, suitable for moderate thermal conductivity requirements (1–3 W/m·K).

Plate-like BN: Though more expensive, its two-dimensional structure aligns under shear forces, creating high in-plane thermal conductivity paths and maintaining excellent electrical insulation, ideal for high-frequency electronics.

AlN: With high thermal conductivity (>170 W/m·K), it requires surface modification to prevent hydrolysis.

Surface Modification of Fillers

Surface modification of inorganic particles is essential because untreated particles have poor compatibility with organic silicone matrices, leading to agglomeration and forming thermal resistance interfaces. Using silane coupling agents (e.g., KH-550, KH-560) to coat the fillers enhances interfacial bonding and reduces phonon scattering at the "filler-matrix" interface, thereby improving overall thermal efficiency. Some advanced formulations even use core-shell structured fillers (e.g., Al₂O₃@SiO₂) to balance thermal conductivity and dielectric properties.

Matrix Selection

Addition-cured liquid silicone rubber (LSR) is preferred for thermal conductive silicone pads and potting compounds due to its low shrinkage, high purity, and adjustable crosslink density. Its curing process produces no by-products, avoiding bubble formation—an important consideration since air pockets significantly reduce effective thermal conductivity.

Application-Specific Formulations

Different applications require tailored formulations:

Thermal pads between battery modules: Need balanced thermal conductivity (3–8 W/m·K), compressibility, resilience, and flame retardancy.

CPU/GPU thermal interface materials (TIM): Require ultra-thin profiles (<0.1mm), low thermal resistance, and long-term reliability.

LED packaging: Demands high transparency and anti-yellowing properties alongside thermal conductivity.

Engineers adjust parameters like filler particle size distribution (combining coarse and fine particles to increase packing density), crosslink density (affecting elastic modulus), and additive types (such as flame retardants and antioxidants) to meet these diverse needs.

Advantages of Thermal Conductive Silicone Rubber

Despite enhanced thermal conductivity, thermal conductive silicone rubber retains other benefits of silicone rubber, including electrical insulation (volume resistivity > 10¹³ Ω·cm), broad operating temperature range (-50°C to 200°C), flexibility, and compressibility, which help fill microscopic gaps at interfaces and significantly reduce contact thermal resistance—a capability rigid thermal materials cannot match.

In summary, designing thermal conductive silicone rubber involves finding the optimal balance among thermal conductivity, electrical insulation, flexibility, and processability. It transforms what was once an insulating material into an efficient thermal pathway, silently ensuring the cool operation of every chip.



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