How To Achieve The Perfect Combination of High Thermal Conductivity, Flexibility, And High Rebound of Thermal Pads?

The rapid development of information technology has accelerated the arrival of the intelligent era, and various electronic products as carriers are constantly being updated and replaced. The evolution trend of high power, high integration, and miniaturization has made heat dissipation and reliability issues increasingly difficult, and gradually become bottlenecks in electronic product design. Through thermal management, high-power systems or equipment can effectively control and manage the heat generated to ensure that the system equipment operates at an acceptable temperature level, ultimately guaranteeing the reliability, performance, and lifespan of the system.

Thermal Interface Material (TIM) is a material used to improve heat transfer between two surfaces, typically a heat source (such as a computer processor) and a heat sink (such as a metal heat sink or other cooling system).

About TIM

Interface thermal resistance is one of the important studies in heat dissipation, which determines the heat dissipation ability of semiconductor chips, electronic products, mobile phones, automotive batteries, etc., thereby affecting their performance and stability. Air is a typical poor conductor of heat, and there are many interface gaps on the microscopic solid surface at the interface contact. Due to the presence of air, the heat dissipation efficiency is very low.

If the air is effectively eliminated to make the contact between the device and the heat sink closer, the interface contact thermal resistance can be reduced, and an efficient heat transfer channel can be established, thereby maximizing the efficiency of heat dissipation. TIM is a product that conducts heat between two or more solid surfaces. Filling TIM between various structures on the heat dissipation path can effectively discharge the air between the gaps, accelerate the conduction of hot spot heat by increasing the contact area, effectively improve the heat transfer between the two surfaces, and enhance the efficiency of the thermal management system.

TIM is usually a heat dissipation material made of polymer materials as the matrix and filled with thermally conductive particles. As the size of chips decreases, integration and power density continue to increase, the heat generated during chip operation increases, leading to a continuous rise in chip temperature, which seriously affects the performance, reliability and lifespan of the final electronic components. The thermal interface material needs to have high thermal conductivity and high flexibility to ensure that it can fully fill the gaps on the contact surface under low installation pressure conditions, ensuring that the contact thermal resistance between the thermal interface material and the contact surface is very small, while ensuring insulation and non toxicity. Thermal conductive fillers are divided into three categories: metal, ceramic, and carbon materials, while the matrix materials are mostly silicone oil, rubber, and resin materials. Common thermal conductive interface materials include thermal conductive silicone grease, thermal conductive gel, thermal conductive pad, phase change materials, etc.

At present, the main research directions are probably multifunctional, oriented structure thermal conductive materials, and non silicon system thermal conductive materials. The multifunctionality of thermal conductive materials mainly refers to the addition of other functions or higher requirements for performance other than thermal conductivity on the premise of having thermal conductivity. The purpose of multifunctionality is mainly to meet the needs of different thermal conductivity scenarios, to achieve functional integration, optimize device structure, and further improve comprehensive performance while dissipating heat.

For example, one of the traditional methods to improve the thermal conductivity of a substrate is to increase the content of fillers, as it has high thermal conductivity, high rebound, and flexibility. Traditional metal and ceramic thermal conductive fillers have high mechanical strength, and increasing their content can lead to a decrease in the flexibility and elasticity of the matrix, limiting the material's processability and application in some special scenarios. Therefore, developing thermal interface materials with high thermal conductivity, flexibility, and high resilience has become one of the key focuses of development. So how does the industry solve this problem?

Is it impossible to balance the high thermal conductivity, high rebound, and Shore hardness of TIM?

As is well known, the main challenge to the reliability of electronic products comes from the coefficient of thermal expansion (CTE), which is commonly referred to as' thermal expansion and contraction '. CTE is a characteristic of the material itself, and due to the mismatch of CTE between electronic devices, bending deformation occurs when the operating temperature rises or falls, as shown in the following figure:

Over time, repeated deformation and compression of the thermal interface material between gaps can cause gaps to appear, thereby affecting the heat dissipation effect. And many high-power devices, in response to requirements such as' energy conservation and emission reduction 'and' carbon neutrality ', will activate tidal mode, which means that during peak demand periods, the power will be fully turned on, and during low demand periods such as late night to early morning, the system power will be reduced or shut down, which is equivalent to performing temperature cycling aging on the workpiece every day. This undoubtedly exacerbates the impact of CTE.

The electronic manufacturing industry has been using traditional silicon-based thermal conductive pads for decades, which are mainly composed of organic silicon resin and various types of thermal conductive fillers. The natural characteristic brought by this composition is that the finished product itself does not have compression rebound characteristics at all. After long-term compression of chips and heat dissipation modules at the microscopic level, coupled with the impact of harsh working conditions,

especially high temperatures, on the material body, over time, irreversible interface detachment "Delamination" will gradually appear on a certain contact surface, resulting in a significant increase in interface thermal resistance and the inability to continue providing a good heat conduction path, ultimately leading to thermal failure of the chip or even the entire machine.

In response to such issues, the industry is gradually paying attention to thermal conductive materials with high rebound characteristics. However, as mentioned earlier, in the traditional development path of thermal conductive materials, the rebound characteristics of the material are constrained by the following factors, becoming an insurmountable obstacle:

With the evolution of raw material properties and the upgrading of testing methods, many thermal conductive material manufacturers have been able to effectively control the hardness of thermal pads in recent years, thus meeting various heat dissipation application scenarios for customers. However, within the hardness range of Shore 00, it is a challenge for the entire industry to make thermal interface materials with high rebound characteristics, especially in ultra-high thermal conductivity scenarios. Due to the increase in the content of thermal conductive powder, the "powder characteristics" of the material are obvious. To achieve a balance between thermal conductivity, rebound, and Shore 00 hardness, it has almost become an "impossible triangle".

Ultra high thermal conductivity pads, due to the need for sufficiently high "loading" of thermal conductivity fillers, can cause rapid deterioration of the mechanical properties (softness and resilience) of the material during long-term service in harsh environments, making it unable to meet long-term reliability requirements.