In cutting-edge fields such as 5G communications, artificial intelligence (AI), electric vehicles (EVs), and data centers, electronic devices are evolving rapidly toward higher power density, smaller size, and more complex designs. This trend presents a core challenge: thermal management. When the power consumption of chips, batteries, or power devices exceeds the kilowatt level, traditional heat dissipation solutions (such as metal heat sinks, air cooling, or liquid cooling) can no longer independently address local hot spot issues. At this juncture, Thermal Interface Materials (TIMs), acting as the "bridge" connecting heat sources and heat dissipation systems, have become a key technology to ensure the reliable operation of devices.
1. Definition: Bridging the "Last Nanometer" of Heat Conduction
The essence of TIMs is materials that fill micro-gaps and optimize heat conduction paths. In electronic packaging, although heat sources (such as CPUs, GPUs, and IGBT modules) and heat sinks appear to be in close contact, there are actually a large number of microscopically uneven surfaces. These gaps are filled with air, whose thermal conductivity is only about 0.026 W/m·K, much lower than that of metals (e.g., copper at 401 W/m·K) or semiconductor materials. By filling these gaps, TIMs reduce thermal resistance to the order of 0.01–1 K·cm²/W, ensuring efficient heat transfer from heat sources to heat dissipation systems.
TIMs come in various forms, including:
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Thermal Grease: Semi-fluid, suitable for scenarios requiring frequent disassembly (e.g., PC assembly), but prone to pump-out or drying/cracking;
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Thermal Pad: Pre-formed solid sheet, easy for automated assembly, but with high requirements for thickness uniformity;
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Liquid Metal: Thermal conductivity up to 10–80 W/m·K, but needs to address electrochemical corrosion issues;
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Phase Change Material (PCM): Achieves temporary heat storage through melting and heat absorption, suitable for pulsed heat load scenarios;
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Thermal Gel: Combines the characteristics of grease and pads, balancing fillability and stability.
Traditional TIMs mostly use silicone oil, epoxy resin, or acrylic as the matrix, filled with ceramic particles such as aluminum oxide (Al₂O₃), zinc oxide (ZnO), or aluminum nitride (AlN). However, as the heat flux density of electronic devices exceeds 100 W/cm², the thermal conductivity of these materials (usually 1–10 W/m·K) has approached its limit. This has spurred the research and development of advanced TIMs, whose core direction is the introduction of carbon-based materials and boron nitride (BN) to achieve an order-of-magnitude improvement in thermal conductivity.
2. Two Core Technology Routes for Advanced TIMs: Carbon-Based vs. Boron Nitride
2.1 Carbon-Based TIMs: Unrivaled Thermal Conductivity "Pioneers"
The thermal conduction mechanism of carbon materials (such as graphene, carbon nanotubes, and graphite) stems from the conjugated π bonds formed by their sp² hybrid orbitals, which allow phonons (lattice vibrations) to propagate at ultra-high speeds. For example, the in-plane thermal conductivity of graphene can reach 5300 W/m·K, far exceeding that of copper; the axial thermal conductivity of carbon nanotubes is even higher, approaching 6000 W/m·K. This anisotropic (direction-dependent) thermal conductivity characteristic endows carbon-based TIMs with unique advantages in specific scenarios:
Graphite Sheet
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Prepared by Chemical Vapor Deposition (CVD) or high-temperature calendering process, with a thickness as thin as 10–50 μm;
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In-plane thermal conductivity up to 1500 W/m·K, but vertical (Z-axis) thermal conductivity only 10–30 W/m·K;
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Widely used in smartphones, tablets, and EV battery packs to horizontally diffuse heat and prevent local overheating;
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Modified graphite sheets (e.g., surface-deposited metal layers or nitrogen-doped) can improve vertical thermal conductivity, but at the cost of increased cost.
Graphene-Enhanced TIMs
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Dispersing a small amount of graphene (mass fraction 1–5%) in a polymer matrix can significantly improve the thermal conductivity of the composite material;
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For example, the thermal conductivity of graphene/silicone oil composites can reach 8–15 W/m·K, 3–5 times that of pure silicone oil;
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The challenge lies in the easy agglomeration of graphene, which needs to be solved by surface modification (e.g., grafting silane coupling agents) or ultrasonic dispersion technology.
