Application Research of Microchannel Cooling in IGBT

Microchannel cooling is the etching of microscale channels on a substrate. The heat is carried away by the working fluid flowing in the microchannel after being conducted through the substrate. It is an efficient cooling method, and has great development prospects in the fields of electronic computer chip cooling, aerospace and other fields. Since the concept was put forward, it has been widely concerned by scholars at home and abroad, but the heat transfer mechanism of microchannels is relatively complicated and has not yet been clarified. In order to explore the application prospect of microchannel cooling technology in the field of thermal management of microelectronic devices, the existing research was analyzed and evaluated.

1. Microchannel Size Structure

The criteria for distinguishing macro- and micro-scale channels is a very important topic. This not only affects the channel design of theoretical research, the scope of application of correlation, etc., but also has important guiding significance for the selection and processing of channel size in practical applications. Early scholars have carried out a lot of research on the difference between microchannels and conventional channels. KANDLIKAR gives the diameter range of different types of channels based on the hydraulic diameter. However, the THOME report pointed out that it is not entirely reasonable to distinguish micro and macro scales only based on the channel size, but the fluid properties must be considered at the same time. Because there are many parameters that change and affect the flow as the channel diameter decreases. In order to distinguish microchannels from conventional channels, some researchers put forward different standards for the definition of microchannels.

The division method based on channel hydraulic diameter is shown in Table 1. This division method was proposed by LEE and KANDLIKAR et al. This standard comprehensively considers the application of heat exchangers and the mechanical manufacturing technology at that time, and gives corresponding references.

In flow boiling, the threshold from macroscale channel to microscale channel has not yet formed a generally accepted standard. Some scholars have summarized the two-phase flow experimental data of microchannels (100 μm ≤ D ≤ 4.2 mm) and various test fluids (deionized water, FC-72, H2O, R11, etc.). Based on the analysis of relevant studies, it is concluded that the threshold from the macro-scale channel to the micro-scale channel is related to the bubble escape diameter and bubble aggregation, and the definition of the boundary between the macro-scale and the micro-scale needs to consider the bubble limit. From the point of view of force, the detachment of bubbles in flow boiling in a tube is controlled by surface tension and buoyancy. Some scholars have proposed a division method using a restriction number (Co, Confinement Number). The Co number represents the relative size of surface tension and gravity in the channel. When Co > 0.5, the heat transfer and flow characteristics are significantly different from those observed in large channels. Therefore, Co > 0.5 can be considered as the division standard of microchannels. This division method of quoting the limit number Co is a typical representative of the division method of microchannels judged by bubble force analysis, and it is proposed based on certain experimental conditions. The standards obtained under different experimental conditions are different, so it is generally used as a reference for theoretical research under similar experimental conditions.

At present, in the field of industrial applications, it is widely believed that channels with a hydraulic diameter D ≤ 1 mm can be called microchannels.

The structure of the microchannel has a great influence on the heat transfer performance of the microchannel. Reasonable microchannel geometry design is the key to heat transfer enhancement. Scholars have done a lot of research on microchannel structures, involving different microchannel fabrication, flow patterns, pressure drop characteristics, and heat transfer characteristics under different channel structures.

Starting from the earliest proposed parallel multi-channel silicon channel structure, people have studied the structure and shape of various microchannels to improve their heat transfer performance. In terms of cross-sectional shapes of microchannels, scholars have explored microchannels with cross-sectional shapes such as circular, triangular, rectangular, and trapezoidal shapes, and analyzed the heat dissipation performance under different cross-sectional shapes. It was found that the difference in cross-sectional shape has a great influence on the heat dissipation performance. In addition to regular-shaped cross-sections, scholars have also studied irregular-shaped structures, such as a concave microchannel in the cross-section, which is called BCT (buried channel technology). Manufactured using buried channel processing technology on a silicon substrate. The groove depth and width are 75μm and 5μm respectively, which provide a new path for fluid flow in the microchannel. Some scholars proposed a new type of microchannel with concave Ω grooves on the sidewall, with a geometric size of 200 μm×253 μm. The channels are not connected to each other. The experimental results show that the microchannels with grooves can promote the nucleation of bubbles and significantly increase the critical heat flux. It helps alleviate the instability of rapid bubble growth and eases the instability of fluid boiling. Some scholars have designed a reentrant porous microchannel with Ω-shaped microchannels by using powder sintering technology, which is called RPM (reentrantporous microchannels). Hydraulic diameter 786 μm. Experiments show that the structure can greatly improve the heat transfer performance of microchannel single-phase flow and two-phase flow, and can reduce the instability of two-phase flow. Some scholars have carried out three-dimensional numerical analysis and optimization on the parameters of the sidewall grooves of the microchannel, and obtained a trapezoidal groove with a groove tip length ratio of 0.5. The microchannel with groove depth ratio of 0.4, groove pitch ratio of 3.334, and groove direction ratio of 0 has the best heat transfer performance and minimum flow resistance. In order to improve the flow inside the microchannel and improve the heat transfer performance, researchers have also designed many microchannel heat exchangers with different flow channel forms, such as corrugated form, pin fin form, cylindrical oblique fin, staggered fin, double fin, etc. Microchannel heat exchange such as layers and microchannel heat exchangers with small cavities, etc. Scholars have also conducted research on some special-structure flow channels, and found that the use of some special-structure flow channels may have the effect of enhancing heat transfer. For example, the linear microchannel is improved, and the rectangular cross-section wave microchannel is proposed. The single-phase simulation results show that the wavy microchannel can generate eddy currents, improve the convective heat transfer coefficient, and have a smaller pressure drop than the linear microchannel. This study also shows that the relative amplitude change along the flow direction does not have much impact on the compactness and efficiency of the microchannel. Reducing the wavelength of the wave microchannel can make the temperature distribution of the equipment more uniform and reduce the generation of local overheating. Three types of porous interconnected microchannel networks were fabricated by copper powder sintering and wire electrical discharge machining. In the test, it is concluded that the 0.4 mm porous interconnected microchannel has the best heat transfer performance and the ability to alleviate the instability of the two-phase flow. In addition, some scholars have designed bionic microchannel topology, such as leaf vein microchannel topology, human trachea tree topology, spider web topology, river network structure, honeycomb structure and insect wing vein structure.

