1. Definition and Primary Applications of Thermal Paste

Thermal grease, accurately termed Thermal Silicone Grease or Thermal Compound, is a paste-like TIM composed of silicone oil as a base fluid, filled with thermally conductive particles (e.g., alumina, zinc oxide, boron nitride, aluminum nitride).

Its primary function is to fill microscopic air gaps between a heat source (e.g., CPU, GPU, power semiconductor) and a heat sink. By displacing the air—a poor thermal conductor with a conductivity of approximately 0.026 W/(m·K)—it establishes an efficient pathway for heat conduction. High-performance thermal greases can possess thermal conductivity values ranging from 1 to over 10 W/(m·K), significantly reducing the overall interfacial thermal resistance.

Key Application Scenarios Include:

• Consumer Electronics: Cooling for CPUs/GPUs in personal computers and main chips in smartphones.
• Telecommunications Equipment: Heat dissipation for core chips in servers, base stations, and routers.
• New Energy Vehicles: Cooling for power devices in e-drive control units (IGBTs), onboard chargers (OBCs), and battery management systems (BMS).
• Industrial and Power Electronics: Heat dissipation in variable-frequency drives, inverters, and high-power supply modules.

2. Historical Development of Thermal Grease

The evolution of thermal grease is intrinsically linked to the advancement of the semiconductor industry. In the 1960s-70s, with the advent of integrated circuits, chip power density began to increase, making simple mechanical contact insufficient for heat dissipation. Early TIMs were rudimentary forms of greases or pads.

1970s-1990s: Foundational Research and Commercialization

This period involved systematic research into the effects of various fillers (e.g., metal powders, oxides) on the thermal properties of composite materials. Silicone-based greases filled with micron-sized alumina or zinc oxide became the mainstream solution. Companies like Dow Corning (later integrated into Dow Inc.) and Shin-Etsu Chemical Co., Ltd., leveraging their deep expertise in silicone chemistry, pioneered the development of standardized, reliable commercial products (e.g., Dow Corning 340), establishing the industry's foundation [1]. These early products typically had thermal conductivities below 1 W/(m·K) but effectively addressed initial cooling needs.

Late 1990s-Early 2000s: Performance Breakthroughs and Standardization

The explosion of the PC and server industries, coupled with rapidly rising CPU power, demanded higher-performing TIMs. The industry began adopting ceramic fillers with higher intrinsic conductivity, such as boron nitride (BN) and aluminum nitride (AlN). Concurrently, test method standardization gained prominence. The ASTM D5470 standard – "Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials" – gained widespread acceptance, providing a unified benchmark for comparing products [2].

2000s-Present: Diversification and High-Performance Competition

Since the turn of the century, particularly in China, the TIM industry has experienced rapid growth. Driven by policy support and market demand, several Chinese companies have risen swiftly. They not only captured significant market share in the mid-to-low tier but also achieved continuous breakthroughs in the high-performance segment through sustained R&D investment. Research has focused on the nano-sizing and hybridization of fillers, and modification of the base oil, aiming for higher conductivity, lower thermal resistance, and superior long-term stability.

3. Overview of Major Global Thermal Grease Brands

3.1 Leading International Brands

Dow Inc.

As a global leader in silicones and chemical materials, Dow possesses profound technical heritage through its acquisition of Dow Corning. Its thermal grease portfolio is extensive, renowned for high reliability and long-term stability, making it suitable for demanding applications in automotive and industrial sectors. A representative product, Dow TC-5625, is known for its excellent pump-out resistance and electrical insulation properties. Dow's products often emphasize balanced overall performance rather than pursuing a single extreme parameter.

Shin-Etsu Chemical (Japan)

A global leader in silicon wafers and silicones. In the TIM sector, Shin-Etsu enjoys an exceptional reputation for cutting-edge technology and superior product performance, especially in the high-end CPU cooling market. Classic products like Shin-Etsu X-23-7762 have long been highly regarded. The more recent Shin-Etsu 7921 is a preferred choice for many branded computers and servers due to its very low volatility and excellent durability. Shin-Etsu often sets industry benchmarks for thermal conductivity and resistance.

3.2 The Rapid Development of Chinese Brands

Over the past 20 years, Chinese TIM companies have transitioned from being followers to participants and, in some areas, leaders. Initially competing through imitation and low cost, the focus has now shifted to independent R&D and innovation.