Carbon Nanotube (CNT) TIMs
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The theoretical thermal conductivity of single-walled carbon nanotubes (SWCNT) can reach 6000 W/m·K, but in actual composites, due to interface thermal resistance, the thermal conductivity is usually 20–50 W/m·K;
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Advantage: Can be designed as isotropic or anisotropic thermal conductivity, suitable for complex heat flow paths;
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High cost (>100 USD/gram), currently only used in aerospace or high-end military electronic equipment.
Limitations
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Weak vertical thermal conductivity: The anisotropy of carbon materials leads to low Z-axis thermal conductivity, making it difficult to directly conduct heat from the heat source to the heat sink;
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High electrical conductivity: Graphene and carbon nanotubes are good conductors, requiring insulation coatings or polymer encapsulation to prevent short circuits;
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Cost fluctuations: The preparation process of high-quality graphene and carbon nanotubes is complex, and prices are limited by production capacity.
2.2 Boron Nitride (BN) TIMs: All-Round "Insulating and Thermal Conductive King"
Hexagonal boron nitride (h-BN), with a layered structure similar to graphite, is known as "white graphene." Its core advantage lies in simultaneously having high thermal conductivity, high insulation, and high-temperature resistance:
Thermal Conductivity Performance
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The lattice thermal conductivity of h-BN is about 300–600 W/m·K (in-plane direction), which is lower than graphene but much higher than traditional ceramic fillers;
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By controlling particle size (e.g., nano-scale h-BN can improve interface contact), the thermal conductivity of composites can reach 10–30 W/m·K;
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Vertical thermal conductivity is better than carbon materials, suitable for scenarios requiring Z-axis heat transfer.
Electrical Insulation
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The band gap width of h-BN reaches 5.9 eV, making it a natural wide-bandgap semiconductor with a volume resistivity >10¹⁴ Ω·cm;
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Can be directly used in high-voltage power electronic modules (such as IGBT, SiC MOSFET) without additional insulation layers.
Thermal Stability
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Can withstand high temperatures up to 1000°C in air and even 2000°C in an inert atmosphere;
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Suitable for extreme environments such as aerospace and nuclear energy.
Chemical Inertness
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Does not react with most acids, bases, or organic solvents, with no degradation risk during long-term use;
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Good biocompatibility, can be used in medical electronic equipment.
Application Cases
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EV Batteries: BN-enhanced silicone gaskets are used between battery modules to achieve both thermal conductivity and electrical insulation;
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5G Base Stations: BN/polyimide composite films wrap power amplifiers to solve high heat density issues in the millimeter-wave band;
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Data Centers: BN-filled liquid metal TIMs are used between CPUs and cold plates, with a thermal conductivity of 40 W/m·K, 4 times higher than traditional materials.
Limitations
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High cost: The price of high-purity h-BN powder is about 50–65 USD/kg, 10–20 times that of aluminum oxide;
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Difficult processing: h-BN particles are prone to agglomeration, requiring ball milling, surface modification, or in-situ growth technology to optimize dispersibility;
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Low density: The density of h-BN is only 2.1 g/cm³, and excessive filling may lead to reduced mechanical strength of the composite material.
3. Application Scenarios of Advanced TIMs: From Consumer Electronics to Industrial-Grade Thermal Management
3.1 Electric Vehicles (EV): "Thermal Guardians" of Batteries and E-Drive Systems
The battery pack and e-drive system of EVs are two major heat sources:
Battery Thermal Management
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During fast charging or high-rate discharge, the local temperature of lithium-ion batteries can exceed 60°C, causing capacity degradation or even thermal runaway;
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Graphite sheets are used between battery modules to horizontally diffuse heat to the entire battery pack;
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BN-enhanced TIM gaskets are used between battery cells and liquid cooling plates to ensure thermal conductivity while preventing leakage.
E-Drive System
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The power density of SiC MOSFET and IGBT modules reaches 500 W/cm², requiring TIMs to transfer heat to pin-fin heat sinks;
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Liquid metal/BN composite TIMs can achieve a thermal conductivity >30 W/m·K, meeting the requirements of 800V high-voltage platforms.
