The Black Technology of Thermal Management – Flexible Heat Pipes

With the advent of the AI era, electronic devices (foldable mobile phones, wearable devices, VR glasses) are developing towards miniaturization, flexibility and high integration, with continuous improvement in power density. Traditional cooling solutions (graphite thermal pads, conventional heat pipes) cannot adapt to complex geometric shapes and dynamic deformation scenarios due to their rigid structures, and their heat dissipation efficiency is difficult to meet the demand.

Therefore, developing a thermal management technology with high-efficiency heat dissipation, thinness, flexibility, stability and reliability has become a key issue. The "flexible heat pipe" can perfectly fit the relevant application scenarios, so today let's talk about what kind of thermal management black technology the "flexible heat pipe" is.

Heat pipe technology is a heat transfer component called "heat pipe" invented by George Grover of the Los Alamos National Laboratory in the United States in 1963. It makes full use of the principles of heat conduction and the rapid heat transfer properties of phase change media, and rapidly transfers the heat of heat-generating objects to the outside of the heat source through the heat pipe, with thermal conductivity exceeding any known metal.

A "flexible heat pipe" is a type of heat pipe. A traditional heat pipe is a copper tube with a wick made by sintering copper powder or copper mesh, which enables the phase change process of the internal liquid. However, it can be found that traditional heat pipes are straight or bent on a horizontal plane and cannot be folded or bent, because folding will lead to the closure of the internal passage, making the internal liquid unable to circulate for phase change and losing its original high heat dissipation function. In contrast, a "flexible heat pipe" is a foldable and bendable heat pipe, whose advantage is that it can be used in some electronic devices that require folding and bending, and can stably perform phase change heat transfer after folding and bending.

Advantages and Characteristics of Flexible Heat Pipes

1. High thermal conductivity: The equivalent thermal conductivity reaches 5,000–200,000 W/(m·K), 10–200 times that of copper/aluminum, and the heat transfer capacity attenuation is less than 10% when bent at 180° (under optimized design).
2. Structural flexibility: The minimum bendable radius is 1.6 cm, adapting to L/U-shaped layouts or foldable devices. It features anti-vibration design to avoid mechanical fatigue failure.
3. Lightweight: The polymer matrix model has a weight of less than 1g/cm³ and can be as thin as 0.4mm.
5. Flexible heat pipes can be classified into the following three types according to their structural forms:
6. (1) Ultra-thin heat pipes that achieve small-amplitude bending deformation by using metal elastic deformation;
7. (2) "Three-section" flexible heat pipes that achieve bending deformation by using metal bellows or flexible polymer pipes;
8. (3) Polymer matrix flexible flat heat pipes that achieve bending deformation by using flexible films as the shell.
10. In addition, they can also be divided into three categories according to the matrix material: metal matrix flexible heat pipes, polymer-metal composite matrix flexible heat pipes and polymer matrix flexible heat pipes.

1. Ultra-thin Heat Pipes

Ultra-thin heat pipes are a general term for heat pipes with a thickness of less than 2 mm. They use high thermal conductivity metal materials such as copper and aluminum for the shell, and can achieve small-amplitude bending deformation by utilizing the elastic deformation of metal materials. Benefiting from the mature packaging technology of metal shells, this type of flexible phase change heat transfer component has realized mass production and application, and achieved remarkable results in the heat dissipation field of electronic devices such as tablet computers, laptops and smart phones.

However, limited by the high strength of metal materials, the deformation of this type of flexible phase change heat transfer component is generally small. In practical applications, it is often bent once according to the installation space and does not undergo repeated bending during use. Therefore, this type of flexible heat pipe is difficult to adapt to the repeated bending and folding requirements in the heat dissipation field of foldable screen and flexible screen electronic devices.

Furukawa of Japan launched a metal shell flexible heat pipe named "PERA-FLEX" in 2004. The product uses copper foil as the shell and deionized water as the working medium, with two thickness specifications of 0.7 mm and 0.55 mm, and its ultimate power is 15 W and 8 W respectively.

2. Three-section Flexible Heat Pipes

Three-section flexible heat pipes refer to heat pipes whose shell is formed by splicing two metal pipes and one section of polymer flexible pipe or metal bellows. The two metal pipes are located at both ends of the heat pipe as the evaporation section and condensation section respectively, and the polymer pipe or metal bellows is located in the middle of the heat pipe as the adiabatic section to realize the flexible bending function of the heat pipe.

