Heat Dissipation

Thermally Conductive Silicone Compounds, Gap Fillers, and TIM Materials

Thermal interface materials (TIM) are the conduction path between heat-generating components (CPUs, GPUs, power modules, LEDs) and heat sinks. Without TIM, microscopic surface roughness traps insulating air pockets at the interface, reducing thermal transfer by 90% or more. With properly applied TIM, junction-to-heatsink thermal resistance can be reduced to 0.05–0.20 K·cm²/W — close to the bulk thermal resistance of the silicone or grease layer alone.

Silicone-based TIMs dominate the market because silicones combine three properties no other polymer chemistry matches:

Low modulus (conforms to surface roughness even at thin bondlines)
Service temperature -50 °C to +200 °C (covers all electronics and most automotive applications)
Chemical inertness (no migration into electronics components, no degradation under power cycling)

The four major silicone-TIM product categories are: thermal grease (for CPU/GPU thermal pasting), thermal pads (pre-formed elastomer sheets), thermal gap-fillers (dispensable two-component cures), and thermally conductive RTV adhesives.

Thermal Conductivity by Filler Loading

Pure silicone has thermal conductivity of only 0.16 W/m·K. To reach the 1–10 W/m·K range required for power electronics, the silicone is loaded with thermally conductive fillers:

Filler	Thermal Conductivity (W/m·K)	Typical Loading	Notes
Aluminum oxide (Al₂O₃)	25–40	65–80 wt%	Cost-effective baseline
Aluminum nitride (AlN)	150–220	50–70 wt%	High-performance, moisture-sensitive
Boron nitride (BN)	250–300	30–50 wt%	Premium, electrically insulating
Zinc oxide (ZnO)	50–60	50–70 wt%	Cost-effective for grease
Magnesium oxide (MgO)	30–60	60–75 wt%	Cost between Al₂O₃ and AlN
Silver	400	Low loading (< 30 wt%)	Electrically conductive

The achievable system thermal conductivity is determined by both the filler thermal conductivity and the loading: more filler raises k but also raises viscosity, eventually reaching a paste that cannot be dispensed. Most commercial 1–6 W/m·K TIMs use Al₂O₃ at 65–75 wt% loading; premium 8–14 W/m·K TIMs use BN or AlN at 40–60 wt% loading.

Product Categories

Thermal grease (typical k = 1.0–6.0 W/m·K): silicone fluid (300–1000 cSt) plus thermally conductive filler in a non-curing paste. Applied as a thin (50–200 μm) layer between CPU/GPU and heatsink. Re-pasteable for service. Major commercial examples: Arctic Silver, Noctua NT-H1, Thermal Grizzly.

Thermal pads (typical k = 1.0–7.0 W/m·K): cured silicone elastomer sheets (Shore 00 30–60) loaded with thermally conductive filler. Pre-cut to size and applied between component and heatsink. Eliminates the application skill required for grease. Major sources: Bergquist, Henkel Sil-Pad, 3M.

Thermal gap-fillers (typical k = 1.0–8.0 W/m·K): two-component RTV-2 silicones that mix at point of dispense and cure to a soft (Shore 00 10–40) elastomer in 30–60 minutes. Used for irregular gaps where pads can't conform. Major sources: Bergquist Gap Pad, Henkel Sil-Gel, Dow Dowsil.

Thermal RTV adhesives (typical k = 0.6–2.5 W/m·K): one-component RTV-1 silicones with thermal filler, used to bond LED arrays to heatsinks or to seal heatsink-to-PCB interfaces. Lower thermal conductivity than gap-fillers because adhesive strength requires higher silicone fraction.

Filler Selection Considerations

Al₂O₃ (alumina): workhorse filler for cost-sensitive TIMs. Spherical alumina (e.g., Showa Denko AS-50) provides better packing than angular alumina, reaching higher loading and higher k.

AlN (aluminum nitride): highest k of any commercial non-precious filler, but reacts with moisture to form ammonia and aluminum hydroxide. Surface-treated AlN grades are required for long-term reliability.

BN (boron nitride): anisotropic thermal conductivity (in-plane k = 200–400 W/m·K, through-plane k = 1–4 W/m·K). Premium TIM uses platelets or BN agglomerates aligned to optimize through-plane heat flow.

ZnO (zinc oxide): low cost, no electrical conductivity issues. Used in commodity thermal grease but rarely in high-end TIM.

Silver: highest thermal conductivity but electrically conductive — risk of shorting if it contacts adjacent traces. Used only where electrical isolation is built into the design.

Application and Standards

TIM is selected and applied based on:

Bondline thickness (BLT): thinner is better for thermal performance; typical electronic-grade TIM achieves BLT < 100 μm for grease, 100–500 μm for pads, 100–3000 μm for gap-fillers
Pressure: applied pressure affects BLT; typical assembly pressure is 0.05–0.20 MPa for grease, 0.05–0.5 MPa for pads
Service life: silicone TIMs are typically warranted for 5–10 years of normal service; degradation modes include silicone migration, filler settling, and thermal cycling fatigue
Outgassing: low-outgassing grades (TML under 1.0%, CVCM under 0.1% per ASTM E595) are required for aerospace and vacuum applications

Major standards: IEC 62321 (RoHS for electronics), ASTM D5470 (thermal conductivity test), ASTM E1461 (thermal diffusivity). Automotive electronics typically require AEC-Q200 qualification.

Sourcing and Cost

Silicone TIM prices vary widely with thermal conductivity:

1.0 W/m·K commodity grease: $15–50 USD/kg
3.0 W/m·K thermal pad: $80–200 USD/kg
6.0 W/m·K gap-filler: $150–400 USD/kg
12.0 W/m·K BN-loaded premium: $400–1500 USD/kg

For commodity electronics, Chinese-supplied silicone TIMs offer 30–50% cost savings vs branded alternatives. For automotive/EV power electronics, OEM qualification typically demands branded TIM with full reliability documentation.