3.2 5G and Data Centers: Addressing "Exponential" Heat Flux Density
The heat flux density of server CPUs/GPUs in 5G base stations and data centers has exceeded 300 W/cm², which is difficult for traditional TIMs to handle:
5G Base Stations
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The efficiency of power amplifiers (PAs) in the millimeter-wave band (24–100 GHz) is only 30–40%, and the remaining energy is converted into heat;
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Graphene/BN hybrid TIMs are used between PAs and heat dissipation cavities, with a thermal conductivity of 25 W/m·K, reducing temperature by 15–20°C;
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Laser diodes in optical modules require TIMs to transfer heat to microchannel coolers, and BN-based TIMs can avoid electrical signal interference.
Data Centers
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In liquid-cooled servers, the TIM between the CPU and cold plate needs to withstand >10 bar pressure, while the thermal conductivity >20 W/m·K;
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Carbon nanotube/polymer composite TIMs can maintain low contact thermal resistance under high pressure, making them the first choice for liquid cooling systems.
3.3 Consumer Electronics: Balancing Thinness and High Performance
Devices such as smartphones and laptops require TIMs to be thin, flexible, and efficient:
Smartphones
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The power of SoCs (System-on-Chips) reaches 10–15 W, but the heat dissipation space is only 2–3 mm³;
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Combination solution of graphite sheets + silicone gaskets: graphite sheets horizontally diffuse heat to the middle frame, and silicone gaskets transfer heat to the screen or back cover;
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High-end models have begun to adopt graphene-enhanced TIMs, with thermal conductivity increased to 12 W/m·K, reducing surface temperature by 3–5°C.
Laptops
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Thin and light laptops adopt the "vapor chamber + TIM" solution: the vapor chamber quickly diffuses heat to a large area, and the TIM transfers heat to the fan outlet;
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BN-filled Phase Change Materials (PCMs) are used between the CPU and vapor chamber to cope with short-term high loads through melting and heat absorption.
3.4 Advanced Driver Assistance Systems (ADAS): "Invisible Guardians" of Safety and Reliability
ADAS sensors (such as lidar, cameras) and AI processors need to work stably in a wide temperature range of -40°C to +125°C:
Lidar
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The pulse power of the emission module reaches 100 W, requiring TIMs to transfer heat to heat dissipation fins;
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BN-based TIMs can avoid electromagnetic interference and withstand vibration and impact.
AI Processors
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Automotive-grade chips need to pass AEC-Q100 certification, and TIMs need to meet a 10-year service life;
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Carbon-based TIMs may cause signal noise due to electrical conductivity, making BN-based TIMs a safer choice.
4. Challenges and Future Trends: From Material Innovation to System Integration
Although significant progress has been made in advanced TIMs, they still face multiple challenges:
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Cost: The price of carbon nanotubes and BN needs to drop to within 5 times that of traditional materials to achieve large-scale replacement;
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Reliability: Under long-term high temperatures, the polymer matrix may age, leading to reduced thermal conductivity or pump-out failure;
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Standardization: Currently, there is a lack of unified standards for TIM testing methods (such as ASTM D5470, ISO 8894), affecting performance comparison;
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Environmental Friendliness: Some TIMs contain halogens or heavy metals, requiring the development of recyclable or biodegradable green materials.
Future Trends:
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Mixed Materials: Combining carbon-based (high thermal conductivity) and BN (high insulation) to develop "dual-function" TIMs;
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Nanostructures: Using 3D printing or self-assembly technology to build directional thermal conduction channels, breaking the anisotropic limit;
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Smart TIMs: Integrating phase change materials or shape memory polymers to achieve dynamic thermal management (e.g., automatically adjusting thermal conductivity according to temperature);
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Matrix-Free TIMs: Developing pure carbon or pure BN solid blocks to completely eliminate interface thermal resistance.
5. Conclusion and Call to Action: Seize the High Ground of Thermal Management Technology
As electronic devices evolve toward higher power, smaller size, and more complex designs, advanced TIMs have become a core element to ensure device reliability. Carbon-based materials have advantages in raw thermal conductivity, while BN stands out in specific fields due to its stability and insulation. However, cost, reliability, and standardization issues still need to be addressed, driving research on mixed materials, nanostructures, and smart TIMs.