Three-section flexible heat pipes have good flexibility, can realize repeated bending with large deformation, and can meet the heat dissipation requirements of occasions with high heat dissipation power, long transmission distance and large structural size. However, the connection structure between the adiabatic section and the evaporation/condensation section results in a large difference between the inner and outer diameters of the heat pipe, leading to a large overall diameter of the heat pipe. Its structural characteristics limit the development of three-section flexible heat pipes towards miniaturization and thinness.

At the same time, all three-section flexible heat pipes adopt cylindrical shells, and there is no three-section flexible heat pipe with a flat shell structure at present, which affects the contact between the heat pipe and heat dissipation components. These defects limit the application of three-section flexible heat pipes in the heat dissipation field of compact electronic devices, and developing thin and flexible phase change heat dissipation devices is the key to solving the current heat dissipation problems of flexible screen electronic devices.

3. Polymer Matrix Flexible Heat Pipes

Ultra-thin flexible heat pipes are made with high temperature resistant and high-performance polymer materials to form polymer films as the matrix. The good flexibility of polymer films is utilized to realize the bending deformation of heat pipes. The heat pipes maintain good heat transfer performance under multiple deformation conditions, and also have the advantages of light weight, ultra-thinness and good insulation, which can better solve the heat dissipation problems of compact flexible electronic devices.

At present, there are relatively few studies on polymer matrix flexible heat pipes, and the factors restricting the development of polymer shell flexible heat pipes are mainly as follows:

① The packaging process of polymer films is complex, with unstable packaging effect and poor durability, resulting in a short service life of polymer shell flexible heat pipes;

② The air tightness of polymer films is poor, and gas molecules can pass through the gaps between polymer molecules. In practical applications, the method of compounding polymer films with metal films is usually adopted to ensure the air tightness of the heat pipe shell, which reduces the flexibility of the heat pipe;

③ The hydrophilicity of polymer films is poor, and there are relatively few studies on the compatibility between polymers and working media.

At present, most flexible heat pipes adopt the combination of copper wire mesh wick and deionized water working medium. The core of a flexible heat pipe is to drive the circulation of the working medium through capillary force: the evaporation section absorbs heat and vaporizes → the vapor flows to the condensation section to release heat and condense → the liquid flows back through the capillary action of the wick, forming a closed cycle.

Different from traditional heat pipes, it adopts a flexible structure (such as bellows or polymer matrix), allowing bending (up to 360°), vibration or relative displacement, while maintaining low thermal resistance (typical value 0.002–0.32℃/W).

The most critical structural component in both traditional heat pipes and "flexible heat pipes" is the wick, which is an important link to realize phase change. In the manufacturing process of traditional phase change heat transfer components, the wick and the metal shell are usually sintered together by a high-temperature sintering process to reduce the contact thermal resistance. However, polymer films cannot withstand excessively high sintering temperatures (>500 ℃). Therefore, flexible heat pipes with polymer films as the matrix often adopt electrodeposition connection process or hot pressing process to connect the wire mesh with the shell.

Research team made a stainless steel wire mesh wick with gradient wettability. First, the stainless steel wire mesh is treated with a mixed solution of Al₂O₃ and NiNO₃; then it is immersed in the prepolymer solution and pulled out at a constant speed; after the prepolymer solution on the surface of the stainless steel wire mesh is dried, it is placed in an atmosphere sintering furnace and kept at 800 ℃ for 1 hour, finally forming a stainless steel wire mesh wick with Fe₂O₃ nanowires on the surface.

Some other teams have used polymer wicks, but such practical cases are very rare. However, polymer powder sintered wicks have been experimentally studied and applied in the field of loop heat pipes. The table above shows the types of polymer powder sintered wicks and their working media. With low thermal conductivity, small pore distribution and strong capillary force, loop heat pipes using polymer wicks have achieved high ultimate power, and their heat transfer performance even exceeds that of loop heat pipes using metal powder sintered wicks.

Wire mesh wicks will not be damaged during the bending deformation of heat pipes, and have good flexibility and capillary performance, making them the main wicks adopted in current ultra-thin heat pipes and flexible heat pipes. However, compared with polymer materials, metal wire meshes have disadvantages such as poor flexibility, high density and low bending fatigue strength, which affect the bending radius, weight and bending life of flexible heat pipes. In addition, metal wire meshes are often used in the field of signal shielding, and the large-area metal wire mesh wicks used in flat heat pipes may affect the signal transmission of electronic devices.

