Current status, challenges and development trends of vehicle-level power semiconductor technology

Release on : May 7, 2019

On April 25, 2019, Liu Guoyou, a semiconductor technology expert and deputy chief engineer of the semiconductor business unit of Zhuzhou CRRC Times Electric Co., Ltd., gave a speech on "Application of Power Semiconductor New Materials and Technical Difficulties" in the course of the future course of the 100-member conference. And interact with the participants on the spot. This article is edited and organized on the basis of the course.

This article discusses the technology and application prospects of vehicle-grade power semiconductors through the following four aspects:

1. Power semiconductor technology and applications;

Second, the application of IGBT technology in new energy vehicles;

Third, the car IGBT technology challenges and solutions;

Fourth, the development trend of vehicle power device technology.

Power semiconductor technology and applications

Recently, the field of semiconductor chips is very lively. The chip is the "food" of industry, the basic core industry that supports the development of the whole society and economic development, and the lifeblood of the national economy. As the "CPU" of the energy Internet of Things, power semiconductors are bridges between weak current control and high-power operation, enabling energy transmission, conversion and control. Power semiconductors have unique structures, mechanisms, and manufacturing processes that incorporate more and more microelectronic manufacturing processes that differ from the integrated circuit chips we encounter in our daily lives.

Evolution of power semiconductor material technology
Power semiconductor devices are inseparable from the support of materials. Semiconductor materials have experienced three generations since the 1940s: the first generation is elemental semiconductors, the main materials are germanium (Ge), silicon (Si); the second generation is compound semiconductors, the main material is gallium arsenide (GaAs) Tin sulfide (InP); the third generation is a wide bandgap semiconductor, the main materials are silicon carbide (SiC), nitrided (GaN), which has a wider band gap, a higher critical breakdown electric field, and higher Its thermal conductivity makes it ideal for high pressure, high temperature and high frequency applications. The second-generation compound semiconductors are mainly used in microwave radio frequency, and the materials used in high-power semiconductors are mainly the first generation and the third generation.

Evolution of power semiconductor technology
Since the invention of power semiconductor devices in the United States in the 1940s, the technological evolution has also gone through three generations: the first generation includes manifolds, silicon diodes, and thyristors, represented by silicon-based thyristors. The main feature is semi-control, which can only control conduction. The second generation is mainly based on silicon materials, including MOSFET, GTO, IGBT and IGCT. The main feature is that it can not only control the turn-on, but also has the ability to turn off autonomously, which can realize free switching and higher frequency. The third generation of semiconductors, mainly wide-bandgap materials, including SiC, GaN materials, etc. Although SiC materials and devices began to study in the 1980s, the real rapid development is still after 2000, although there are SiC SBD and MOSFET products, but the technology is not yet mature.

At present, the mainstream, more used semiconductor devices, one is the thyristor, which can output larger power, but the frequency is relatively low, mainly used for DC transmission and high-power low-frequency power supply; the second is IGCT, which is GTO and gate The drive circuit is integrated in a low-sensitivity manner, which can improve the shutdown performance. The device is currently mainly used for high-power motor drives, including ship drives, offshore wind power, etc. The third is IGBT, which has broken through the technical bottleneck since the 1990s. Industrialization and large-scale application have been realized. At present, high-power IGBTs can achieve up to 6500V, and their application methods are wide, including rail transit, photovoltaic power generation, automotive electronics, etc., which are the current mainstream switching devices.

Progress in IGBT technology and its application in new energy vehicles

IGBT is a composite power semiconductor device developed on the basis of MOS. It is a combination of traditional power electronics technology and microelectronic technology. The IGBT is a voltage-controlled power switching device with an operating frequency between MOS and BJT and a large power capacity. Its biggest technical challenge is to withstand the harsh application of microelectronics' fine structure and to withstand the various electromagnetic thermo-mechanical stresses of power switches and switching processes in traditional power electronics.

IGBT was invented in the early 1980s. In the past 30 years, its power density has increased by 3 times, the loss is only 1/3 of the original, the reliability is getting higher and higher, the chip is getting smaller and smaller, but the control power is getting more and more. Big.

Technical challenges in the development of IGBT
There are three technical bottlenecks in the development of current IGBTs: one is to reduce the conduction loss; the other is to reduce the turn-on loss and turn-on loss; the third is robustness, so that all parameters are run within the safe working area (SOA), including Forward Safety Work Area (FBSOA), Reverse Bias Safe Work Area (RBSOA), Switch Safety Work Area (SSOA), and Short Circuit Safety Work Area (SCSOA). Balancing and optimizing these three relationships can enable IGBTs to develop toward higher power, higher density, and higher reliability.

