Cooling Method for IGBT Module

"Moore's Law" accurately predicted the rapid development of the semiconductor industry, but with the rapid development of electronic packaging technology and micromachining technology, the size and integration of transistors are getting closer and closer to the physical limit. The precision of existing lithography technology is limited, coupled with the frequent occurrence of problems such as leakage and heat dissipation, Moore's Law may no longer be able to accurately lead the development pace of the semiconductor industry in the future. The heat dissipation problem that has become increasingly prominent but has not been completely resolved has received widespread attention in the industry. The thermal management of highly integrated mechanical and electronic devices has become a bottleneck for the continuous development of the electronics industry and even the entire field of mechanical manufacturing and electronic control engineering. The heat dissipation capability of traditional air cooling technology can no longer meet the heat dissipation requirements of high heat flow electronic equipment. At the same time, as the types and application fields of electronic devices become more diverse, the operating conditions become more complex, and the working environment becomes more sophisticated, which requires the corresponding thermal management system to achieve efficient heat dissipation under different working conditions. Therefore, how to choose a suitable cooling technology and rationally design a thermal management system that is stable, reliable and flexible to adapt to different working scenarios has become one of the main problems to be solved in the field of high heat flow cooling.

IGBT is a new type of power semiconductor self-turn-off device, which has the advantages of small driving power, simple driving circuit, low steady-state loss, high input impedance, strong short-circuit withstand capacity and current carrying capacity, and is widely used in frequency converters, traction drives , AC motors, household appliances and other fields. It is a representative of a new generation of electronic components. The working performance of IGBT is greatly affected by temperature. During the working process, a large amount of heat is generated due to frequent disconnection. Once it is not dissipated in time, the internal temperature of the module will increase, resulting in changes in its semiconductor physical constants and internal parameters of the device. This makes performance indicators such as on-state voltage drop, switching off speed, off voltage peak, current tailing time and loss worse, and eventually causes the IGBT module to fail to work normally and reduces its working life. In addition, the large temperature difference inside the module will also cause thermal stress to cause thermal runaway, reducing the reliability of the module. Therefore, the heat dissipation problem of the IGBT module needs to be solved urgently. In the case that air cooling cannot meet the heat dissipation requirements of the module, single-phase water-cooled cold plates have become the mainstream cooling method for the current large-power IGBT module heat dissipation system. However, in recent years, the process of miniaturization and high power of IGBT has been accelerated. Due to the increase of module heat load, the single-phase water-cooled cold plate is gradually unable to meet the rapidly increasing heat dissipation demand, and the forced convective heat transfer of circulating water inside the cold plate has an inevitable low temperature uniformity problem. For this reason, many scholars began to focus on flow boiling heat transfer. Microchannel flow boiling heat transfer has the advantages of strong heat transfer capacity, high heat transfer coefficient, good temperature uniformity and less working fluid charge. It is an ideal heat dissipation method for IGBT heat dissipation. In recent years, with the progress of micromachining technology and the reduction of the processing cost of microchannels, the flow boiling heat transfer in microchannels has great development prospects.

1. Structure and thermal resistance analysis of IGBT module

Most of the products currently on the market are modular IGBT products. It is a modular semiconductor product that is packaged by IGBT chips and FWD through a specific circuit bridge. It has the characteristics of energy saving, convenient installation and maintenance, and stable heat dissipation. The packaged IGBT module is mainly composed of chip, copper circuit layer, insulating ceramic layer, copper layer, and substrate, as shown in Figure 1.