• Technological Advancement: Chinese firms have made significant progress in surface treatment and formulation technology for high-conductivity fillers. Product thermal conductivity has increased from 1-2 W/(m·K) to over 10 W/(m·K) in some cases, reaching the global upper tier.
• Market Application: Leveraging local supply chain advantages and responsive service, Chinese brands have successfully entered the supply chains of top domestic companies like Huawei, ZTE, BYD, and Lenovo, gradually expanding their international presence

4. The Critical Importance of Thermal Resistance: Its Relationship with Thermal Conductivity and Calculation

4.1 The Limitation of Thermal Conductivity and the Primacy of Thermal Resistance

A common misconception is to consider "Thermal Conductivity" as the sole performance indicator. Thermal Conductivity (λ) is an intrinsic material property, defined as the heat flux per unit area under a unit temperature gradient, with units of W/(m·K). It describes only the material's inherent ability to conduct heat.

However, in practice, thermal grease exists as a thin layer between two surfaces. Its ultimate cooling efficacy depends not only on (λ) but also critically on the bond line thickness (d) and the quality of interfacial contact. Thermal Resistance (R) is the parameter that comprehensively incorporates both material property and geometrical form, directly reflecting the total "resistance" to heat flow through the interface. Lower thermal resistance signifies better cooling performance.

Thermal Resistance (R) is defined as the temperature difference (ΔT) across a heat flow path divided by the heat flow rate (P, power), with units of °C/W or K/W. The formula is:

Where:

• R: Total Thermal Resistance (K/W)
• ΔT: Temperature difference across the TIM layer (K)
• P: Heat flow power (W)

For the thermal grease layer itself, its specific thermal resistance (R_TIM) can be calculated based on its thickness and conductivity:

Where:

• R_TIM: Thermal Resistance of the TIM layer (K/W)
• d: Average thickness of the TIM layer (m)
• λ: Thermal Conductivity of the grease (W/(m·K))
• A: Coverage area of the grease (m²)

4.2 Interpretation of the Relationship

From the formula R_TIM = d / (λ × A), it is clear that:

• Thermal Resistance (R) is directly proportional to the thickness (d). A thinner applied layer results in lower thermal resistance, all else being equal.
• Thermal Resistance (R) is inversely proportional to Thermal Conductivity (λ). A higher (λ) yields a lower (R).
• Consequently, a product with high thermal conductivity may exhibit higher effective thermal resistance if it results in a thicker bond line due to its viscosity or application method, compared to a product with slightly lower conductivity that forms an ultra-thin, stable layer.

5. Comparative Table of Low Thermal Resistance Products from Global Brands

The following table compiles data for products advertised as having low thermal resistance from Dow and Shin-Etsu. It is crucial to note that direct comparison of these values requires caution, as test conditions (e.g., pressure, temperature, equipment) may vary between manufacturers. All data are sourced from official company technical datasheets and are provided for reference purposes only.

Note: Brands like Shin-Etsu and Dow often report Thermal Impedance in Imperial units (K·in²/W). For comparative purposes only, this can be approximated to Thermal Resistance (K/W) assuming a standard contact area of 1 in² (6.45 cm²): R (K/W) ≈ R (K·in²/W). Actual R is inversely proportional to area. 1 K·in²/W = 0.00064516 K·m²/W.

6. Future Development Directions for Thermal Grease

Future development of thermal grease technology will likely progress in the following directions:

1. Pursuit of Higher Thermal Conductivity and Lower Thermal Resistance

R&D will continue to focus on novel fillers like graphene and carbon nanotubes. The challenges lie in dispersion stability within the silicone matrix and cost reduction. Achieving lower thermal resistance will hinge on constructing more efficient thermal pathways through filler morphology control (spheroidization) and particle size blending (micro + nano).

2. Enhancement of Comprehensive Reliability

Beyond low initial thermal resistance, long-term stability is paramount. This includes:

• Low Bleed/Non-Pump-Out: Preventing silicone oil from separating under prolonged heat or being pumped out from the interface during thermal cycling.
• Long-Term Thermal Stability: Avoiding performance degradation from curing or drying out at high temperatures.
• Aging Resistance: Ensuring predictable performance degradation over the product's operational lifetime.

3. Optimization for Application Processes

Developing formulations suitable for high-volume automated dispensing processes (e.g., stencil printing, jetting). This requires greases with tailored viscosity, thixotropy, and spreadability. Phase Change Materials (PCMs), solid at room temperature for handling but liquefying at operating temperatures to fill gaps, represent an alternative direction.

4. Sustainability and Safety

With increasingly stringent environmental regulations (e.g., REACH, RoHS), developing more eco-friendly alternatives, such as silicone-free formulations, bio-based oils, or greases with low volatile organic compound (VOC) content, is a long-term trend.