White Paper on High Efficiency Phase Change Heat Transfer VC, Strong Bow for 5G Communication Heat Dissipation

ZTE Communication Co., Ltd., Shenzhen Hongfuhan Technology Co., Ltd

Abstract

Due to the use of latent heat of medium, phase change heat transfer technology represented by heat pipe and VC (Vapor Chamber) has significantly higher heat transfer coefficient and heat dissipation capacity than thermal conductivity and convection, which is the key technology to solve the increasing demand of product heat dissipation. In the prospect that chip power consumption and heat flux continue to rise, the development and application of phase change heat transfer technology such as VC really determines the reliability and performance upgrade space of communication products, which has a vital significance.

Keywords: 5G, phase change heat transfer,VC, homogenized heat plate, uniform temperature plate cooling module

1 Evolution of radiator technology

Heat dissipation is an important link to ensure the long-term safe and reliable operation of electronic equipment and products. The development of communication and information technology has promoted the heat dissipation or thermal design to become a systematic industry. The research and development of electric power, security, consumer electronics, automobiles, LED and other fields also pay more and more attention to the heat dissipation performance of products, in order to have more advantages in the market competitiveness.

At present, 5G communication and information products are developing towards the goal of larger capacity, higher performance, energy saving and low noise. The equipment is becoming more and more integrated, the single chip function is more powerful, the power consumption increases greatly, but the layout is more compact, the heat flux increases exponentially, and the heat dissipation technology is facing severe challenges.

The traditional heat dissipation system mainly relies on single-phase material to conduct heat from the device to the radiator surface, and then the air dissipates the heat to the environment through natural convection (natural heat dissipation system) or forced convection (forced air cooling system). The efficiency of heat conduction depends on and is limited by the inherent thermal conductivity of the material.

The phase change heat transfer technology represented by heat pipe and VC (Vapor Chamber) uses the medium to evaporate in the heating area and condense in the cooling area, absorb or release the corresponding latent heat of phase change at the same time, and realize the rapid diffusion or migration of heat by alternating cycles. The absorption and release of latent heat is a rapid and efficient process, and two-phase heat transfer usually uses working fluids with large latent heat, so the heat transfer efficiency is very high. The equivalent thermal conductivity can reach more than 2000 W/m · K, which is far more than pure metal materials such as gold, silver, copper, aluminum (200~400 W/m · K), and can support the heat transfer requirements of higher power consumption and higher heat flow density that traditional radiators cannot meet. At the same time, it can be matched with a variety of cold source forms (natural convection, forced air cooling, liquid cooling, radiation, etc.), and its application forms are flexible and diverse.

Fig. 1 Phase change heat transfer principle

From the heat pipe that was first put forward and has been widely used, it has evolved into VC soaking plate, Thermosyphon thermosyphon, LTS loop thermosyphon, LHP loop heat pipe and other forms, which are widely applicable to all kinds of products. Aiming at the problems of high power consumption, high heat flow density, poor temperature uniformity and so on, it has become the focus and development direction of the current heat dissipation field to solve the heat dissipation needs that cannot be met by traditional radiators.

FIG. 2 Morphology of phase change heat transfer radiator (Figure source network)

2 The development of VC soaking plate technology

VC soaking plate is a kind of phase change heat transfer product which is widely used in communication and electronics industry except heat pipe. Typical VC is a flat closed form, which is composed of shell, capillary structure, support structure and working medium. Through evaporation and condensation of working medium and capillary transport, it achieves efficient heat conduction and diffuses heat from the concentrated area to the whole structure plane.

FIG. 3 Structure principle of VC soaking plate

Benefiting from the advantages of large area capillary characteristics and two-dimensional or even three-dimensional thermal diffusion, VC has a higher heat flux carrying capacity, especially for the cooling of electronic devices with a heat flux of more than 50W/cm2, the temperature equalization effect is significantly better than that of pure metal or heat sink substrate, which can greatly improve the efficiency of the radiator. With the development trend of chip heat flux exceeding 100W/cm2, VC is undoubtedly the key technology to support the performance upgrade of communication equipment.

Like ordinary heat pipe shell, VC shell is usually made of metal materials. At present, the vast majority of VC used on the ground are made of copper sheet by stamping. Because of the good thermal conductivity of copper, good mechanical processing performance and welding performance, the forming process is relatively simple and the precision is high. In the field of consumer electronics, military industry or aerospace, in order to meet the further demand for high strength, ultra-thin or lightweight, stainless steel (high strength, corrosion resistance, low cost), titanium (high strength, low density, corrosion resistance) and other materials have also been developed to some extent as VC shells. Furthermore, in order to meet the market demand for cost and weight reduction, the industry has gradually carried out the exploration of aluminum based phase change heat transfer devices.

Table 1 Performance Comparison of Titanium, Copper and Aluminum

The selection of working medium is based on working temperature zone, material compatibility, thermophysical properties and other factors. The working medium with the best compatibility with copper is water, which has excellent thermophysical properties, is safe, non-toxic, and easy to get and handle. The working medium matched with aluminum is mainly refrigerant, which has a mature civil foundation as a cooling medium. Methanol, ethanol, acetone and other working fluids are also commonly used in various VC performance studies, but they are rarely used in practice due to toxicity, flammability and explosiveness.