The three parts of copper circuit layer, insulating ceramic layer and copper layer mainly play the role of heat transfer, insulation and thermal stress relief. Multiple IGBT chips can be packaged inside the IGBT module. By connecting multiple IGBT chips in parallel in the module, high current handling capability can be achieved, so as to avoid the problem of reducing the yield of IGBT chips while increasing the active area. Compared with single-chip modules, packaged modules with multiple IGBT chips inside have a more complex structure and have higher requirements for thermal management. IGBT module is a power device with high calorific value and its performance is greatly affected by temperature. In actual operation, the junction temperature must be controlled within a reasonable range to ensure normal operation. Excessively high operating temperature will change its semiconductor physical constants and internal parameters of the device, resulting in the failure of the IGBT module to work normally. In severe cases, it even affects its working life. Generally speaking, when the junction temperature of the IGBT chip exceeds 125 °C, its performance will drop sharply, and even the IGBT will be damaged. What's more, the thermal stress in the IGBT module due to the large temperature difference between the internal chips may cause thermal runaway and reduce the reliability of the module. Therefore, the thermal management of the packaging module not only needs to ensure that the temperature of each chip does not exceed the rated value, but also needs to pay special attention to the temperature difference of the chips at different positions.

As a power semiconductor self-turn-off device, the IGBT module has a certain power loss in the conduction work and the on/off process, which is usually called the on-state loss and switching loss. The on-state loss usually depends on the effective voltage and current during the conduction process, while the switching loss mainly depends on the switching characteristics and switching frequency of the IGBT device. The existence of on-state loss and switching loss is the most important factor leading to the heating problem of the IGBT module. At the same time, the temperature change inside the module due to heating also reacts on the on-state loss and switching loss, which affects the working performance of the module. The transfer path of the heat generated by power loss inside the IGBT is related to its package composition, that is, chip→chip soldering layer→copper circuit layer→ceramic layer→copper layer→system soldering layer→substrate→radiator.

2. IGBT Cooling Technology

At present, the IGBT cooling methods that have been widely used in the market include air cooling technology, heat pipe cooling and circulating water cooling technology. The heat dissipation technology commonly used in IGBT and the applicable heat flux range of the cooling technology that is a hot research topic in the field of IGBT thermal management are summarized, as shown in Figure 2.

Air cooling technology uses air convection heat transfer to remove heat to achieve the purpose of heat dissipation, which can be divided into passive natural convection air cooling and active forced convection air cooling. Natural convection air cooling is mainly due to the density difference caused by the temperature difference of the air at different locations. The generated buoyancy is the driving force, which drives the surrounding air to flow away heat. The radiator of this cooling method has a simple structure and is easy to maintain, and was widely used in the early days. But its heat transfer ability is poor, so it can only be used to cool devices with low power, low calorific value, and heat flux not exceeding 0.08 W/cm 2 . With the integration of IGBT power devices and the development of high power, the demand for cooling is increasing day by day. In order to meet the heat dissipation requirements and improve heat exchange efficiency, a fan or fan is installed on the IGBT device to promote air forced convection. The thermal resistance of forced convection air cooling can be reduced to 1/5 to 1/15 of that of natural convection air cooling, and the heat dissipation capacity is greatly increased. However, due to the addition of fans/fans and other devices, it is necessary to reasonably design the air duct and perform regular maintenance, which reduces the reliability of the system and the integration of devices, and it is accompanied by relatively large noise during operation.

In order to improve the cooling efficiency of air cooling technology, a radiator is usually installed on the IGBT module to increase the heat exchange area, and the common radiator is a finned radiator. The heat dissipation efficiency of the air-cooled radiator is affected by the fin structure, size, arrangement design, fan position and speed, and ambient temperature. After a lot of research and optimization, the air-cooled heat sink, especially the parallel aluminum fin heat sink, is the most commonly used heat sink in current IGBT cooling due to its simple design and mature manufacturing process. However, due to the problems of small air specific volume and low thermal conductivity, even forced convection air cooling has limited heat dissipation capacity. It cannot well cope with the heat dissipation requirements of the current IGBT integrated modules with high heat flux density and fast instantaneous temperature rise. In addition, problems such as temperature non-uniformity, noise, and system reliability also greatly limit the further development of air cooling technology.

In order to optimize the performance of air-cooled radiators, it is common to add heat pipes on top of them. The heat pipe cooling technology for IGBT heat dissipation is optimized on the basis of air cooling, and its typical heat pipe radiator structure is shown in Figure 3.