Although the polymer powder sintered wicks used in loop heat pipes do not have flexibility, they provide a new direction for preparing flexible polymer wicks. Preparing polymer matrix wicks, studying the compatibility between polymer wicks and working media, and exploring the wetting mechanism of polymer wicks are the keys to developing high-performance flexible heat pipes.

III. Development and Application of Flexible Heat Pipes

Ultra-thin heat pipes, three-section flexible heat pipes and polymer matrix flexible heat pipes have different application fields and their own advantages and disadvantages.

The manufacturing process of ultra-thin heat pipes is basically mature with good heat transfer effect, but poor flexibility, suitable for heat dissipation occasions that only require one-time bending; repeated bending will seriously reduce their service life and reliability.

Three-section flexible heat pipes are suitable for heat dissipation occasions requiring long-distance transmission, with good flexibility and heat transfer performance, but the large diameter and bending radius of the heat pipes make it difficult to meet the miniaturization requirements of wearable flexible electronic devices.

Although polymer film matrix flexible flat heat pipes have shortcomings such as low heat transfer efficiency, short service life and high cost, they have become a reliable choice to solve the heat dissipation problems of flexible electronic devices by virtue of their advantages such as good flexibility, extremely light weight, ultra-thin thickness, good insulation performance and adaptability, and have become the main development trend of flexible heat pipes.

Recently, teams such as Yang Xiaoping and Ma Xiang from Xi'an Jiaotong University proposed a new type of ultra-thin flexible loop heat pipe (UFLHP) with an evaporator thickness of only 0.7 mm. The research results were published in the journal International Communications in Heat and Mass Transfer under the title "A novel ultra-thin flexible loop-type heat pipe for application in wearable electronic devices cooling".

The UFLHP adopts a trapezoidal serrated nickel felt main core and an oxidized spiral woven copper mesh (SWM) secondary core. Through single/double layer SWM experiments and multi-operation parameter research, it is found that the UFLHP with double-layer secondary core performs the best, with a heat flux of up to 7 W/cm², a thermal resistance as low as 0.987 K/W, and an equivalent thermal conductivity of 24204.4 W/(m·K). It can stably handle 5W heat input even in a 90° bending state (thermal resistance 2.22 K/W).

The established theoretical model can predict the steady-state temperature and thermal resistance, with a prediction error of evaporator temperature less than 15% under high power, providing a new solution for high-efficiency thermal management of wearable devices.

IV. Summary

Heat pipe/vapor chamber heat dissipation technology is an effective means to solve the current heat dissipation problems of high-performance electronic devices. Copper-based heat pipes/vapor chambers have basically achieved ultra-thinness and are gradually developing towards a thickness of less than 0.3 mm; at the same time, aluminum-based phase change heat transfer devices are gradually replacing conventional-sized copper-based phase change heat transfer devices to achieve light weight.

In recent years, with the explosive growth of various flexible wearable electronic devices such as curved screen smart watches, foldable screen mobile phones, flexible screen laptops and AR/VR glasses, traditional copper and aluminum-based rigid phase change heat transfer devices can no longer meet the structural design and heat dissipation requirements of emerging flexible electronic devices, and flexibility has become a new development direction of phase change heat dissipation devices today.

Compared with traditional phase change heat transfer devices, flexible heat pipes, especially polymer matrix flexible and ultra-thin flat heat pipes suitable for flexible wearable electronic devices, have a short research time and few research results, lacking high-reliability packaging processes and systematic theoretical research, resulting in problems such as short service life, low equivalent thermal conductivity and high cost of flexible phase change heat transfer components, which have not yet been truly industrialized. Therefore, related technologies such as flexible heat pipes/vapor chambers need further in-depth research to meet the rigid heat dissipation demand of emerging flexible electronic devices.

• Written by

  CoolingThermal Engineering Team

  CoolingThermal is an automation equipment manufacturer based in Kunshan, China, specializing in heat pipe and vapor chamber production equipment since 2017. Our engineering team designs, builds, and commissions complete production lines covering forming, degassing, welding, testing, and assembly processes. The technical content on this blog is written by the same team that develops the equipment — based on real production experience, not secondary research.