Package form
At present, IGBTs mainly have the following package types:

The first is the TO standard plastic package module structure, which is simpler in structure and lower in cost, mainly in the form of a single chip or an IGBT and a diode, but the thermal cycle life of this package is limited.

The second is the industrial/automotive standard power module structure. At present, as long as the power exceeds several kilowatts, the module package will be adopted, which is characterized by good heat dissipation, relatively good power cycle and thermal cycle life, and process. More mature than other forms, the cost is relatively moderate.

The third is the crimp IGBT. This package has no solder layer or wire bonding. Its biggest feature is its large power capacity, high safe working area (SOA), and fault short-circuit characteristics. It is suitable for series applications. Can withstand higher voltages, but the process is more complicated and the cost is higher.

IGBT application
The following describes the IGBT reliability requirements with the application of high-speed rail and pure electric vehicles:

The IGBT commonly used in high-speed rail is 1500A/3300V welded module. The long group consists of 256 modules, including 9216 chips. The converted chip area is about 1.68m2, and there are nearly 150 million IGBT cells, as long as one of the cells fails. This high-speed rail will fail, so the IGBT reliability requirements are very high.

The automotive IGBT power is much smaller, mainly used for motor drive, DC/DC boost converter, bidirectional DC/AC inverter, and DC/DC buck converter for charging terminal. The motor controller uses a 6-unit module. The IGBT includes 36 chips, about 2.4 million cells, any one cell fails, the car will also fail, and the reliability requirements for the IGBT are also very high. In addition, new energy vehicles have higher technical requirements for power semiconductors in terms of loss, power density and cost.

At present, not all devices can be applied to new energy vehicles. In the new energy vehicles, the IGBT faces complex use environment and application conditions: First, the power cycle fluctuation of the vehicle working condition is more complicated than that of the high-speed rail; the second is the long-term working environment with high vibration, high humidity and high temperature; the third is the assembly volume. , weight and manufacturing costs are strictly limited; Fourth, the long-term life of automotive modules is longer than 20km or longer than 15 years.

Technical requirements for power semiconductors
Automotive-grade power semiconductor module standards have long been missing, and only a few standards have been available for reference in recent years. The automotive IGBT is to meet the requirements of the latest automotive grade standard LV324/AQG324, and the second is the group standard of China IGBT Alliance and Zhongguancun Wide Band Gap Alliance. These standards set higher standards in terms of temperature shock, power cycling, temperature cycling, junction temperature and other aspects related to full life cycle reliability.

At present, IGBT technology can provide a complete solution for automotive applications to meet the diverse needs of the market, including higher power density, higher reliability, higher operating junction temperature, lower cost and customized ability.

As the largest application market for new energy vehicles in the world, China will have nearly 1.3 million vehicles in 2018 and 2 million new energy vehicles by 2020, providing a very good market space for the automotive IGBT market.

Technical challenges and solutions for car-level IGBTs

New energy vehicles place very high demands on IGBTs, requiring them to have more powerful and efficient power processing capabilities, while also reducing their own excessive power consumption and unnecessary heat generation to improve the performance of the vehicle.

Technical problems and challenges
The main technical problems encountered at present: one is the low loss caused by driving mileage and emission requirements; the second is the high power density and heat dissipation caused by lightweight; the third is the requirements of harsh operating environment and extreme working conditions. The safe working area and long-term reliability issues; Fourth, the low cost of the automotive industry's ecological requirements.

At present, the technical challenges faced by car-level IGBTs are as follows: First, the design and manufacture of chips, the chip is difficult to balance before low power consumption, high reliability and high power capacity; second, the whole process of chip packaging, to solve efficient heat dissipation and reliability. Wait for performance verification to ensure long-term reliable operation of the car; third, how to ensure the normal operation of IGBT under certain special conditions through drive control.

Car gauge IGBT topology
At present, there are three kinds of IGBT topologies: single-tube modules, 2 in 1 modules, and 6 in 1 modules, which form a complete 3-phase full-wave rectification circuit, which can realize different integration application requirements in automobiles through flexible circuit topology.

Enterprise technology roadmap
(1) Infineon

Since 2013, Infineon has made many contributions to the advancement of automotive-grade IGBT chip technology, including IGBT3 and HybridPACK 2. The voltage is increased from 650V to 750V, the current density is increased from 150A/cm2 to 270A/cm2, and the power density is about 2kw/mm2, which is higher than the high-iron IGBT power density.

(2) Mitsubishi

Mitsubishi has done a lot of research and development work in the chip and packaging, and its double-sided cooling technology is relatively leading.