Heat pipes have the advantages of low heat transfer temperature difference, small size, and require no mechanical maintenance. Generally, the heat pipe is not used alone as a radiator, but is usually embedded in the fins of the air-cooled radiator, and uses its efficient phase change heat transfer to quickly transfer the heat from the IGBT module substrate to the air to achieve the purpose of heat dissipation.

Compared with the forced convection air cooling technology, the introduction of the heat pipe greatly improves the performance of the radiator. The heat pipe radiator has high reliability and low risk of working fluid leakage. Therefore, it has a certain application foundation in the current IGBT thermal management market. However, most heat pipe radiators, like air-cooled radiators, need to cooperate with external fans/fans to achieve higher heat dissipation efficiency. The working efficiency of the heat pipe radiator is also affected by the fan type, wind speed, ambient temperature, etc., and there are problems such as regular maintenance and noise during operation. In addition, after adding the heat pipe structure, the overall size of the radiator increases. For example, a cylindrical heat pipe radiator combined with fins is usually only suitable for heat dissipation scenarios with a large space, which is not conducive to improving the compactness and integration of IGBT modules.

When the power density of the IGBT module increases and the air channel design, reliability, noise index and other conditions are limited, it is relatively difficult to implement the air cooling technology and the heat pipe cooling technology. It cannot well meet the equipment operation and heat dissipation requirements. As a result, water cooling technology came to the stage. Water has good thermal conductivity, large specific heat capacity, and almost no pollution. Compared with air-cooled heat dissipation, the use of water-cooled radiators (or water-cooled plates) has higher heat dissipation efficiency, smaller volume, and easier layout of the heat dissipation system. It is more suitable for cooling system of larger power IGBT modules. Therefore, circulating water cooling has become the mainstream cooling method for the cooling system of larger power IGBT modules. The circulating water cooling radiator can be divided into the following two types according to the packaging form between the radiator and the IGBT module. One is a separate heat sink formed by combining the IGBT module and the water-cooled plate as two independent components. Use the water circulation in the cold plate to take away the heat of the IGBT module. The separate IGBT water-cooled radiator is easy to install, but it also causes contact thermal resistance on the contact surface between the IGBT module and the cold plate. Moreover, the greater the IGBT heat generation, the greater the impact of the thermal contact resistance on the performance of the heat sink. Therefore, in practical applications, it is necessary to apply thermal conductive silicone grease to the contact surface to reduce the thermal contact resistance. The other is to directly package the IGBT on the finned water-cooled plate substrate. This heat sink eliminates the contact thermal resistance between the substrate and the cold plate, and has higher heat dissipation performance. Studies have shown that the thermal resistance of this integrated form of the module and the heat sink is 33% lower than that of the separated heat sink. However, this form of radiator brings inconvenience to disassembly and assembly, and also increases the risk of the cooling liquid contacting the internal chips and circuit boards, so the requirements for the electrical insulation of the cooling water are more stringent. At present, the application of circulating water cooling technology in IGBT is relatively mature, and scholars have carried out a lot of research on water cooling system and structural design optimization.

Although circulating water cooling technology has many advantages, the problem of low temperature uniformity cannot be ignored. Especially for IGBT chip, its power conversion efficiency will increase with the decrease of junction temperature of IGBT chip. Poor temperature uniformity will lead to different junction temperatures between IGBT chips at different positions, making each IGBT chip have different power conversion efficiencies, resulting in different power outputs. This is very detrimental to the operation and reliability of the module, and even causes thermal runaway and damages the device in severe cases. In the face of increasing heat load, the single-phase cooling performance of traditional cold plate circulating water is seriously affected by the relative position of the inlet and outlet. The cooling performance will be greatly reduced near the outlet, and the temperature non-uniformity will be more prominent.

In order to alleviate this problem, traditional circulating water single-phase cooling usually adopts the method of increasing the pump work. Increase the mass flow of water in the cooling system, but this method increases power consumption and the effect is not ideal. Therefore, it is urgent to develop a new cooling technology with excellent heat dissipation performance and good temperature uniformity.

