How to Properly Apply Thermal Silicone Grease

With the return of soldered thermal solutions for high-end Intel 9th series CPUs, thermal silicone grease (not silicone gel!) has once again gained attention among PC enthusiasts. Today, let's focus on essential knowledge about thermal silicone grease.

We know that whether it's the CPU, GPU, or the heatsink surfaces in contact with them, they are far from as smooth, flat, or pure as we might imagine. When the heatsink and chip surfaces come into contact, they are uneven, with many grooves or gaps filled with air. Air has poor thermal conductivity, so another substance is needed to reduce thermal resistance; otherwise, the heatsink's performance will be significantly compromised, and it might not effectively dissipate heat.

To address these gaps between contact surfaces, thermal interface materials come into play. Their role is to fill the various gaps between the two contact surfaces, increasing the contact area between the heat source and the heatsink. Thermal silicone grease is one of the most common thermal interface materials.

As the intermediary layer between the heatsink and CPU/GPU chips, the heat must pass through thermal silicone grease for effective transfer to the heatsink. Therefore, the quality of the silicone grease has a crucial impact on the entire thermal management system. Despite enthusiasts' relentless pursuit of the heatsink, there has been a lack of sufficient understanding of thermal silicone grease by many.

What is Thermal Silicone Grease?

In international and domestic literature on thermal silicone grease products, it is sometimes called thermal paste or thermal compound. In English, thermal silicone grease can be referred to in various ways, such as Thermal Grease, Thermal Compound, or Thermal Paste. For convenience, we will use "thermal silicone grease" as a general term, sometimes abbreviated as "silicone grease" when referring to specific products.

Strictly speaking, thermal silicone grease is just one type of silicone grease. There are other types, such as insulating silicone grease, lubricating silicone grease, optically transparent silicone grease, etc. Silicone grease, often referred to as the MSG of the industry, has wide applications in electrical insulation, lubrication, mold release, rust prevention, corrosion protection, water resistance, shock resistance, and more.

Silicone grease is a product of secondary processing of silicone oil, mainly composed of high molecular compounds (organosilicon compounds) with silicon atoms in the main chain. The primary organic silicon high polymer is polydimethylsiloxane. Polydimethylsiloxane is a high molecular compound formed by the polymerization of many monomers with bonding, typically referred to as siloxanes. Its structural feature includes a basic skeleton with alternating silicon and oxygen atoms, and each silicon atom is connected to organic groups. Silicon-oxygen bonds in polydimethylsiloxane have high stability, and the organic groups include methyl, longer alkyl, fluoroalkyl, phenyl, vinyl, and some other groups.

Common cosmetic ingredients are essentially the same as the main components of thermal silicone grease, i.e., polydimethylsiloxane.

Silicone grease shares similar characteristics with silicone oil, boasting excellent heat resistance, electrical insulation, weather resistance, water repellency, physiological inertia, and low surface tension. It also has a low viscosity-temperature coefficient and high compressive strength. Its outstanding thermal stability and oxidation stability make it resistant to deterioration even at 150°C in prolonged contact with air and slow oxidation when in contact with oxygen and chlorine at 200°C. It generally operates in a temperature range of -50 to 150°C, exhibiting good lubrication properties on various substrates without corrosive effects.

These characteristics make silicone grease the optimal choice for a thermal interface material. Its low surface tension allows it to spread well into the gaps on the surfaces of the chip and heatsink. Its thermal stability ensures normal operation at high temperatures, and electrical insulation guarantees the safety of other electronic components. To enhance its thermal conductivity, functional fillers such as metal oxides are added, resulting in thermal silicone grease.

Silicone grease is naturally white, but the addition of different fillers may give it various colors, such as the common gray or golden yellow. While the quality of silicone grease contributes to its performance, the primary determinant is the difference in added fillers. Terms like nano-silicone grease and diamond silicone grease originate from these added fillers.

It's essential to note the distinction between silicone grease and silicone gel. Silicone gel, as used in industries like breast augmentation, has no direct relation to thermal silicone grease.

Performance Parameters of Thermal Silicone Grease

As a chemical substance, thermal silicone grease has several performance parameters reflecting its characteristics. Understanding these parameters can help assess the performance of a thermal silicone grease product.

1. Thermal Conductivity:

The thermal conductivity is measured in W/m·K (or W/m·°C), representing the heat conduction power when a temperature difference of 1 Kelvin (K=°C+273.15) exists over a distance of 1 meter along the axis of a column with a cross-sectional area of 1 square meter. A higher value indicates faster heat transfer and better thermal conductivity. Various materials have significantly different thermal conductivities. Metals have the highest thermal conductivity, followed by non-metals and liquids, with gases having the lowest. Materials with thermal conductivities less than or equal to 0.055W/m·K are considered highly efficient insulating materials, while those greater than or equal to 500W/m·K are considered highly efficient conductive materials. The thermal conductivity of common thermal silicone grease products is generally above 1W/m·K, with excellent ones reaching 6W/m·K or more—over 200 times that of air. However, compared to metals like copper and aluminum, the thermal conductivity of thermal silicone grease is only about 1/100.

2. Thermal Conductance:

Thermal conductance refers to the amount of heat transferred through a 1-square meter area in 1 hour with a fluid temperature difference of 1°C (or 1 K) under stable heat transfer conditions. It is measured in W/m²·K (or W/m²·°C). It is crucial to note that thermal conductance and thermal conductivity are distinct concepts.

3. Thermal Resistance:

Thermal resistance indicates how effectively an object impedes heat conduction. It is measured in °C/W, representing the temperature difference at the ends of the thermal conduction path when the continuous heat transfer power is 1W. Lower thermal resistance is better, as, under the same environmental temperature and thermal power, lower thermal resistance results in a lower temperature for the heating object. The magnitude of thermal resistance is highly dependent on the materials used in the thermal silicone grease.

4. Viscosity:

Viscosity measures the internal resistance to flow within a fluid. It is expressed in poise or Pa·s for dynamic viscosity. For thermal silicone grease, a viscosity of around 2500 poise provides good spreading, allowing it to easily spread around the chip's surface under certain pressure while maintaining a certain viscosity, preventing excess grease from flowing during compression. However, few thermal silicone grease products provide this performance parameter.

5. Operating Temperature Range:

Due to the characteristics of silicone grease itself, its operating temperature range is broad. The operating temperature is an important parameter ensuring that the thermal silicone grease is in a solid or liquid state. At excessively high temperatures, the fluid volume of the silicone grease.