According to the numerical calculation results, compared with the rectangular chip, except for the river network structure, the topological structure of each biomimetic microchannel has a stronger heat dissipation capacity than the ordinary rectangular flat microchannel. And with the increase of the heat flux density of the chip, the difference in the heat dissipation capacity of various microchannel structures is more obvious. Therefore, in the application of high heat flux density chips, the microchannel topology has a great influence on the heat dissipation effect of the chip. According to experiments, it is found that the spider web structure has the advantages of large heat dissipation specific surface area, high average convective heat transfer coefficient, and good fluid flow performance. Its comprehensive heat dissipation performance is optimal, and the inlet and outlet pressure drop is lower, which has good engineering application value.

Extensive research on the microchannel structure has demonstrated the excellent heat dissipation potential and broad development prospects of the microchannel cooling technology, and also laid a solid foundation for the further popularization and application of the subsequent microchannel cooling technology. In addition to the size structure, whether the phase change of the fluid in the channel is also one of the important factors affecting the heat dissipation ability of the microchannel. According to whether the fluid phase changes or not, microchannel technology can be divided into two types: microchannel single-phase cooling and microchannel flow boiling (two-phase) cooling. In the following, the existing studies on single-phase and two-phase microchannels are reviewed and analyzed.

2. Microchannel Single-phase Cooling Technology

Single-phase cooling means that the cooling medium remains in the same state (usually liquid) throughout the cooling process, without boiling or condensation. Compared with the conventional scale single-phase cooling device (system), the micro-channel single-phase cooling device (system) has a larger heat transfer area and micro-scale effect under the same volume, and the overall heat dissipation performance is stronger. It can be seen that water is the focus of the microchannel single-phase experimental research. At present, there are great expectations for the application of microchannel cooling technology in the field of electronic chip cooling. Therefore, many experimental studies directly use electronic chips as heat sources to analyze the heat transfer performance of microchannel heat sinks. Most of the microchannel lengths tested in experiments are in the range of 10-20mm.

In contrast, there are few studies on modular IGBT module products. The overall size of the IGBT package module is relatively large compared to computer electronic chips. It is usually necessary to design microchannels with a length greater than 50 mm, which is unfavorable for single-phase cooling. Because a longer channel will cause a larger temperature difference between the inlet and outlet, it may increase the risk of thermal runaway of the IGBT package module due to uneven temperature. This is a problem that cannot be ignored when micro-channel single-phase cooling technology is applied to IGBT thermal management. Some scholars have carried out research on this issue, such as developing an integrated vapor chamber microchannel liquid cooling radiator for high-power IGBTs in order to enhance the temperature uniformity of microchannel single-phase cooling. Integrate the micro-channel heat sink with the vapor chamber. By comparing the integrated micro-channel heat sink with the simple micro-channel heat sink, the excellent comprehensive performance of the integrated micro-channel heat sink is verified. This provides important insights into the application of microchannel single-phase cooling heat sinks for thermal management of high-power IGBTs.