Spray cooling technology is a very efficient new liquid cooling technology. It uses a nozzle to atomize the cooling liquid into a group of micro-droplets and then violently hits the surface of the heat source to form a thin film of cooling liquid on the surface. With the flow of the liquid film, evaporation and the formation, growth, and detachment of the bubbles in the liquid film, the rapid heat dissipation effect is achieved. Spray cooling has strong heat transfer capacity, good temperature uniformity of heat transfer surface, and small demand for working fluid, which is an effective high heat flow cooling method. Since the concept of spray cooling was proposed in the 1980s, domestic and foreign scholars have carried out a lot of research work on theory and experiment.

A large number of studies have shown that the spray cooling technology has high heat dissipation efficiency and has a good development prospect in the field of high heat flow heat dissipation. However, due to the complexity of the spray cooling process and the interaction of many influencing factors, it is difficult to solve the mathematical model, which brings great difficulties to both theoretical analysis and experimental research. At present, the research on spray cooling mainly adopts the combination of model research, numerical simulation and experimental research. So far, no clear conclusions about the heat transfer mechanism and influencing factors of spray cooling have been obtained. In addition, the design of the spray cooling device is difficult, including the structure of the nozzle, the distance between the nozzle and the surface, the inclination angle, and the spray flow rate. A large number of parameter support is required, which brings great challenges to the design and promotion of heat sinks, which in turn limits its expansion and application in the thermal management market.

Jet impingement cooling is a cooling method in which the coolant directly impacts the surface of the heat source at a high speed under the pressure difference of the nozzle to achieve efficient heat exchange. The typical jet impingement flow field is divided into three regions: free jet region, stagnation point region, and wall jet region. Among them, the stagnation point region is the main action area of jet cooling, and the fluid will form a very thin velocity and temperature boundary layer in the stagnation point region. The temperature gradient, axial velocity gradient and pressure gradient in the boundary layer are very large, so that the The parameters change drastically, resulting in high local heat transfer coefficients.

Due to the complexity of the jet cooling flow field structure, it is usually impossible to obtain accurate quantitative conclusions on the flow and heat transfer characteristics only by theoretical analysis. At present, a combination of formula derivation, numerical simulation and experimental research is generally used to obtain relatively accurate quantitative conclusions.

Due to the limited range of single-hole jet impact, it is easy to cause low temperature uniformity on the surface of the heat source, which is very unfavorable for large-area heat sources. To solve this problem, jet arrays with multiple nozzles are usually used. Figure 4 is a distributed back flow jet array device, which can greatly improve the temperature uniformity of the equipment.

Existing research results show that the heat dissipation capability of jet array cooling is excellent. Especially in reducing local hot spots and increasing the overall temperature uniformity of large-area equipment. But it is similar to spray cooling, the heat transfer mechanism and flow field characteristics are extremely complex, and the design is difficult. In addition, when the jet impingement cooling device works for a long time, the impact force of the high-speed fluid will be potentially destructive to the surface of the IGBT module, which restricts the large-scale application of the jet impingement cooling technology in the electronic device cooling industry.

3. Summarize

With the increase of heat load of IGBT modules in recent years, the air cooling method is gradually unable to meet the rapidly growing demand for heat dissipation. Although heat pipe cooling technology and circulating water cooling technology have improved the heat dissipation capacity, they do not have obvious advantages in the field of high heat flow in the future. IGBTs are in urgent need of advanced thermal management techniques. Although spray cooling technology and jet impingement cooling technology can achieve high heat dissipation capacity. However, their heat transfer mechanism is complex, and a relatively unified heat transfer theory and heat transfer law have not yet been obtained. In addition, the nozzle design is difficult, making it unlikely that they will be marketed in a short period of time. In contrast, the microchannel cooling technology has the advantages of compact structure, strong heat transfer capacity, high heat transfer coefficient, less working fluid charge, and good temperature uniformity. Compared with jet cooling and spray cooling radiators, microchannel radiators are easier to achieve further popularization and application, and have great development prospects in the field of thermal management of electronic devices.