Capillary core (or liquid absorption core) is an important part of capillary driven heat transfer or thermal diffusion devices such as heat pipe and VC. Its structure directly affects the performance of heat transfer and heat transfer and the carrying capacity of heat flow density. Due to the flat shape characteristics, VC mainly adopts four types of capillary cores: screen type, groove type, sintered type and composite type.

Fig. 4 Morphology of wire mesh type, groove type and sintered capillary wick (source network)

In fact, these types can be regarded as the basic structure of the capillary core. In order to further improve the thermal diffusivity and heat flux carrying capacity of VC, many researches are devoted to optimizing the geometric structure of the capillary core on a larger scale.

VC used in the early stage is the most classic pure metal support column, and the support column only plays the role of structural reinforcement. After that, a layer of powder sintered capillary ring is coated on the support column, or the metal column is directly replaced by the capillary powder column, and the pure wool fine column arrangement or the metal column and the capillary column mixed arrangement are adopted. In this way, the working fluid condensed on the cold side can backflow to the evaporation area on the hot side through the capillary ring or capillary column. The backflow path is greatly shortened, the liquid supplement rate is increased, and the heat transfer capacity of VC is therefore enhanced.

VC with higher performance is often seen in the densification of local capillary structures in the evaporation area corresponding to the heat source location. In addition to enhancing the capillary force and liquid reflux, the surface of these capillary structures simultaneously expands the evaporation area and improves the evaporation rate. From this point of view, the design also includes the densified pure metal structure, which is covered with a layer of capillary material outside. Because the thermal conductivity of pure metal, especially pure copper, is higher than that of capillary structure, the internal pure metal conducts heat to the surface capillary structure with higher efficiency, and the strength of pure metal is also better. This type of design is various, and the bearing capacity of VC heat flux can reach 30~100W/cm2.

At present, there are some more advanced special capillary structures in the stage of research, development and application, such as the radial channel structure of etching processing. The liquid is supplemented to the heat source area through the evaporation layer and a series of lateral convergence channels, which greatly improves the heat flux bearing limit of VC. Or use the bionic design, learn from the unique heat and mass transfer mechanism of plant leaf vein structure, effectively balance the contradiction between permeability and capillary force, and obtain lower thermal resistance and excellent temperature uniformity.

Fig. 5 Different types of VC inner cavity support and capillary structure (map source network)

Different from the mature process of heat pipe, VC process is still under exploration. Although many domestic and foreign companies have carried out mass production, there are still problems such as high manufacturing cost, low welding process efficiency, poor stability, deformation and reliability of good products.

Most VC manufacturers focus on screen technology. The copper wire mesh is easy to weave, the quality of the finished product is stable, and the production efficiency is high, but the capillary force is small, so it will face greater performance limitations when dealing with high heat flow density and counter gravity scenes.

3 Summary and outlook

Over the past 20 years, VC has made considerable progress in application. Some key technical nodes include:

1) Application of composite capillary core. The contradiction between VC capillary force and working fluid flow resistance is coordinated through the combination of capillary materials with different apertures;

2) Application of capillary column/ring. On the basis of the support column, the sintered capillary column/ring is added to replace or cooperate, which not only ensures that VC has sufficient strength, but also greatly shortens the return path of condensate and improves the heat transfer performance of VC;

3) The lightweight demand for VC in the field of consumer electronics has led researchers and manufacturers to invest a lot of research on different materials, processes and processes;

4) The development of diffusion welding technology has further improved the performance and appearance of VC, as well as the yield.

The development trend of high power consumption and high heat flow density chips puts forward higher requirements for VC's temperature equalization performance. The optimal design of VC must improve the efficiency of heat conduction and gas-liquid transportation from the material and structure aspects while improving the capillary performance, so as to significantly reduce the thermal resistance of VC. Only when the working heat flow density is doubled or even several times, The temperature difference from the heat source to the VC cold surface is still equivalent to the current low heat flux application conditions.

On the theoretical level, although many researches focus on reducing the thermal resistance, improving the drying limit and critical heat flux, and achieving higher heat flux carrying capacity, in order to more accurately predict and evaluate the heat transport process and limit inside VC, it is necessary to find appropriate methods to simulate the gas-liquid interface in different capillary structures, which still requires more basic theoretical work to deeply analyze the physical mechanism of VC.

At the application level, the characterization of VC's heat transfer capacity and thermal resistance needs to be more refined, and a more perfect experimental database should be established to summarize the rules of VC affected by working conditions, so as to more accurately grasp the effect of VC's application in the system and improve the reliability of product design.

At the same time, the improvement and optimization of materials and processes for various structural types of phase change heat transfer also depend on the joint exploration of the industry to build it into a strong bow of heat dissipation technology and contribute more to the development of 5G communication.