3. Microchannel flow boiling (two-phase) cooling technology

Flow boiling (two-phase) cooling is mainly a cooling method in which heat is taken away through the phase transition heat absorption during fluid flow boiling. Relying on the heat transfer characteristics of flow boiling and the micro-scale effect of microchannels, flow boiling cooling in microchannels has the advantages of compact radiator structure, strong heat transfer capacity, high heat transfer coefficient, good temperature uniformity and less working fluid charge. . The use of microchannel flow boiling cooling is one of the excellent potential solutions to alleviate the large temperature difference between inlet and outlet in single-phase cooling. Some of the application research results of microchannel flow boiling cooling in the field of IGBT thermal management are as follows. Aiming at the microchannel flow boiling heat transfer under the condition of large size and high heat flux density of the IGBT module, the influence of different heating directions on the microchannel flow boiling heat transfer was studied experimentally with R134a as the cooling medium. The research results show that there are two heat transfer mechanisms in the microchannel: nucleate boiling and forced convective boiling. The heat transfer performance of bottom heating is better than that of top heating. And the mass flow rate of the fluid in the channel and the heat flux density of the heating surface have an important influence on the microchannel wall temperature and heat transfer coefficient. The research also summarizes the modified heat transfer correlation based on the experimental data, which can accurately predict the heat transfer coefficient when the top is heated, with an average error of 16.6%. Some scholars have established cooling experiment systems under natural circulation power and forced circulation power suitable for IGBT modules. The start-up and heat transfer characteristics of the microchannel unit-natural circulation cooling system, the characteristics and rules of flow boiling heat transfer of R134a in the microchannel, the transformation of the R134a flow pattern and the transformation of the heat transfer mechanism in the microchannel were explored. The microchannel structure was optimized based on experimental data and theoretical research. At present, there are few related studies on the application of microchannel flow boiling cooling to the heat dissipation of IGBT modules. There is a lack of research on the structural design of the microchannel flow boiling heat sink with a larger size structure for the IGBT module, a longer channel length, the actual heat dissipation effect, and the selection of the mass flow rate under different heat flux densities. The actual effect of microchannel flow boiling cooling on the heat dissipation effect and temperature uniformity improvement of the IGBT module needs further research and verification.

Although there are few studies on the application of microchannel flow boiling to IGBT heat dissipation related fields, scholars have carried out extensive research on the heat transfer mechanism and factors affecting heat transfer performance of microchannel flow boiling heat transfer and achieved certain research results.

Microchannel flow boiling heat transfer is more complicated than single-phase flow heat transfer, and there are many influencing factors. The research direction is broad, involving the evolution of boiling heat transfer flow pattern, bubble dynamics, boiling heat transfer and heat transfer characteristics, heat transfer reliability and flow stability research. Boiling heat transfer has the interaction between bubbles and liquid. There are different flow patterns inside the channel, and different flow patterns will directly affect the heat transfer effect on the surface of the microchannel, which brings great challenges to the study of heat transfer mechanism. Flow boiling heat transfer in regular-scale channels is affected by two basic mechanisms. One is the dominant mechanism of nucleate boiling related to the formation of bubbles at the wall surface and the dynamics of the bubbles. The other is a convection-dominated mechanism related to conduction and convection through the liquid film. The existing research results on the heat transfer performance of microchannels show that the flow boiling heat transfer in microchannels also has the influence of these two heat transfer mechanisms. The nucleate boiling dominant mechanism strongly depends on the heat flux, while the convective dominant mechanism strongly depends on the mass flow rate. The combined action of the two mechanisms makes the heat transfer characteristics of flow boiling very complicated. In addition to the micro-scale influence, most of the traditional flow boiling heat transfer correlations are not enough to predict the flow boiling heat transfer in micro-scale channels. It is difficult to obtain a unified description of the heat transfer mechanism under different working fluids, different channel structures, and different flow patterns, and further research is needed. The experimental research results of microchannel flow boiling cooling are summarized. Scholars have carried out a large number of experimental studies under different working fluids, different channel numbers, and different working conditions. The law provides a solid experimental basis.

In general, most of the current research is to explore the heat transfer mechanism as the main purpose. Many scholars have studied the heat transfer characteristics and mechanism of microchannel flow boiling in the way of uniform heating. However, the difference between the uniform heating scene and the non-uniform heating scene such as the IGBT module makes the heat transfer characteristics of the corresponding microchannel flow boiling heat sink and the factors to be considered in the design may be different. Moreover, the complexity of the heat transfer mechanism of microchannel flow boiling still makes it impossible to draw convincing and clear conclusions. The characteristics of microchannel flow boiling heat transfer are different under different working fluids, different channel structures, different application scenarios and experimental conditions. At the same time, the heat dissipation design of the IGBT module not only pays attention to the temperature uniformity among the internal chips, but also needs to consider the relationship between the temperature distribution of different chips and the relative positions of the inlet and outlet, the reasonable length of the microchannel, and the partial drying of the fluid in the microchannel. phenomenon and other issues. The solution of these problems will be the key to the further popularization and application of microchannel flow boiling cooling in the field of thermal management of IGBT modules.