(3) Middle car

Since 2008, CRRC has entered the field of IGBT. After 10 years of development, the application of IGBT on high-speed rail has been realized, and the application of power grid has begun to be mass-produced. Since 2012, CRRC has begun to develop the research and development of car-level IGBTs. At present, it has developed two generations of car-level IGBT technology, including the 5th generation high-performance trench gate and the 6th generation fine trench gate IGBT chip, and the current density reaches 275A/cm2, which is equivalent to foreign advanced level, has begun to develop next-generation higher current density chips based on reverse-directed RET technology.

Car IGBT technology solutions
(1) High performance FRD

The use of high-performance FRD can effectively reduce the oscillation and interference problems and stabilize the operation of the car-level IGBT. At present, CRRC FRD is mainly based on the advanced PIC structure, through the proton and electron irradiation, to control the life of the minority, and achieve a good match with the performance of the IGBT. Another solution is to use SiC SBD to realize the freewheeling control of IGBT. This method can reduce the loss of IGBT module by 30%, but the disadvantage is that the reverse recovery of SBD is fast, which will cause a large current impact on the turn-on of IGBT. Affect long-term reliability. Therefore, if you can find a better performance FRD, it is not recommended to use a mixture of two different frequency devices, which will affect the overall performance and reliability.

(2) Layout optimization and low-sensitivity design

Through layout optimization and low-sensitivity design, the influence of EMI under high-frequency application conditions can be reduced, the current sharing of the chip can be improved, the voltage overshoot of the switching process can be reduced, and the power density can be improved.

(3) Insulation structure

IGBTs are mainly used to achieve negative and positive insulation through ceramic linings. Currently used linings include AlN ceramic linings, Al2O3 ceramic linings, and Si3N4 ceramic linings. For cost control, most automotive-grade IGBTs are packaged using Al2O3 ceramic liners. However, the difference in thermal expansion coefficient between Al2O3 and silicon and copper is large, which causes the fatigue of the material and the degradation of the soldering layer during use, thus affecting the service life of the entire module.

(4) Optimizing the packaging material system

The improvement of IGBT electrical performance has encountered bottlenecks. It can improve the performance and reliability of IGBT by improving thermal design, optimize the packaging material system, reduce the interface of package interconnection, and reduce the mismatch of thermal expansion coefficient as much as possible. Integrated lining to improve the characteristics of the IGBT module.

(5) Efficient cooling technology

There are also technical measures on the substrate to improve the performance of the car-level IGBT: direct liquid cooling with Pin-fin structure, double-sided water cooling technology, can reduce the thermal resistance of 20%-30%.

(6) module package reliability design

The biggest problem in the reliability of IGBT use is the solder layer. Long-term operation will cause material degradation, which will cause thermal resistance and conduction voltage drop to rise. In response to this problem, there is a new solution: through the low-temperature sintering silver junction technology, to achieve ultra-high melting point, excellent thermal conductivity, so as to obtain better thermal cycle and power cycle capability.

Copper process technology also improves reliability. The high-speed rail module uses copper metallization + copper wire stitching; the automotive module uses copper foil sintered + copper wire bonding. The power cycling capability can be increased by a factor of 10 by this copper process technology.

Another way to improve reliability is double-sided soldering. At present, the packaging of all modules is realized based on the single-sided heat dissipation mode, and the double-sided soldering allows the chip to be directly connected to the lining plate, eliminating the lead interconnection and improving the power cycle capability by 50%.

The power terminal is where the IGBT passes the maximum current and generates the maximum heat. The quality of the terminal connection directly affects the application reliability of the IGBT module. It is also one of the main failure modes of the IGBT, and it is relatively high among all the factors that cause the failure of the IGBT. The traditional process is to realize the interconnection of the terminal and the lining plate through the welding piece. The new technology mainly reduces the contact thermal resistance by the ultrasonic welding of the metal segment, improves the heat dissipation capability, increases the current carrying capacity, and improves the mechanical vibration in the vehicle environment. And the resistance to shock.

(7) Package mode

At present, the car-level IGBT mainly has three package modes: standard package mode - Type 1, pin-wing package - Type 2, planar package - Type 3.

The C-Class IGBT modules include the S1 module, the S2 module, the S3 module and the L1 module, which respectively mark the HP2, DC6 and HPDrive of Infineon, including all parameters and performance and external interface dimensions. There has not been a large-scale promotion. L1 is a flat package module that enables double-sided heat dissipation and is currently undergoing sample verification.

Fourth, the development trend of vehicle-level power device technology

In the future, motor controllers will be further miniaturized and lightweight, requiring high power and high efficiency, lower chip loss, higher current density, and higher frequency and operating temperature. At the same time, the module is required to have better heat dissipation, stronger solder joints and solder joints, and higher long-term reliability.

Current mainstream technology solutions include: fine-grained technology to achieve finer trenches to achieve higher current density; IGBT and FRD pass